• [アーティクル]

Enumerable.FirstOrDefault<TSource> Method (IEnumerable<TSource>)

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Returns the first element of a sequence, or a default value if the sequence contains no elements.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function FirstOrDefault(Of TSource) ( _
    source As IEnumerable(Of TSource) _
) As TSource

public static TSource FirstOrDefault<TSource>(
    this IEnumerable<TSource> source
)

Type Parameters

  • TSource
    The type of the elements of source.

Parameters

Return Value

Type: TSource
default (TSource) if source is empty; otherwise, the first element in source.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<TSource>. When you use instance method syntax to call this method, omit the first parameter.

Exceptions

Exception Condition
ArgumentNullException

source is nulla null reference (Nothing in Visual Basic).

Remarks

The default value for reference and nullable types is nulla null reference (Nothing in Visual Basic).

The FirstOrDefault method does not provide a way to specify a default value. If you want to specify a default value other than default(TSource), use the DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) method as described in the Example section.

Examples

The following code example demonstrates how to use FirstOrDefault<TSource>(IEnumerable<TSource>) on an empty array.

      ' Create an empty array.
      Dim numbers() As Integer = {}

      ' Select the first element in the array, or a default value
      ' if there are not elements in the array.
      Dim first As Integer = numbers.FirstOrDefault()

      ' Display the output.
      outputBlock.Text &= first & vbCrLf

      ' This code produces the following output:
      '
      ' 0


      int[] numbers = { };
      int first = numbers.FirstOrDefault();
      outputBlock.Text += first + "\n";

      /*
       This code produces the following output:

       0
      */

Sometimes the value of default(TSource) is not the default value that you want to use if the collection contains no elements. Instead of checking the result for the unwanted default value and then changing it if necessary, you can use the DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) method to specify the default value that you want to use if the collection is empty. Then, call First<TSource>(IEnumerable<TSource>) to obtain the first element. The following code example uses both techniques to obtain a default value of 1 if a collection of numeric months is empty. Because the default value for an integer is 0, which does not correspond to any month, the default value must be specified as 1 instead. The first result variable is checked for the unwanted default value after the query has finished executing. The second result variable is obtained by using DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) to specify a default value of 1.

      Dim months As New List(Of Integer)(New Integer() {})

      ' Setting the default value to 1 after the query.
      Dim firstMonth1 As Integer = months.FirstOrDefault()
      If firstMonth1 = 0 Then
         firstMonth1 = 1
      End If
      outputBlock.Text &= String.Format("The value of the firstMonth1 variable is {0}", firstMonth1) _
                          & vbCrLf

      ' Setting the default value to 1 by using DefaultIfEmpty() in the query.
      Dim firstMonth2 As Integer = months.DefaultIfEmpty(1).First()
      outputBlock.Text &= String.Format("The value of the firstMonth2 variable is {0}", firstMonth2) & _
                           vbCrLf

      ' This code produces the following output:
      '
      ' The value of the firstMonth1 variable is 1
      ' The value of the firstMonth2 variable is 1


      List<int> months = new List<int> { };

      // Setting the default value to 1 after the query.
      int firstMonth1 = months.FirstOrDefault();
      if (firstMonth1 == 0)
      {
         firstMonth1 = 1;
      }
      outputBlock.Text += String.Format("The value of the firstMonth1 variable is {0}", firstMonth1) + "\n";

      // Setting the default value to 1 by using DefaultIfEmpty() in the query.
      int firstMonth2 = months.DefaultIfEmpty(1).First();
      outputBlock.Text += String.Format("The value of the firstMonth2 variable is {0}", firstMonth2) + "\n";

      /*
       This code produces the following output:

       The value of the firstMonth1 variable is 1
       The value of the firstMonth2 variable is 1
      */

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.