System.InvalidOperationException class

This article provides supplementary remarks to the reference documentation for this API.

InvalidOperationException is used in cases when the failure to invoke a method is caused by reasons other than invalid arguments. Typically, it's thrown when the state of an object cannot support the method call. For example, an InvalidOperationException exception is thrown by methods such as:

• IEnumerator.MoveNext if objects of a collection are modified after the enumerator is created. For more information, see Changing a collection while iterating it.
• ResourceSet.GetString if the resource set is closed before the method call is made.
• XContainer.Add, if the object or objects to be added would result in an incorrectly structured XML document.
• A method that attempts to manipulate the UI from a thread that is not the main or UI thread.

InvalidOperationException uses the HRESULT COR_E_INVALIDOPERATION, which has the value 0x80131509.

For a list of initial property values for an instance of InvalidOperationException, see the InvalidOperationException constructors.

Common causes of InvalidOperationException exceptions

The following sections show how some common cases in which in InvalidOperationException exception is thrown in an app. How you handle the issue depends on the specific situation. Most commonly, however, the exception results from developer error, and the InvalidOperationException exception can be anticipated and avoided.

Updating a UI thread from a non-UI thread

Often, worker threads are used to perform some background work that involves gathering data to be displayed in an application's user interface. However. most GUI (graphical user interface) application frameworks for .NET, such as Windows Forms and Windows Presentation Foundation (WPF), let you access GUI objects only from the thread that creates and manages the UI (the Main or UI thread). An InvalidOperationException is thrown when you try to access a UI element from a thread other than the UI thread. The text of the exception message is shown in the following table.

UI frameworks for .NET implement a dispatcher pattern that includes a method to check whether a call to a member of a UI element is being executed on the UI thread, and other methods to schedule the call on the UI thread:

• In WPF apps, call the Dispatcher.CheckAccess method to determine if a method is running on a non-UI thread. It returns true if the method is running on the UI thread and false otherwise. Call one of the overloads of the Dispatcher.Invoke method to schedule the call on the UI thread.
• In UWP apps, check the CoreDispatcher.HasThreadAccess property to determine if a method is running on a non-UI thread. Call the CoreDispatcher.RunAsync method to execute a delegate that updates the UI thread.
• In Windows Forms apps, use the Control.InvokeRequired property to determine if a method is running on a non-UI thread. Call one of the overloads of the Control.Invoke method to execute a delegate that updates the UI thread.

The following examples illustrate the InvalidOperationException exception that is thrown when you attempt to update a UI element from a thread other than the thread that created it. Each example requires that you create two controls:

• A text box control named textBox1. In a Windows Forms app, you should set its Multiline property to true.
• A button control named threadExampleBtn. The example provides a handler, ThreadsExampleBtn_Click, for the button's Click event.

In each case, the threadExampleBtn_Click event handler calls the DoSomeWork method twice. The first call runs synchronously and succeeds. But the second call, because it runs asynchronously on a thread pool thread, attempts to update the UI from a non-UI thread. This results in a InvalidOperationException exception.

WPF apps

private async void threadExampleBtn_Click(object sender, RoutedEventArgs e)
{
    textBox1.Text = String.Empty;

    textBox1.Text = "Simulating work on UI thread.\n";
    DoSomeWork(20);
    textBox1.Text += "Work completed...\n";

    textBox1.Text += "Simulating work on non-UI thread.\n";
    await Task.Run(() => DoSomeWork(1000));
    textBox1.Text += "Work completed...\n";
}

private async void DoSomeWork(int milliseconds)
{
    // Simulate work.
    await Task.Delay(milliseconds);

    // Report completion.
    var msg = String.Format("Some work completed in {0} ms.\n", milliseconds);
    textBox1.Text += msg;
}

The following version of the DoSomeWork method eliminates the exception in a WPF app.

private async void DoSomeWork(int milliseconds)
{
    // Simulate work.
    await Task.Delay(milliseconds);

    // Report completion.
    bool uiAccess = textBox1.Dispatcher.CheckAccess();
    String msg = String.Format("Some work completed in {0} ms. on {1}UI thread\n",
                               milliseconds, uiAccess ? String.Empty : "non-");
    if (uiAccess)
        textBox1.Text += msg;
    else
        textBox1.Dispatcher.Invoke(() => { textBox1.Text += msg; });
}

Windows Forms apps

List<String> lines = new List<String>();

private async void threadExampleBtn_Click(object sender, EventArgs e)
{
    textBox1.Text = String.Empty;
    lines.Clear();

    lines.Add("Simulating work on UI thread.");
    textBox1.Lines = lines.ToArray();
    DoSomeWork(20);

    lines.Add("Simulating work on non-UI thread.");
    textBox1.Lines = lines.ToArray();
    await Task.Run(() => DoSomeWork(1000));

    lines.Add("ThreadsExampleBtn_Click completes. ");
    textBox1.Lines = lines.ToArray();
}

private async void DoSomeWork(int milliseconds)
{
    // simulate work
    await Task.Delay(milliseconds);

    // report completion
    lines.Add(String.Format("Some work completed in {0} ms on UI thread.", milliseconds));
    textBox1.Lines = lines.ToArray();
}

Dim lines As New List(Of String)()
Private Async Sub threadExampleBtn_Click(sender As Object, e As EventArgs) Handles Button1.Click
    TextBox1.Text = String.Empty
    lines.Clear()

    lines.Add("Simulating work on UI thread.")
    TextBox1.Lines = lines.ToArray()
    DoSomeWork(20)

    lines.Add("Simulating work on non-UI thread.")
    TextBox1.Lines = lines.ToArray()
    Await Task.Run(Sub() DoSomeWork(1000))

    lines.Add("ThreadsExampleBtn_Click completes. ")
    TextBox1.Lines = lines.ToArray()
End Sub

Private Async Sub DoSomeWork(milliseconds As Integer)
    ' Simulate work.
    Await Task.Delay(milliseconds)

    ' Report completion.
    lines.Add(String.Format("Some work completed in {0} ms on UI thread.", milliseconds))
    textBox1.Lines = lines.ToArray()
End Sub

The following version of the DoSomeWork method eliminates the exception in a Windows Forms app.

