System.IDisposable interface

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

The primary use of the IDisposable interface is to release unmanaged resources. The garbage collector automatically releases the memory allocated to a managed object when that object is no longer used. However, it's not possible to predict when garbage collection will occur. Furthermore, the garbage collector has no knowledge of unmanaged resources such as window handles, or open files and streams.

Use the Dispose method of this interface to explicitly release unmanaged resources in conjunction with the garbage collector. The consumer of an object can call this method when the object is no longer needed.

Because the IDisposable.Dispose implementation is called by the consumer of a type when the resources owned by an instance are no longer needed, you should either wrap the managed object in a SafeHandle (the recommended alternative), or you should override Object.Finalize to free unmanaged resources in the event that the consumer forgets to call Dispose.

For a detailed discussion about how this interface and the Object.Finalize method are used, see the Garbage Collection and Implementing a Dispose Method topics.

Use an object that implements IDisposable

If your app simply uses an object that implements the IDisposable interface, you should call the object's IDisposable.Dispose implementation when you are finished using it. Depending on your programming language, you can do this in one of two ways:

• By using a language construct such as the using statement in C# and Visual Basic, and the use statement or using function in F#.
• By wrapping the call to the IDisposable.Dispose implementation in a try/finally block.

The C#, F#, and Visual Basic Using statement

If your language supports a construct such as the using statement in C#, the Using statement in Visual Basic, or the use statement in F#, you can use it instead of explicitly calling IDisposable.Dispose yourself. The following example uses this approach in defining a WordCount class that preserves information about a file and the number of words in it.

using System;
using System.IO;
using System.Text.RegularExpressions;

public class WordCount
{
    private String filename = String.Empty;
    private int nWords = 0;
    private String pattern = @"\b\w+\b";

    public WordCount(string filename)
    {
        if (!File.Exists(filename))
            throw new FileNotFoundException("The file does not exist.");

        this.filename = filename;
        string txt = String.Empty;
        using (StreamReader sr = new StreamReader(filename))
        {
            txt = sr.ReadToEnd();
        }
        nWords = Regex.Matches(txt, pattern).Count;
    }

    public string FullName
    { get { return filename; } }

    public string Name
    { get { return Path.GetFileName(filename); } }

    public int Count
    { get { return nWords; } }
}

open System.IO
open System.Text.RegularExpressions

type WordCount(filename) =
    let txt = 
        if File.Exists filename |> not then
            raise (FileNotFoundException "The file does not exist.")

        use sr = new StreamReader(filename)
        sr.ReadToEnd()

    let pattern = @"\b\w+\b"
    
    let nWords = Regex.Matches(txt, pattern).Count

    member _.FullName = filename

    member _.Name = Path.GetFileName filename

    member _.Count = nWords

Imports System.IO
Imports System.Text.RegularExpressions

Public Class WordCount
   Private filename As String
   Private nWords As Integer
   Private pattern As String = "\b\w+\b" 

   Public Sub New(filename As String)
      If Not File.Exists(filename) Then
         Throw New FileNotFoundException("The file does not exist.")
      End If   
      
      Me.filename = filename
      Dim txt As String = String.Empty
      Using sr As New StreamReader(filename)
         txt = sr.ReadToEnd()
      End Using
      nWords = Regex.Matches(txt, pattern).Count
   End Sub
   
   Public ReadOnly Property FullName As String
      Get
         Return filename
      End Get   
   End Property
   
   Public ReadOnly Property Name As String
      Get
         Return Path.GetFileName(filename)
      End Get   
   End Property
   
   Public ReadOnly Property Count As Integer
      Get
         Return nWords
      End Get
   End Property
End Class

The using statement (use expression in F#) is actually a syntactic convenience. At compile time, the language compiler implements the intermediate language (IL) for a try/finally block.

For more information about the using statement, see the Using Statement or using Statement topics.

The Try/Finally block

If your programming language does not support a construct like the using statement in C# or Visual Basic, or the use statement in F#, or if you prefer not to use it, you can call the IDisposable.Dispose implementation from the finally block of a try/finally statement. The following example replaces the using block in the previous example with a try/finally block.

using System;
using System.IO;
using System.Text.RegularExpressions;

public class WordCount2
{
    private String filename = String.Empty;
    private int nWords = 0;
    private String pattern = @"\b\w+\b";

    public WordCount2(string filename)
    {
        if (!File.Exists(filename))
            throw new FileNotFoundException("The file does not exist.");

        this.filename = filename;
        string txt = String.Empty;
        StreamReader? sr = null;
        try
        {
            sr = new StreamReader(filename);
            txt = sr.ReadToEnd();
        }
        finally
        {
            if (sr != null) sr.Dispose();
        }
        nWords = Regex.Matches(txt, pattern).Count;
    }

    public string FullName
    { get { return filename; } }

    public string Name
    { get { return Path.GetFileName(filename); } }

    public int Count
    { get { return nWords; } }
}

open System.IO
open System.Text.RegularExpressions

type WordCount2(filename) =
    let txt = 
        if File.Exists filename |> not then
            raise (FileNotFoundException "The file does not exist.")

