Using objects that implement IDisposable

The common language runtime's garbage collector (GC) reclaims the memory used by managed objects. Typically, types that use unmanaged resources implement the IDisposable or IAsyncDisposable interface to allow the unmanaged resources to be reclaimed. When you finish using an object that implements IDisposable, you call the object's Dispose or DisposeAsync implementation to explicitly perform cleanup. You can do this in one of two ways:

  • With the C# using statement or declaration (Using in Visual Basic).
  • By implementing a try/finally block, and calling the Dispose or DisposeAsync method in the finally.

Important

The GC does not dispose your objects, as it has no knowledge of IDisposable.Dispose() or IAsyncDisposable.DisposeAsync(). The GC only knows whether an object is finalizable (that is, it defines an Object.Finalize() method), and when the object's finalizer needs to be called. For more information, see How finalization works. For additional details on implementing Dispose and DisposeAsync, see:

Objects that implement System.IDisposable or System.IAsyncDisposable should always be properly disposed of, regardless of variable scoping, unless otherwise explicitly stated. Types that define a finalizer to release unmanaged resources usually call GC.SuppressFinalize from either their Dispose or DisposeAsync implementation. Calling SuppressFinalize indicates to the GC that the finalizer has already been run and the object shouldn't be promoted for finalization.

The using statement

The using statement in C# and the Using statement in Visual Basic simplify the code that you must write to cleanup an object. The using statement obtains one or more resources, executes the statements that you specify, and automatically disposes of the object. However, the using statement is useful only for objects that are used within the scope of the method in which they are constructed.

The following example uses the using statement to create and release a System.IO.StreamReader object.

using System.IO;

class UsingStatement
{
    static void Main()
    {
        var buffer = new char[50];
        using (StreamReader streamReader = new("file1.txt"))
        {
            int charsRead = 0;
            while (streamReader.Peek() != -1)
            {
                charsRead = streamReader.Read(buffer, 0, buffer.Length);
                //
                // Process characters read.
                //
            }
        }
    }
}
Imports System.IO

Module UsingStatement
    Public Sub Main()
        Dim buffer(49) As Char
        Using streamReader As New StreamReader("File1.txt")
            Dim charsRead As Integer
            Do While streamReader.Peek() <> -1
                charsRead = streamReader.Read(buffer, 0, buffer.Length)
                ' 
                ' Process characters read.
                '
            Loop
        End Using
    End Sub
End Module

With C# 8, a using declaration is an alternative syntax available where the braces are removed, and scoping is implicit.

using System.IO;

class UsingDeclaration
{
    static void Main()
    {
        var buffer = new char[50];
        using StreamReader streamReader = new("file1.txt");

        int charsRead = 0;
        while (streamReader.Peek() != -1)
        {
            charsRead = streamReader.Read(buffer, 0, buffer.Length);
            //
            // Process characters read.
            //
        }
    }
}

Although the StreamReader class implements the IDisposable interface, which indicates that it uses an unmanaged resource, the example doesn't explicitly call the StreamReader.Dispose method. When the C# or Visual Basic compiler encounters the using statement, it emits intermediate language (IL) that is equivalent to the following code that explicitly contains a try/finally block.

using System.IO;

class TryFinallyGenerated
{
    static void Main()
    {
        var buffer = new char[50];
        StreamReader? streamReader = null;
        try
        {
            streamReader = new StreamReader("file1.txt");
            int charsRead = 0;
            while (streamReader.Peek() != -1)
            {
                charsRead = streamReader.Read(buffer, 0, buffer.Length);
                //
                // Process characters read.
                //
            }
        }
        finally
        {
            // If non-null, call the object's Dispose method.
            streamReader?.Dispose();
        }
    }
}
Imports System.IO

Module TryFinallyGenerated
    Public Sub Main()
        Dim buffer(49) As Char
        Dim streamReader As New StreamReader("File1.txt")
        Try
            Dim charsRead As Integer
            Do While streamReader.Peek() <> -1
                charsRead = streamReader.Read(buffer, 0, buffer.Length)
                ' 
                ' Process characters read.
                '
            Loop
        Finally
            If streamReader IsNot Nothing Then DirectCast(streamReader, IDisposable).Dispose()
        End Try
    End Sub
End Module

The C# using statement also allows you to acquire multiple resources in a single statement, which is internally equivalent to nested using statements. The following example instantiates two StreamReader objects to read the contents of two different files.

using System.IO;

class SingleStatementMultiple
{
    static void Main()
    {
        var buffer1 = new char[50];
        var buffer2 = new char[50];

        using StreamReader version1 = new("file1.txt"),
                           version2 = new("file2.txt");

        int charsRead1, charsRead2 = 0;
        while (version1.Peek() != -1 && version2.Peek() != -1)
        {
            charsRead1 = version1.Read(buffer1, 0, buffer1.Length);
            charsRead2 = version2.Read(buffer2, 0, buffer2.Length);
            //
            // Process characters read.
            //
        }
    }
}

Try/finally block

Instead of wrapping a try/finally block in a using statement, you may choose to implement the try/finally block directly. It may be your personal coding style, or you might want to do this for one of the following reasons:

  • To include a catch block to handle exceptions thrown in the try block. Otherwise, any exceptions thrown within the using statement are unhandled.
  • To instantiate an object that implements IDisposable whose scope is not local to the block within which it is declared.

The following example is similar to the previous example, except that it uses a try/catch/finally block to instantiate, use, and dispose of a StreamReader object, and to handle any exceptions thrown by the StreamReader constructor and its ReadToEnd method. The code in the finally block checks that the object that implements IDisposable isn't null before it calls the Dispose method. Failure to do this can result in a NullReferenceException exception at run time.

using System;
using System.Globalization;
using System.IO;

class TryExplicitCatchFinally
{
    static void Main()
    {
        StreamReader? streamReader = null;
        try
        {
            streamReader = new StreamReader("file1.txt");
            string contents = streamReader.ReadToEnd();
            var info = new StringInfo(contents);
            Console.WriteLine($"The file has {info.LengthInTextElements} text elements.");
        }
        catch (FileNotFoundException)
        {
            Console.WriteLine("The file cannot be found.");
        }
        catch (IOException)
        {
            Console.WriteLine("An I/O error has occurred.");
        }
        catch (OutOfMemoryException)
        {
            Console.WriteLine("There is insufficient memory to read the file.");
        }
        finally
        {
            streamReader?.Dispose();
        }
    }
}
Imports System.Globalization
Imports System.IO

Module TryExplicitCatchFinally
    Sub Main()
        Dim streamReader As StreamReader = Nothing
        Try
            streamReader = New StreamReader("file1.txt")
            Dim contents As String = streamReader.ReadToEnd()
            Dim info As StringInfo = New StringInfo(contents)
            Console.WriteLine($"The file has {info.LengthInTextElements} text elements.")
        Catch e As FileNotFoundException
            Console.WriteLine("The file cannot be found.")
        Catch e As IOException
            Console.WriteLine("An I/O error has occurred.")
        Catch e As OutOfMemoryException
            Console.WriteLine("There is insufficient memory to read the file.")
        Finally
            If streamReader IsNot Nothing Then streamReader.Dispose()
        End Try
    End Sub
End Module

You can follow this basic pattern if you choose to implement or must implement a try/finally block, because your programming language doesn't support a using statement but does allow direct calls to the Dispose method.

IDisposable instance members

If a class holds an IDisposable implementation as an instance member, either a field or a property, the class should also implement IDisposable. For more information, see implement a cascade dispose.

See also