private async void DoSomeWork(int milliseconds)
{
    // simulate work
    await Task.Delay(milliseconds);

    // Report completion.
    bool uiMarshal = textBox1.InvokeRequired;
    String msg = String.Format("Some work completed in {0} ms. on {1}UI thread\n",
                               milliseconds, uiMarshal ? String.Empty : "non-");
    lines.Add(msg);

    if (uiMarshal) {
        textBox1.Invoke(new Action(() => { textBox1.Lines = lines.ToArray(); }));
    }
    else {
        textBox1.Lines = lines.ToArray();
    }
}

Private Async Sub DoSomeWork(milliseconds As Integer)
    ' Simulate work.
    Await Task.Delay(milliseconds)

    ' Report completion.
    Dim uiMarshal As Boolean = TextBox1.InvokeRequired
    Dim msg As String = String.Format("Some work completed in {0} ms. on {1}UI thread" + vbCrLf,
                                      milliseconds, If(uiMarshal, String.Empty, "non-"))
    lines.Add(msg)

    If uiMarshal Then
        TextBox1.Invoke(New Action(Sub() TextBox1.Lines = lines.ToArray()))
    Else
        TextBox1.Lines = lines.ToArray()
    End If
End Sub

Changing a collection while iterating it

The foreach statement in C#, for...in in F#, or For Each statement in Visual Basic is used to iterate the members of a collection and to read or modify its individual elements. However, it can't be used to add or remove items from the collection. Doing this throws an InvalidOperationException exception with a message that is similar to, "Collection was modified; enumeration operation may not execute."

The following example iterates a collection of integers attempts to add the square of each integer to the collection. The example throws an InvalidOperationException with the first call to the List<T>.Add method.

using System;
using System.Collections.Generic;

public class IteratingEx1
{
    public static void Main()
    {
        var numbers = new List<int>() { 1, 2, 3, 4, 5 };
        foreach (var number in numbers)
        {
            int square = (int)Math.Pow(number, 2);
            Console.WriteLine("{0}^{1}", number, square);
            Console.WriteLine("Adding {0} to the collection...\n", square);
            numbers.Add(square);
        }
    }
}
// The example displays the following output:
//    1^1
//    Adding 1 to the collection...
//
//
//    Unhandled Exception: System.InvalidOperationException: Collection was modified;
//       enumeration operation may not execute.
//       at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
//       at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
//       at Example.Main()

open System

let numbers = ResizeArray [| 1; 2; 3; 4; 5 |]
for number in numbers do
    let square = Math.Pow(number, 2) |> int
    printfn $"{number}^{square}"
    printfn $"Adding {square} to the collection...\n"
    numbers.Add square

// The example displays the following output:
//    1^1
//    Adding 1 to the collection...
//
//
//    Unhandled Exception: System.InvalidOperationException: Collection was modified
//       enumeration operation may not execute.
//       at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
//       at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
//       at <StartupCode$fs>.main()

Imports System.Collections.Generic

Module Example6
    Public Sub Main()
        Dim numbers As New List(Of Integer)({1, 2, 3, 4, 5})
        For Each number In numbers
            Dim square As Integer = CInt(Math.Pow(number, 2))
            Console.WriteLine("{0}^{1}", number, square)
            Console.WriteLine("Adding {0} to the collection..." + vbCrLf,
                           square)
            numbers.Add(square)
        Next
    End Sub
End Module
' The example displays the following output:
'    1^1
'    Adding 1 to the collection...
'    
'    
'    Unhandled Exception: System.InvalidOperationException: Collection was modified; 
'       enumeration operation may not execute.
'       at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
'       at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
'       at Example.Main()

You can eliminate the exception in one of two ways, depending on your application logic:

• If elements must be added to the collection while iterating it, you can iterate it by index using the for (for..to in F#) statement instead of foreach, for...in, or For Each. The following example uses the for statement to add the square of numbers in the collection to the collection.

  using System;
  using System.Collections.Generic;
  
  public class IteratingEx2
  {
      public static void Main()
      {
          var numbers = new List<int>() { 1, 2, 3, 4, 5 };
  
          int upperBound = numbers.Count - 1;
          for (int ctr = 0; ctr <= upperBound; ctr++)
          {
              int square = (int)Math.Pow(numbers[ctr], 2);
              Console.WriteLine("{0}^{1}", numbers[ctr], square);
              Console.WriteLine("Adding {0} to the collection...\n", square);
              numbers.Add(square);
          }
  
          Console.WriteLine("Elements now in the collection: ");
          foreach (var number in numbers)
              Console.Write("{0}    ", number);
      }
  }
  // The example displays the following output:
  //    1^1
  //    Adding 1 to the collection...
  //
  //    2^4
  //    Adding 4 to the collection...
  //
  //    3^9
  //    Adding 9 to the collection...
  //
  //    4^16
  //    Adding 16 to the collection...
  //
  //    5^25
  //    Adding 25 to the collection...
  //
  //    Elements now in the collection:
  //    1    2    3    4    5    1    4    9    16    25
  

  open System
  open System.Collections.Generic
  
  let numbers = ResizeArray [| 1; 2; 3; 4; 5 |]
  
  let upperBound = numbers.Count - 1
  for i = 0 to upperBound do
      let square = Math.Pow(numbers[i], 2) |> int
      printfn $"{numbers[i]}^{square}"
      printfn $"Adding {square} to the collection...\n"
      numbers.Add square
  
  printfn "Elements now in the collection: "
  for number in numbers do
      printf $"{number}    "
  // The example displays the following output:
  //    1^1
  //    Adding 1 to the collection...
  //
  //    2^4
  //    Adding 4 to the collection...
  //
  //    3^9
  //    Adding 9 to the collection...
  //
  //    4^16
  //    Adding 16 to the collection...
  //
  //    5^25
  //    Adding 25 to the collection...
  //
  //    Elements now in the collection:
  //    1    2    3    4    5    1    4    9    16    25
  

  Imports System.Collections.Generic
  
  Module Example7
      Public Sub Main()
          Dim numbers As New List(Of Integer)({1, 2, 3, 4, 5})
          Dim upperBound = numbers.Count - 1
  