        let sr = new StreamReader(filename)
        try
            sr.ReadToEnd()
        finally
            sr.Dispose()

    let pattern = @"\b\w+\b"
    
    let nWords = Regex.Matches(txt, pattern).Count

    member _.FullName = filename

    member _.Name = Path.GetFileName filename

    member _.Count = nWords

Imports System.IO
Imports System.Text.RegularExpressions

Public Class WordCount2
   Private filename As String
   Private nWords As Integer
   Private pattern As String = "\b\w+\b" 

   Public Sub New(filename As String)
      If Not File.Exists(filename) Then
         Throw New FileNotFoundException("The file does not exist.")
      End If   
      
      Me.filename = filename
      Dim txt As String = String.Empty
      Dim sr As StreamReader = Nothing
      Try
         sr = New StreamReader(filename)
         txt = sr.ReadToEnd()
      Finally
         If sr IsNot Nothing Then sr.Dispose() 
      End Try
      nWords = Regex.Matches(txt, pattern).Count
   End Sub
   
   Public ReadOnly Property FullName As String
      Get
         Return filename
      End Get   
   End Property
   
   Public ReadOnly Property Name As String
      Get
         Return Path.GetFileName(filename)
      End Get   
   End Property
   
   Public ReadOnly Property Count As Integer
      Get
         Return nWords
      End Get
   End Property
End Class

For more information about the try/finally pattern, see Try...Catch...Finally Statement, try-finally, try...finally Expression, or try-finally Statement.

Implement IDisposable

You should implement IDisposable if your type uses unmanaged resources directly or if you wish to use disposable resources yourself. The consumers of your type can call your IDisposable.Dispose implementation to free resources when the instance is no longer needed. To handle cases in which they fail to call Dispose, you should either use a class derived from SafeHandle to wrap the unmanaged resources, or you should override the Object.Finalize method for a reference type. In either case, you use the Dispose method to perform whatever cleanup is necessary after using the unmanaged resources, such as freeing, releasing, or resetting the unmanaged resources. For more information about implementing IDisposable.Dispose, see the Dispose(bool) method overload.

IDisposable and the inheritance hierarchy

A base class with subclasses that should be disposable must implement IDisposable as follows. You should use this pattern whenever you implement IDisposable on any type that isn't sealed (NotInheritable in Visual Basic).

• It should provide one public, non-virtual Dispose() method and a protected virtual Dispose(Boolean disposing) method.
• The Dispose() method must call Dispose(true) and should suppress finalization for performance.
• The base type should not include any finalizers.

The following code fragment reflects the dispose pattern for base classes. It assumes that your type does not override the Object.Finalize method.

using System;
using System.IO;
using System.Runtime.InteropServices;

class BaseClass1 : IDisposable
{
    // Flag: Has Dispose already been called?
    bool disposed = false;
    // Instantiate a FileStream instance.
    FileStream fs = new FileStream("test.txt", FileMode.OpenOrCreate);

    // Public implementation of Dispose pattern callable by consumers.
    public void Dispose()
    {
        Dispose(disposing: true);
        GC.SuppressFinalize(this);
    }

    // Protected implementation of Dispose pattern.
    protected virtual void Dispose(bool disposing)
    {
        if (disposed)
            return;

        if (disposing)
        {
            fs.Dispose();
            // Free any other managed objects here.
            //
        }

        disposed = true;
    }
}

open System
open System.IO

type BaseClass1() =
    // Flag: Has Dispose already been called?
    let mutable disposed = false

    // Instantiate a FileStream instance.
    let fs = new FileStream("test.txt", FileMode.OpenOrCreate)

    interface IDisposable with
        // Public implementation of Dispose pattern callable by consumers.
        member this.Dispose() =
            this.Dispose true
            GC.SuppressFinalize this

    // Implementation of Dispose pattern.
    abstract Dispose: bool -> unit
    override _.Dispose(disposing) =
        if not disposed then
            if disposing then
                fs.Dispose()
                // Free any other managed objects here.
            disposed <- true

Imports System.IO
Imports System.Runtime.InteropServices

Class BaseClass1 : Implements IDisposable
   ' Flag: Has Dispose already been called?
   Dim disposed As Boolean = False
   ' Instantiate a FileStream instance.
   Dim fs As FileStream = New FileStream("test.txt", FileMode.OpenOrCreate)

   ' Public implementation of Dispose pattern callable by consumers.
   Public Sub Dispose() _
              Implements IDisposable.Dispose
      Dispose(disposing:=True)
      GC.SuppressFinalize(Me)
   End Sub

