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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.

Warning

It is a breaking change to add the IDisposable interface to an existing class. Because pre-existing consumers of your type cannot call Dispose, you cannot be certain that unmanaged resources held by your type will be released.

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.

Important

In .NET Framework, the C++ compiler supports deterministic disposal of resources and does not allow direct implementation of the Dispose method.

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.

Note

Documentation for types that implement IDisposable note that fact and include a reminder to call its Dispose implementation.

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; } }
}

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; } }
}

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.

Important

If you are defining a base class that uses unmanaged resources and that either has, or is likely to have, subclasses that should be disposed, you should implement the IDisposable.Dispose method and provide a second overload of Dispose, as discussed in the next section.

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;
    }
}

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);
    }
}

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);
    }
}