          For ctr As Integer = 0 To upperBound
              Dim square As Integer = CInt(Math.Pow(numbers(ctr), 2))
              Console.WriteLine("{0}^{1}", numbers(ctr), square)
              Console.WriteLine("Adding {0} to the collection..." + vbCrLf,
                             square)
              numbers.Add(square)
          Next
  
          Console.WriteLine("Elements now in the collection: ")
          For Each number In numbers
              Console.Write("{0}    ", number)
          Next
      End Sub
  End Module
  ' The example displays the following output:
  '    1^1
  '    Adding 1 to the collection...
  '    
  '    2^4
  '    Adding 4 to the collection...
  '    
  '    3^9
  '    Adding 9 to the collection...
  '    
  '    4^16
  '    Adding 16 to the collection...
  '    
  '    5^25
  '    Adding 25 to the collection...
  '    
  '    Elements now in the collection:
  '    1    2    3    4    5    1    4    9    16    25
  

  Note that you must establish the number of iterations before iterating the collection either by using a counter inside the loop that will exit the loop appropriately, by iterating backward, from Count - 1 to 0, or, as the example does, by assigning the number of elements in the array to a variable and using it to establish the upper bound of the loop. Otherwise, if an element is added to the collection on every iteration, an endless loop results.

• If it is not necessary to add elements to the collection while iterating it, you can store the elements to be added in a temporary collection that you add when iterating the collection has finished. The following example uses this approach to add the square of numbers in a collection to a temporary collection, and then to combine the collections into a single array object.

  using System;
  using System.Collections.Generic;
  
  public class IteratingEx3
  {
      public static void Main()
      {
          var numbers = new List<int>() { 1, 2, 3, 4, 5 };
          var temp = new List<int>();
  
          // Square each number and store it in a temporary collection.
          foreach (var number in numbers)
          {
              int square = (int)Math.Pow(number, 2);
              temp.Add(square);
          }
  
          // Combine the numbers into a single array.
          int[] combined = new int[numbers.Count + temp.Count];
          Array.Copy(numbers.ToArray(), 0, combined, 0, numbers.Count);
          Array.Copy(temp.ToArray(), 0, combined, numbers.Count, temp.Count);
  
          // Iterate the array.
          foreach (var value in combined)
              Console.Write("{0}    ", value);
      }
  }
  // The example displays the following output:
  //       1    2    3    4    5    1    4    9    16    25
  

  open System
  open System.Collections.Generic
  
  let numbers = ResizeArray [| 1; 2; 3; 4; 5 |]
  let temp = ResizeArray()
  
  // Square each number and store it in a temporary collection.
  for number in numbers do
      let square = Math.Pow(number, 2) |> int
      temp.Add square
  
  // Combine the numbers into a single array.
  let combined = Array.zeroCreate<int> (numbers.Count + temp.Count)
  Array.Copy(numbers.ToArray(), 0, combined, 0, numbers.Count)
  Array.Copy(temp.ToArray(), 0, combined, numbers.Count, temp.Count)
  
  // Iterate the array.
  for value in combined do
      printf $"{value}    "
  // The example displays the following output:
  //       1    2    3    4    5    1    4    9    16    25
  

  Imports System.Collections.Generic
  
  Module Example8
      Public Sub Main()
          Dim numbers As New List(Of Integer)({1, 2, 3, 4, 5})
          Dim temp As New List(Of Integer)()
  
          ' Square each number and store it in a temporary collection.
          For Each number In numbers
              Dim square As Integer = CInt(Math.Pow(number, 2))
              temp.Add(square)
          Next
  
          ' Combine the numbers into a single array.
          Dim combined(numbers.Count + temp.Count - 1) As Integer
          Array.Copy(numbers.ToArray(), 0, combined, 0, numbers.Count)
          Array.Copy(temp.ToArray(), 0, combined, numbers.Count, temp.Count)
  
          ' Iterate the array.
          For Each value In combined
              Console.Write("{0}    ", value)
          Next
      End Sub
  End Module
  ' The example displays the following output:
  '       1    2    3    4    5    1    4    9    16    25
  

Sorting an array or collection whose objects cannot be compared

General-purpose sorting methods, such as the Array.Sort(Array) method or the List<T>.Sort() method, usually require that at least one of the objects to be sorted implement the IComparable<T> or the IComparable interface. If not, the collection or array cannot be sorted, and the method throws an InvalidOperationException exception. The following example defines a Person class, stores two Person objects in a generic List<T> object, and attempts to sort them. As the output from the example shows, the call to the List<T>.Sort() method throws an InvalidOperationException.

using System;
using System.Collections.Generic;

public class Person1
{
    public Person1(string fName, string lName)
    {
        FirstName = fName;
        LastName = lName;
    }

    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class ListSortEx1
{
    public static void Main()
    {
        var people = new List<Person1>();

        people.Add(new Person1("John", "Doe"));
        people.Add(new Person1("Jane", "Doe"));
        people.Sort();
        foreach (var person in people)
            Console.WriteLine("{0} {1}", person.FirstName, person.LastName);
    }
}
// The example displays the following output:
//    Unhandled Exception: System.InvalidOperationException: Failed to compare two elements in the array. --->
//       System.ArgumentException: At least one object must implement IComparable.
//       at System.Collections.Comparer.Compare(Object a, Object b)
//       at System.Collections.Generic.ArraySortHelper`1.SwapIfGreater(T[] keys, IComparer`1 comparer, Int32 a, Int32 b)
//       at System.Collections.Generic.ArraySortHelper`1.DepthLimitedQuickSort(T[] keys, Int32 left, Int32 right, IComparer`1 comparer, Int32 depthLimit)
//       at System.Collections.Generic.ArraySortHelper`1.Sort(T[] keys, Int32 index, Int32 length, IComparer`1 comparer)
//       --- End of inner exception stack trace ---
//       at System.Collections.Generic.ArraySortHelper`1.Sort(T[] keys, Int32 index, Int32 length, IComparer`1 comparer)
//       at System.Array.Sort[T](T[] array, Int32 index, Int32 length, IComparer`1 comparer)
//       at System.Collections.Generic.List`1.Sort(Int32 index, Int32 count, IComparer`1 comparer)
//       at Example.Main()