   ' Protected implementation of Dispose pattern.
   Protected Overridable Sub Dispose(disposing As Boolean)
      If disposed Then Return

      If disposing Then
         fs.Dispose()
         ' Free any other managed objects here.
         '
      End If

      disposed = True
   End Sub
End Class

If you do override the Object.Finalize method, your class should implement the following pattern.

using System;

class BaseClass2 : IDisposable
{
    // Flag: Has Dispose already been called?
    bool disposed = false;

    // Public implementation of Dispose pattern callable by consumers.
    public void Dispose()
    {
        Dispose(disposing: true);
        GC.SuppressFinalize(this);
    }

    // Protected implementation of Dispose pattern.
    protected virtual void Dispose(bool disposing)
    {
        if (disposed)
            return;

        if (disposing)
        {
            // Free any other managed objects here.
            //
        }

        // Free any unmanaged objects here.
        //
        disposed = true;
    }

    ~BaseClass2()
    {
        Dispose(disposing: false);
    }
}

open System

type BaseClass2() =
    // Flag: Has Dispose already been called?
    let mutable disposed = false

    interface IDisposable with
        // Public implementation of Dispose pattern callable by consumers.
        member this.Dispose() =
            this.Dispose true
            GC.SuppressFinalize this

    // Implementation of Dispose pattern.
    abstract Dispose: bool -> unit
    override _.Dispose(disposing) =
        if not disposed then
            if disposing then
                // Free any other managed objects here.
                ()

            // Free any unmanaged objects here.
            disposed <- true

    override this.Finalize() =
        this.Dispose false

Class BaseClass : Implements IDisposable
   ' Flag: Has Dispose already been called?
   Dim disposed As Boolean = False

   ' Public implementation of Dispose pattern callable by consumers.
   Public Sub Dispose() _
              Implements IDisposable.Dispose
      Dispose(disposing:=True)
      GC.SuppressFinalize(Me)
   End Sub

   ' Protected implementation of Dispose pattern.
   Protected Overridable Sub Dispose(disposing As Boolean)
      If disposed Then Return

      If disposing Then
         ' Free any other managed objects here.
         '
      End If

      ' Free any unmanaged objects here.
      '
      disposed = True
   End Sub

   Protected Overrides Sub Finalize()
      Dispose(disposing:=False)
   End Sub
End Class

Subclasses should implement the disposable pattern as follows:

• They must override Dispose(Boolean) and call the base class Dispose(Boolean) implementation.
• They can provide a finalizer if needed. The finalizer must call Dispose(false).

Note that derived classes do not themselves implement the IDisposable interface and do not include a parameterless Dispose method. They only override the base class Dispose(Boolean) method.

The following code fragment reflects the dispose pattern for derived classes. It assumes that your type does not override the Object.Finalize method.

using System;
using System.IO;
using System.Runtime.InteropServices;

class MyDerivedClass : MyBaseClass
{
    // Flag: Has Dispose already been called?
    bool disposed = false;
    // Instantiate a FileStream instance.
    FileStream fs = new FileStream("test.txt", FileMode.OpenOrCreate);

    // Protected implementation of Dispose pattern.
    protected override void Dispose(bool disposing)
    {
        if (disposed)
            return;

        if (disposing)
        {
            fs.Dispose();
            // Free any other managed objects here.
            //
        }

        // Free any unmanaged objects here.
        //

        disposed = true;
        // Call base class implementation.
        base.Dispose(disposing);
    }
}

open Microsoft.Win32.SafeHandles
open System

type MyDerivedClass() =
    inherit MyBaseClass()
    
    // Flag: Has Dispose already been called?
    let mutable disposed = false
    // Instantiate a FileStream instance.
    let fs = new FileStream("test.txt", FileMode.OpenOrCreate)

    // Implementation of Dispose pattern.
    override _.Dispose(disposing) =
        if not disposed then
            if disposing then
                fs.Dispose()
                // Free any other managed objects here.

            // Free any unmanaged objects here.
            disposed <- true
            // Call base class implementation.
            base.Dispose disposing

Imports System.IO
Imports System.Runtime.InteropServices

Class DerivedClass2 : Inherits BaseClass2
   ' Flag: Has Dispose already been called?
   Dim disposed As Boolean = False
   ' Instantiate a FileStream instance.
   Dim fs As FileStream = New FileStream("test.txt", FileMode.OpenOrCreate)

   ' Protected implementation of Dispose pattern.
   Protected Overrides Sub Dispose(disposing As Boolean)
      If disposed Then Return

      If disposing Then
         fs.Dispose()
         ' Free any other managed objects here.
         '
      End If

      ' Free any unmanaged objects here.
      '
      disposed = True

      ' Call base class implementation.
      MyBase.Dispose(disposing)
   End Sub
End Class