Memory Management and Garbage Collection in the .NET Framework

https://msdn.microsoft.com/en-us/library/hh156531(v=vs.110).aspx

Memory Management and Garbage Collection in the .NET Framework

This section of the documentation provides information about managing memory in the .NET Framework.

In This Section

Cleaning Up Unmanaged Resources
Describes how to properly manage and clean up unmanaged resources..

Garbage Collection
Provides information about the .NET Framework garbage collector.

IDisposable Interface

Provides a mechanism for releasing unmanaged resources.

To browse the .NET Framework source code for this type, see the Reference Source.

Namespace:   System
Assembly:  mscorlib (in mscorlib.dll)

Syntax

C#

C++

F#

VB

[ComVisibleAttribute(true)]
public interface IDisposable

Methods

	Name	Description
	Dispose()	Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.

Remarks

Note
To view the .NET Framework source code for this type, see the Reference Source. You can browse through the source code online, download the reference for offline viewing, and step through the sources (including patches and updates) during debugging; see instructions.

The primary use of this 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 is 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 the .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.

Using an object that implements IDisposable

Implement IDisposable only if you are using unmanaged resources directly. If your app simply uses an object that implements IDisposable, don't provide an IDisposable implementation. Instead, 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.

• By wrapping the call to the IDisposable.Dispose implementation in a try/catch block.

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

The C# and Visual Basic Using statement

If your language supports a construct such as the using statement in C# and the Using statement in Visual Basic, 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.

C#

VB

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 is actually a syntactic convenience. At compile time, the language compiler implements the intermediate language (IL) for a try/catch block.

For more information about the using statement, see the Using Statement (Visual Basic) or using Statement (C# Reference) topics.

The Try/Catch block

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

C#

VB

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;
      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 (Visual Basic), try-finally (C# Reference), or try-finally Statement (C).

Implementing IDisposable

You should implement IDisposable only if your type uses unmanaged resources directly. 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.

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.

C#

VB

using Microsoft.Win32.SafeHandles;
using System;
using System.Runtime.InteropServices;

class BaseClass : IDisposable
{
   // Flag: Has Dispose already been called?
   bool disposed = false;
   // Instantiate a SafeHandle instance.
   SafeHandle handle = new SafeFileHandle(IntPtr.Zero, true);

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

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

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

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

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

C#

VB

using System;

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

   // Public implementation of Dispose pattern callable by consumers.
   public void Dispose()
   { 
      Dispose(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;
   }

   ~BaseClass()
   {
      Dispose(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.

C#

VB

using Microsoft.Win32.SafeHandles;
using System;
using System.Runtime.InteropServices;

class DerivedClass : BaseClass
{
   // Flag: Has Dispose already been called?
   bool disposed = false;
   // Instantiate a SafeHandle instance.
   SafeHandle handle = new SafeFileHandle(IntPtr.Zero, true);

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

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

      // Free any unmanaged objects here.
      //

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

Examples

The following example demonstrates how to create a resource class that implements the IDisposable interface.

C#

C++

VB

using System;
using System.ComponentModel;

// The following example demonstrates how to create
// a resource class that implements the IDisposable interface
// and the IDisposable.Dispose method.

public class DisposeExample
{
    // A base class that implements IDisposable.
    // By implementing IDisposable, you are announcing that
    // instances of this type allocate scarce resources.
    public class MyResource: IDisposable
    {
        // Pointer to an external unmanaged resource.
        private IntPtr handle;
        // Other managed resource this class uses.
        private Component component = new Component();
        // Track whether Dispose has been called.
        private bool disposed = false;

        // The class constructor.
        public MyResource(IntPtr handle)
        {
            this.handle = handle;
        }

        // Implement IDisposable.
        // Do not make this method virtual.
        // A derived class should not be able to override this method.
        public void Dispose()
        {
            Dispose(true);
            // This object will be cleaned up by the Dispose method.
            // Therefore, you should call GC.SupressFinalize to
            // take this object off the finalization queue
            // and prevent finalization code for this object
            // from executing a second time.
            GC.SuppressFinalize(this);
        }

        // Dispose(bool disposing) executes in two distinct scenarios.
        // If disposing equals true, the method has been called directly
        // or indirectly by a user's code. Managed and unmanaged resources
        // can be disposed.
        // If disposing equals false, the method has been called by the
        // runtime from inside the finalizer and you should not reference
        // other objects. Only unmanaged resources can be disposed.
        protected virtual void Dispose(bool disposing)
        {
            // Check to see if Dispose has already been called.
            if(!this.disposed)
            {
                // If disposing equals true, dispose all managed
                // and unmanaged resources.
                if(disposing)
                {
                    // Dispose managed resources.
                    component.Dispose();
                }

                // Call the appropriate methods to clean up
                // unmanaged resources here.
                // If disposing is false,
                // only the following code is executed.
                CloseHandle(handle);
                handle = IntPtr.Zero;

                // Note disposing has been done.
                disposed = true;

            }
        }

        // Use interop to call the method necessary
        // to clean up the unmanaged resource.
        [System.Runtime.InteropServices.DllImport("Kernel32")]
        private extern static Boolean CloseHandle(IntPtr handle);

        // Use C# destructor syntax for finalization code.
        // This destructor will run only if the Dispose method
        // does not get called.
        // It gives your base class the opportunity to finalize.
        // Do not provide destructors in types derived from this class.
        ~MyResource()
        {
            // Do not re-create Dispose clean-up code here.
            // Calling Dispose(false) is optimal in terms of
            // readability and maintainability.
            Dispose(false);
        }
    }
    public static void Main()
    {
        // Insert code here to create
        // and use the MyResource object.
    }
}