type Person(firstName: string, lastName: string) =
    member val FirstName = firstName with get, set
    member val LastName = lastName with get, set

let people = ResizeArray()

people.Add(Person("John", "Doe"))
people.Add(Person("Jane", "Doe"))
people.Sort()
for person in people do
    printfn $"{person.FirstName} {person.LastName}"
// The example displays the following output:
//    Unhandled Exception: System.InvalidOperationException: Failed to compare two elements in the array. --->
//       System.ArgumentException: At least one object must implement IComparable.
//       at System.Collections.Comparer.Compare(Object a, Object b)
//       at System.Collections.Generic.ArraySortHelper`1.SwapIfGreater(T[] keys, IComparer`1 comparer, Int32 a, Int32 b)
//       at System.Collections.Generic.ArraySortHelper`1.DepthLimitedQuickSort(T[] keys, Int32 left, Int32 right, IComparer`1 comparer, Int32 depthLimit)
//       at System.Collections.Generic.ArraySortHelper`1.Sort(T[] keys, Int32 index, Int32 length, IComparer`1 comparer)
//       --- End of inner exception stack trace ---
//       at System.Collections.Generic.ArraySortHelper`1.Sort(T[] keys, Int32 index, Int32 length, IComparer`1 comparer)
//       at System.Array.Sort[T](T[] array, Int32 index, Int32 length, IComparer`1 comparer)
//       at System.Collections.Generic.List`1.Sort(Int32 index, Int32 count, IComparer`1 comparer)
//       at <StartupCode$fs>.main()

Imports System.Collections.Generic

Public Class Person9
    Public Sub New(fName As String, lName As String)
        FirstName = fName
        LastName = lName
    End Sub

    Public Property FirstName As String
    Public Property LastName As String
End Class

Module Example9
    Public Sub Main()
        Dim people As New List(Of Person9)()

        people.Add(New Person9("John", "Doe"))
        people.Add(New Person9("Jane", "Doe"))
        people.Sort()
        For Each person In people
            Console.WriteLine("{0} {1}", person.FirstName, person.LastName)
        Next
    End Sub
End Module
' The example displays the following output:
'    Unhandled Exception: System.InvalidOperationException: Failed to compare two elements in the array. ---> 
'       System.ArgumentException: At least one object must implement IComparable.
'       at System.Collections.Comparer.Compare(Object a, Object b)
'       at System.Collections.Generic.ArraySortHelper`1.SwapIfGreater(T[] keys, IComparer`1 comparer, Int32 a, Int32 b)
'       at System.Collections.Generic.ArraySortHelper`1.DepthLimitedQuickSort(T[] keys, Int32 left, Int32 right, IComparer`1 comparer, Int32 depthLimit)
'       at System.Collections.Generic.ArraySortHelper`1.Sort(T[] keys, Int32 index, Int32 length, IComparer`1 comparer)
'       --- End of inner exception stack trace ---
'       at System.Collections.Generic.ArraySortHelper`1.Sort(T[] keys, Int32 index, Int32 length, IComparer`1 comparer)
'       at System.Array.Sort[T](T[] array, Int32 index, Int32 length, IComparer`1 comparer)
'       at System.Collections.Generic.List`1.Sort(Int32 index, Int32 count, IComparer`1 comparer)
'       at Example.Main()

You can eliminate the exception in any of three ways:

• If you can own the type that you are trying to sort (that is, if you control its source code), you can modify it to implement the IComparable<T> or the IComparable interface. This requires that you implement either the IComparable<T>.CompareTo or the CompareTo method. Adding an interface implementation to an existing type is not a breaking change.

  The following example uses this approach to provide an IComparable<T> implementation for the Person class. You can still call the collection or array's general sorting method and, as the output from the example shows, the collection sorts successfully.

  using System;
  using System.Collections.Generic;
  
  public class Person2 : IComparable<Person>
  {
      public Person2(String fName, String lName)
      {
          FirstName = fName;
          LastName = lName;
      }
  
      public String FirstName { get; set; }
      public String LastName { get; set; }
  
      public int CompareTo(Person other)
      {
          return String.Format("{0} {1}", LastName, FirstName).
                 CompareTo(String.Format("{0} {1}", other.LastName, other.FirstName));
      }
  }
  
  public class ListSortEx2
  {
      public static void Main()
      {
          var people = new List<Person2>();
  
          people.Add(new Person2("John", "Doe"));
          people.Add(new Person2("Jane", "Doe"));
          people.Sort();
          foreach (var person in people)
              Console.WriteLine("{0} {1}", person.FirstName, person.LastName);
      }
  }
  // The example displays the following output:
  //       Jane Doe
  //       John Doe
  

  open System
  
  type Person(firstName: string, lastName: string) =
      member val FirstName = firstName with get, set
      member val LastName = lastName with get, set
      
      interface IComparable<Person> with
          member this.CompareTo(other) =
              compare $"{this.LastName} {this.FirstName}" $"{other.LastName} {other.FirstName}"
  
  let people = ResizeArray()
  
  people.Add(new Person("John", "Doe"))
  people.Add(new Person("Jane", "Doe"))
  people.Sort()
  for person in people do
      printfn $"{person.FirstName} {person.LastName}"
  // The example displays the following output:
  //       Jane Doe
  //       John Doe
  

  Imports System.Collections.Generic
  
  Public Class Person : Implements IComparable(Of Person)
     Public Sub New(fName As String, lName As String)
        FirstName = fName
        LastName = lName
     End Sub
     
     Public Property FirstName As String
     Public Property LastName As String
     
     Public Function CompareTo(other As Person) As Integer _
            Implements IComparable(Of Person).CompareTo
        Return String.Format("{0} {1}", LastName, FirstName).
              CompareTo(String.Format("{0} {1}", other.LastName, other.FirstName))    
     End Function
  End Class
  
  Module Example10
      Public Sub Main()
          Dim people As New List(Of Person)()
  
          people.Add(New Person("John", "Doe"))
          people.Add(New Person("Jane", "Doe"))
          people.Sort()
          For Each person In people
              Console.WriteLine("{0} {1}", person.FirstName, person.LastName)
          Next
      End Sub
  End Module
  ' The example displays the following output:
  '       Jane Doe
  '       John Doe
  

• If you cannot modify the source code for the type you are trying to sort, you can define a special-purpose sorting class that implements the IComparer<T> interface. You can call an overload of the Sort method that includes an IComparer<T> parameter. This approach is especially useful if you want to develop a specialized sorting class that can sort objects based on multiple criteria.

  The following example uses the approach by developing a custom PersonComparer class that is used to sort Person collections. It then passes an instance of this class to the List<T>.Sort(IComparer<T>) method.

  using System;
  using System.Collections.Generic;
  
  public class Person3
  {
      public Person3(String fName, String lName)
      {
          FirstName = fName;
          LastName = lName;
      }
  
      public String FirstName { get; set; }
      public String LastName { get; set; }
  }
  
  public class PersonComparer : IComparer<Person3>
  {
      public int Compare(Person3 x, Person3 y)
      {
          return String.Format("{0} {1}", x.LastName, x.FirstName).
                 CompareTo(String.Format("{0} {1}", y.LastName, y.FirstName));
      }
  }
  
  public class ListSortEx3
  {
      public static void Main()
      {
          var people = new List<Person3>();
  
          people.Add(new Person3("John", "Doe"));
          people.Add(new Person3("Jane", "Doe"));
          people.Sort(new PersonComparer());
          foreach (var person in people)
              Console.WriteLine("{0} {1}", person.FirstName, person.LastName);
      }
  }
  // The example displays the following output:
  //       Jane Doe
  //       John Doe
  

  open System
  open System.Collections.Generic
  
  type Person(firstName, lastName) =
      member val FirstName = firstName with get, set
      member val LastName = lastName with get, set
  
  type PersonComparer() =
      interface IComparer<Person> with
          member _.Compare(x: Person, y: Person) =
              $"{x.LastName} {x.FirstName}".CompareTo $"{y.LastName} {y.FirstName}"
  
  let people = ResizeArray()
  
  people.Add(Person("John", "Doe"))
  people.Add(Person("Jane", "Doe"))
  people.Sort(PersonComparer())
  for person in people do
      printfn $"{person.FirstName} {person.LastName}"
  // The example displays the following output:
  //       Jane Doe
  //       John Doe
  

  Imports System.Collections.Generic
  
  Public Class Person11
      Public Sub New(fName As String, lName As String)
          FirstName = fName
          LastName = lName
      End Sub
  
      Public Property FirstName As String
      Public Property LastName As String
  End Class
  
  Public Class PersonComparer : Implements IComparer(Of Person11)
      Public Function Compare(x As Person11, y As Person11) As Integer _
            Implements IComparer(Of Person11).Compare
          Return String.Format("{0} {1}", x.LastName, x.FirstName).
               CompareTo(String.Format("{0} {1}", y.LastName, y.FirstName))
      End Function
  End Class
  
  Module Example11
      Public Sub Main()
          Dim people As New List(Of Person11)()
  
          people.Add(New Person11("John", "Doe"))
          people.Add(New Person11("Jane", "Doe"))
          people.Sort(New PersonComparer())
          For Each person In people
              Console.WriteLine("{0} {1}", person.FirstName, person.LastName)
          Next
      End Sub
  End Module
  ' The example displays the following output:
  '       Jane Doe
  '       John Doe
  

• If you cannot modify the source code for the type you are trying to sort, you can create a Comparison<T> delegate to perform the sorting. The delegate signature is

  Function Comparison(Of T)(x As T, y As T) As Integer
  

  int Comparison<T>(T x, T y)
  

  The following example uses the approach by defining a PersonComparison method that matches the Comparison<T> delegate signature. It then passes this delegate to the List<T>.Sort(Comparison<T>) method.

  using System;
  using System.Collections.Generic;
  
  public class Person
  {
     public Person(String fName, String lName)
     {
        FirstName = fName;
        LastName = lName;
     }
  
     public String FirstName { get; set; }
     public String LastName { get; set; }
  }
  
  public class ListSortEx4
  {
     public static void Main()
     {
        var people = new List<Person>();
  
        people.Add(new Person("John", "Doe"));
        people.Add(new Person("Jane", "Doe"));
        people.Sort(PersonComparison);
        foreach (var person in people)
           Console.WriteLine("{0} {1}", person.FirstName, person.LastName);
     }
  
     public static int PersonComparison(Person x, Person y)
     {
        return String.Format("{0} {1}", x.LastName, x.FirstName).
               CompareTo(String.Format("{0} {1}", y.LastName, y.FirstName));
     }
  }
  // The example displays the following output:
  //       Jane Doe
  //       John Doe
  

  open System
  open System.Collections.Generic
  
  type Person(firstName, lastName) =
      member val FirstName = firstName with get, set
      member val LastName = lastName with get, set
  
  let personComparison (x: Person) (y: Person) =
      $"{x.LastName} {x.FirstName}".CompareTo $"{y.LastName} {y.FirstName}"
  
  let people = ResizeArray()
  
  people.Add(Person("John", "Doe"))
  people.Add(Person("Jane", "Doe"))
  people.Sort personComparison
  for person in people do
      printfn $"{person.FirstName} {person.LastName}"
  
  // The example displays the following output:
  //       Jane Doe
  //       John Doe
  

  Imports System.Collections.Generic
  
  Public Class Person12
      Public Sub New(fName As String, lName As String)
          FirstName = fName
          LastName = lName
      End Sub
  
      Public Property FirstName As String
      Public Property LastName As String
  End Class
  
  Module Example12
      Public Sub Main()
          Dim people As New List(Of Person12)()
  
          people.Add(New Person12("John", "Doe"))
          people.Add(New Person12("Jane", "Doe"))
          people.Sort(AddressOf PersonComparison)
          For Each person In people
              Console.WriteLine("{0} {1}", person.FirstName, person.LastName)
          Next
      End Sub
  
      Public Function PersonComparison(x As Person12, y As Person12) As Integer
          Return String.Format("{0} {1}", x.LastName, x.FirstName).
               CompareTo(String.Format("{0} {1}", y.LastName, y.FirstName))
      End Function
  End Module
  ' The example displays the following output:
  '       Jane Doe
  '       John Doe
  

Casting a Nullable<T> that's null to its underlying type

Attempting to cast a Nullable<T> value that is null to its underlying type throws an InvalidOperationException exception and displays the error message, "Nullable object must have a value.

The following example throws an InvalidOperationException exception when it attempts to iterate an array that includes a Nullable(Of Integer) value.

using System;
using System.Linq;

public class NullableEx1
{
   public static void Main()
   {
      var queryResult = new int?[] { 1, 2, null, 4 };
      var map = queryResult.Select(nullableInt => (int)nullableInt);

      // Display list.
      foreach (var num in map)
         Console.Write("{0} ", num);
      Console.WriteLine();
   }
}
// The example displays the following output:
//    1 2
//    Unhandled Exception: System.InvalidOperationException: Nullable object must have a value.
//       at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
//       at Example.<Main>b__0(Nullable`1 nullableInt)
//       at System.Linq.Enumerable.WhereSelectArrayIterator`2.MoveNext()
//       at Example.Main()

open System
open System.Linq

let queryResult = [| Nullable 1; Nullable 2; Nullable(); Nullable 4 |]
let map = queryResult.Select(fun nullableInt -> nullableInt.Value)

// Display list.
for num in map do
    printf $"{num} "
printfn ""
// The example displays the following output:
//    1 2
//    Unhandled Exception: System.InvalidOperationException: Nullable object must have a value.
//       at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
//       at Example.<Main>b__0(Nullable`1 nullableInt)
//       at System.Linq.Enumerable.WhereSelectArrayIterator`2.MoveNext()
//       at <StartupCode$fs>.main()

Imports System.Linq

Module Example13
    Public Sub Main()
        Dim queryResult = New Integer?() {1, 2, Nothing, 4}
        Dim map = queryResult.Select(Function(nullableInt) CInt(nullableInt))

        ' Display list.
        For Each num In map
            Console.Write("{0} ", num)
        Next
        Console.WriteLine()
    End Sub
End Module
' The example displays thIe following output:
'    1 2
'    Unhandled Exception: System.InvalidOperationException: Nullable object must have a value.
'       at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
'       at Example.<Main>b__0(Nullable`1 nullableInt)
'       at System.Linq.Enumerable.WhereSelectArrayIterator`2.MoveNext()
'       at Example.Main()

To prevent the exception:

• Use the Nullable<T>.HasValue property to select only those elements that are not null.
• Call one of the Nullable<T>.GetValueOrDefault overloads to provide a default value for a null value.

The following example does both to avoid the InvalidOperationException exception.

using System;
using System.Linq;

public class NullableEx2
{
   public static void Main()
   {
      var queryResult = new int?[] { 1, 2, null, 4 };
      var numbers = queryResult.Select(nullableInt => (int)nullableInt.GetValueOrDefault());

      // Display list using Nullable<int>.HasValue.
      foreach (var number in numbers)
         Console.Write("{0} ", number);
      Console.WriteLine();

      numbers = queryResult.Select(nullableInt => (int) (nullableInt.HasValue ? nullableInt : -1));
      // Display list using Nullable<int>.GetValueOrDefault.
      foreach (var number in numbers)
         Console.Write("{0} ", number);
      Console.WriteLine();
   }
}
// The example displays the following output:
//       1 2 0 4
//       1 2 -1 4

open System
open System.Linq

let queryResult = [| Nullable 1; Nullable 2; Nullable(); Nullable 4 |]
let numbers = queryResult.Select(fun nullableInt -> nullableInt.GetValueOrDefault())

// Display list using Nullable<int>.HasValue.
for number in numbers do
    printf $"{number} "
printfn ""

let numbers2 = queryResult.Select(fun nullableInt -> if nullableInt.HasValue then nullableInt.Value else -1)
// Display list using Nullable<int>.GetValueOrDefault.
for number in numbers2 do
    printf $"{number} "
printfn ""
// The example displays the following output:
//       1 2 0 4
//       1 2 -1 4

Imports System.Linq

Module Example14
    Public Sub Main()
        Dim queryResult = New Integer?() {1, 2, Nothing, 4}
        Dim numbers = queryResult.Select(Function(nullableInt) _
                                          CInt(nullableInt.GetValueOrDefault()))
        ' Display list.
        For Each number In numbers
            Console.Write("{0} ", number)
        Next
        Console.WriteLine()

        ' Use -1 to indicate a missing values.
        numbers = queryResult.Select(Function(nullableInt) _
                                      CInt(If(nullableInt.HasValue, nullableInt, -1)))
        ' Display list.
        For Each number In numbers
            Console.Write("{0} ", number)
        Next
        Console.WriteLine()

    End Sub
End Module
' The example displays the following output:
'       1 2 0 4
'       1 2 -1 4

Call a System.Linq.Enumerable method on an empty collection

The Enumerable.Aggregate, Enumerable.Average, Enumerable.First, Enumerable.Last, Enumerable.Max, Enumerable.Min, Enumerable.Single, and Enumerable.SingleOrDefault methods perform operations on a sequence and return a single result. Some overloads of these methods throw an InvalidOperationException exception when the sequence is empty, while other overloads return null. The Enumerable.SingleOrDefault method also throws an InvalidOperationException exception when the sequence contains more than one element.

The following table lists the exception messages from the InvalidOperationException exception objects thrown by calls to some System.Linq.Enumerable methods.

How you eliminate or handle the exception depends on your application's assumptions and on the particular method you call.

• When you deliberately call one of these methods without checking for an empty sequence, you are assuming that the sequence is not empty, and that an empty sequence is an unexpected occurrence. In this case, catching or rethrowing the exception is appropriate.

• If your failure to check for an empty sequence was inadvertent, you can call one of the overloads of the Enumerable.Any overload to determine whether a sequence contains any elements.

  Tip

  Calling the Enumerable.Any<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) method before generating a sequence can improve performance if the data to be processed might contain a large number of elements or if operation that generates the sequence is expensive.

• If you've called a method such as Enumerable.First, Enumerable.Last, or Enumerable.Single, you can substitute an alternate method, such as Enumerable.FirstOrDefault, Enumerable.LastOrDefault, or Enumerable.SingleOrDefault, that returns a default value instead of a member of the sequence.

The examples provide additional detail.

The following example uses the Enumerable.Average method to compute the average of a sequence whose values are greater than 4. Since no values from the original array exceed 4, no values are included in the sequence, and the method throws an InvalidOperationException exception.

using System;
using System.Linq;

public class Example
{
   public static void Main()
   {
      int[] data = { 1, 2, 3, 4 };
      var average = data.Where(num => num > 4).Average();
      Console.Write("The average of numbers greater than 4 is {0}",
                    average);
   }
}
// The example displays the following output:
//    Unhandled Exception: System.InvalidOperationException: Sequence contains no elements
//       at System.Linq.Enumerable.Average(IEnumerable`1 source)
//       at Example.Main()

open System
open System.Linq

let data = [| 1; 2; 3; 4 |]
let average = 
   data.Where(fun num -> num > 4).Average();
printfn $"The average of numbers greater than 4 is {average}"
// The example displays the following output:
//    Unhandled Exception: System.InvalidOperationException: Sequence contains no elements
//       at System.Linq.Enumerable.Average(IEnumerable`1 source)
//       at <StartupCode$fs>.main()

Imports System.Linq

Module Example
   Public Sub Main()
      Dim data() As Integer = { 1, 2, 3, 4 }
      Dim average = data.Where(Function(num) num > 4).Average()
      Console.Write("The average of numbers greater than 4 is {0}",
                    average)
   End Sub
End Module
' The example displays the following output:
'    Unhandled Exception: System.InvalidOperationException: Sequence contains no elements
'       at System.Linq.Enumerable.Average(IEnumerable`1 source)
'       at Example.Main()

The exception can be eliminated by calling the Any method to determine whether the sequence contains any elements before calling the method that processes the sequence, as the following example shows.

using System;
using System.Linq;

public class EnumerableEx2
{
    public static void Main()
    {
        int[] dbQueryResults = { 1, 2, 3, 4 };
        var moreThan4 = dbQueryResults.Where(num => num > 4);

        if (moreThan4.Any())
            Console.WriteLine("Average value of numbers greater than 4: {0}:",
                              moreThan4.Average());
        else
            // handle empty collection
            Console.WriteLine("The dataset has no values greater than 4.");
    }
}
// The example displays the following output:
//       The dataset has no values greater than 4.

open System
open System.Linq

let dbQueryResults = [| 1; 2; 3; 4 |]
let moreThan4 = 
    dbQueryResults.Where(fun num -> num > 4)

if moreThan4.Any() then
    printfn $"Average value of numbers greater than 4: {moreThan4.Average()}:"
else
    // handle empty collection
    printfn "The dataset has no values greater than 4."

// The example displays the following output:
//       The dataset has no values greater than 4.

Imports System.Linq

Module Example1
    Public Sub Main()
        Dim dbQueryResults() As Integer = {1, 2, 3, 4}
        Dim moreThan4 = dbQueryResults.Where(Function(num) num > 4)

        If moreThan4.Any() Then
            Console.WriteLine("Average value of numbers greater than 4: {0}:",
                             moreThan4.Average())
        Else
            ' Handle empty collection. 
            Console.WriteLine("The dataset has no values greater than 4.")
        End If
    End Sub
End Module
' The example displays the following output:
'       The dataset has no values greater than 4.

The Enumerable.First method returns the first item in a sequence or the first element in a sequence that satisfies a specified condition. If the sequence is empty and therefore does not have a first element, it throws an InvalidOperationException exception.

In the following example, the Enumerable.First<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) method throws an InvalidOperationException exception because the dbQueryResults array doesn't contain an element greater than 4.

using System;
using System.Linq;

public class EnumerableEx3
{
    public static void Main()
    {
        int[] dbQueryResults = { 1, 2, 3, 4 };

        var firstNum = dbQueryResults.First(n => n > 4);

        Console.WriteLine("The first value greater than 4 is {0}",
                          firstNum);
    }
}
// The example displays the following output:
//    Unhandled Exception: System.InvalidOperationException:
//       Sequence contains no matching element
//       at System.Linq.Enumerable.First[TSource](IEnumerable`1 source, Func`2 predicate)
//       at Example.Main()

open System
open System.Linq

let dbQueryResults = [| 1; 2; 3; 4 |]

let firstNum = dbQueryResults.First(fun n -> n > 4)

printfn $"The first value greater than 4 is {firstNum}"

// The example displays the following output:
//    Unhandled Exception: System.InvalidOperationException:
//       Sequence contains no matching element
//       at System.Linq.Enumerable.First[TSource](IEnumerable`1 source, Func`2 predicate)
//       at <StartupCode$fs>.main()

Imports System.Linq

Module Example2
    Public Sub Main()
        Dim dbQueryResults() As Integer = {1, 2, 3, 4}

        Dim firstNum = dbQueryResults.First(Function(n) n > 4)

        Console.WriteLine("The first value greater than 4 is {0}",
                        firstNum)
    End Sub
End Module
' The example displays the following output:
'    Unhandled Exception: System.InvalidOperationException: 
'       Sequence contains no matching element
'       at System.Linq.Enumerable.First[TSource](IEnumerable`1 source, Func`2 predicate)
'       at Example.Main()

You can call the Enumerable.FirstOrDefault method instead of Enumerable.First to return a specified or default value. If the method does not find a first element in the sequence, it returns the default value for that data type. The default value is null for a reference type, zero for a numeric data type, and DateTime.MinValue for the DateTime type.

The following example calls the Enumerable.FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) method to prevent the InvalidOperationException exception thrown in the previous example.

using System;
using System.Linq;

public class EnumerableEx4
{
    public static void Main()
    {
        int[] dbQueryResults = { 1, 2, 3, 4 };

        var firstNum = dbQueryResults.FirstOrDefault(n => n > 4);

        if (firstNum == 0)
            Console.WriteLine("No value is greater than 4.");
        else
            Console.WriteLine("The first value greater than 4 is {0}",
                              firstNum);
    }
}
// The example displays the following output:
//       No value is greater than 4.

open System
open System.Linq

let dbQueryResults = [| 1; 2; 3; 4 |]

let firstNum = dbQueryResults.FirstOrDefault(fun n -> n > 4)

if firstNum = 0 then
    printfn "No value is greater than 4."
else
    printfn $"The first value greater than 4 is {firstNum}"

// The example displays the following output:
//       No value is greater than 4.

Imports System.Linq

Module Example3
    Public Sub Main()
        Dim dbQueryResults() As Integer = {1, 2, 3, 4}

        Dim firstNum = dbQueryResults.FirstOrDefault(Function(n) n > 4)

        If firstNum = 0 Then
            Console.WriteLine("No value is greater than 4.")
        Else
            Console.WriteLine("The first value greater than 4 is {0}",
                           firstNum)
        End If
    End Sub
End Module
' The example displays the following output:
'       No value is greater than 4.

Call Enumerable.Single or Enumerable.SingleOrDefault on a sequence without one element

The Enumerable.Single method returns the only element of a sequence, or the only element of a sequence that meets a specified condition. If there are no elements in the sequence, or if there is more than one element , the method throws an InvalidOperationException exception.

You can use the Enumerable.SingleOrDefault method to return a default value instead of throwing an exception when the sequence contains no elements. However, the Enumerable.SingleOrDefault method still throws an InvalidOperationException exception when the sequence contains more than one element.

The following table lists the exception messages from the InvalidOperationException exception objects thrown by calls to the Enumerable.Single and Enumerable.SingleOrDefault methods.

In the following example, the call to the Enumerable.Single method throws an InvalidOperationException exception because the sequence doesn't have an element greater than 4.

using System;
using System.Linq;

public class EnumerableEx5
{
    public static void Main()
    {
        int[] dbQueryResults = { 1, 2, 3, 4 };

        var singleObject = dbQueryResults.Single(value => value > 4);

        // Display results.
        Console.WriteLine("{0} is the only value greater than 4", singleObject);
    }
}
// The example displays the following output:
//    Unhandled Exception: System.InvalidOperationException:
//       Sequence contains no matching element
//       at System.Linq.Enumerable.Single[TSource](IEnumerable`1 source, Func`2 predicate)
//       at Example.Main()

open System
open System.Linq

let dbQueryResults = [| 1; 2; 3; 4 |]

let singleObject = dbQueryResults.Single(fun value -> value > 4)

// Display results.
printfn $"{singleObject} is the only value greater than 4"

// The example displays the following output:
//    Unhandled Exception: System.InvalidOperationException:
//       Sequence contains no matching element
//       at System.Linq.Enumerable.Single[TSource](IEnumerable`1 source, Func`2 predicate)
//       at <StartupCode$fs>.main()

Imports System.Linq

Module Example4
    Public Sub Main()
        Dim dbQueryResults() As Integer = {1, 2, 3, 4}

        Dim singleObject = dbQueryResults.Single(Function(value) value > 4)

        ' Display results.
        Console.WriteLine("{0} is the only value greater than 4",
                         singleObject)
    End Sub
End Module
' The example displays the following output:
'    Unhandled Exception: System.InvalidOperationException: 
'       Sequence contains no matching element
'       at System.Linq.Enumerable.Single[TSource](IEnumerable`1 source, Func`2 predicate)
'       at Example.Main()

The following example attempts to prevent the InvalidOperationException exception thrown when a sequence is empty by instead calling the Enumerable.SingleOrDefault method. However, because this sequence returns multiple elements whose value is greater than 2, it also throws an InvalidOperationException exception.

using System;
using System.Linq;

public class EnumerableEx6
{
    public static void Main()
    {
        int[] dbQueryResults = { 1, 2, 3, 4 };

        var singleObject = dbQueryResults.SingleOrDefault(value => value > 2);

        if (singleObject != 0)
            Console.WriteLine("{0} is the only value greater than 2",
                              singleObject);
        else
            // Handle an empty collection.
            Console.WriteLine("No value is greater than 2");
    }
}
// The example displays the following output:
//    Unhandled Exception: System.InvalidOperationException:
//       Sequence contains more than one matching element
//       at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1 source, Func`2 predicate)
//       at Example.Main()

open System
open System.Linq

let dbQueryResults = [| 1; 2; 3; 4 |]

let singleObject = dbQueryResults.SingleOrDefault(fun value -> value > 2)

if singleObject <> 0 then
    printfn $"{singleObject} is the only value greater than 2"
else
    // Handle an empty collection.
    printfn "No value is greater than 2"
    
// The example displays the following output:
//    Unhandled Exception: System.InvalidOperationException:
//       Sequence contains more than one matching element
//       at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1 source, Func`2 predicate)
//       at <StartupCode$fs>.main()

Imports System.Linq

Module Example5
    Public Sub Main()
        Dim dbQueryResults() As Integer = {1, 2, 3, 4}

        Dim singleObject = dbQueryResults.SingleOrDefault(Function(value) value > 2)

        If singleObject <> 0 Then
            Console.WriteLine("{0} is the only value greater than 2",
                             singleObject)
        Else
            ' Handle an empty collection.
            Console.WriteLine("No value is greater than 2")
        End If
    End Sub
End Module
' The example displays the following output:
'    Unhandled Exception: System.InvalidOperationException: 
'       Sequence contains more than one matching element
'       at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1 source, Func`2 predicate)
'       at Example.Main()

Calling the Enumerable.Single method assumes that either a sequence or the sequence that meets specified criteria contains only one element. Enumerable.SingleOrDefault assumes a sequence with zero or one result, but no more. If this assumption is a deliberate one on your part and these conditions are not met, rethrowing or catching the resulting InvalidOperationException is appropriate. Otherwise, or if you expect that invalid conditions will occur with some frequency, you should consider using some other Enumerable method, such as FirstOrDefault or Where.

Dynamic cross-application domain field access

The OpCodes.Ldflda common intermediate language (CIL) instruction throws an InvalidOperationException exception if the object containing the field whose address you are trying to retrieve is not within the application domain in which your code is executing. The address of a field can only be accessed from the application domain in which it resides.

Throw an InvalidOperationException exception

You should throw an InvalidOperationException exception only when the state of your object for some reason does not support a particular method call. That is, the method call is valid in some circumstances or contexts, but is invalid in others.

If the method invocation failure is due to invalid arguments, then ArgumentException or one of its derived classes, ArgumentNullException or ArgumentOutOfRangeException, should be thrown instead.