Interlocked.CompareExchange Method

Definition

Namespace:

System.Threading

Assembly:

System.Threading.dll

Compares two values for equality and, if they are equal, replaces the first value, as an atomic operation.

Overloads

CompareExchange(UIntPtr, UIntPtr, UIntPtr)	Compares two platform-specific handles or pointers for equality and, if they are equal, replaces the first one, as an atomic operation.
CompareExchange(UInt64, UInt64, UInt64)	Compares two 64-bit unsigned integers for equality and, if they are equal, replaces the first value, as an atomic operation.
CompareExchange(UInt32, UInt32, UInt32)	Compares two 32-bit unsigned integers for equality and, if they are equal, replaces the first value, as an atomic operation.
CompareExchange(Single, Single, Single)	Compares two single-precision floating point numbers for equality and, if they are equal, replaces the first value, as an atomic operation.
CompareExchange(Object, Object, Object)	Compares two objects for reference equality and, if they are equal, replaces the first object, as an atomic operation.
CompareExchange(Int64, Int64, Int64)	Compares two 64-bit signed integers for equality and, if they are equal, replaces the first value, as an atomic operation.
CompareExchange(Int32, Int32, Int32)	Compares two 32-bit signed integers for equality and, if they are equal, replaces the first value, as an atomic operation.
CompareExchange(Double, Double, Double)	Compares two double-precision floating point numbers for equality and, if they are equal, replaces the first value, as an atomic operation.
CompareExchange(IntPtr, IntPtr, IntPtr)	Compares two platform-specific handles or pointers for equality and, if they are equal, replaces the first one, as an atomic operation.
CompareExchange<T>(T, T, T)	Compares two instances of the specified reference type T for reference equality and, if they are equal, replaces the first one, as an atomic operation.

CompareExchange(UIntPtr, UIntPtr, UIntPtr)

Source:

Interlocked.cs

Important

This API is not CLS-compliant.

Compares two platform-specific handles or pointers for equality and, if they are equal, replaces the first one, as an atomic operation.

public:
 static UIntPtr CompareExchange(UIntPtr % location1, UIntPtr value, UIntPtr comparand);

[System.CLSCompliant(false)]
public static UIntPtr CompareExchange (ref UIntPtr location1, UIntPtr value, UIntPtr comparand);

[<System.CLSCompliant(false)>]
static member CompareExchange : unativeint * unativeint * unativeint -> unativeint

Public Shared Function CompareExchange (ByRef location1 As UIntPtr, value As UIntPtr, comparand As UIntPtr) As UIntPtr

Parameters

location1

UIntPtr

unativeint

The destination UIntPtr, whose value is compared with the value of comparand and possibly replaced by value.

value

UIntPtr

unativeint

The UIntPtr that replaces the destination value if the comparison results in equality.

comparand

UIntPtr

unativeint

The UIntPtr that is compared to the value at location1.

Returns

UIntPtr

unativeint

The original value in location1.

Attributes

CLSCompliantAttribute

Exceptions

NullReferenceException

The address of location1 is a null pointer.

CompareExchange(UInt64, UInt64, UInt64)

Source:

Interlocked.cs

Important

This API is not CLS-compliant.

Compares two 64-bit unsigned integers for equality and, if they are equal, replaces the first value, as an atomic operation.

public:
 static System::UInt64 CompareExchange(System::UInt64 % location1, System::UInt64 value, System::UInt64 comparand);

[System.CLSCompliant(false)]
public static ulong CompareExchange (ref ulong location1, ulong value, ulong comparand);

[<System.CLSCompliant(false)>]
static member CompareExchange : uint64 * uint64 * uint64 -> uint64

Public Shared Function CompareExchange (ByRef location1 As ULong, value As ULong, comparand As ULong) As ULong

Parameters

location1

UInt64

The destination, whose value is compared with comparand and possibly replaced.

value

UInt64

The value that replaces the destination value if the comparison results in equality.

comparand

UInt64

The value that is compared to the value at location1.

Returns

UInt64

The original value in location1.

Attributes

CLSCompliantAttribute

Exceptions

NullReferenceException

The address of location1 is a null pointer.

CompareExchange(UInt32, UInt32, UInt32)

Source:

Interlocked.cs

Important

This API is not CLS-compliant.

Compares two 32-bit unsigned integers for equality and, if they are equal, replaces the first value, as an atomic operation.

public:
 static System::UInt32 CompareExchange(System::UInt32 % location1, System::UInt32 value, System::UInt32 comparand);

[System.CLSCompliant(false)]
public static uint CompareExchange (ref uint location1, uint value, uint comparand);

[<System.CLSCompliant(false)>]
static member CompareExchange : uint32 * uint32 * uint32 -> uint32

Public Shared Function CompareExchange (ByRef location1 As UInteger, value As UInteger, comparand As UInteger) As UInteger

Parameters

location1

UInt32

The destination, whose value is compared with comparand and possibly replaced.

value

UInt32

The value that replaces the destination value if the comparison results in equality.

comparand

UInt32

The value that is compared to the value at location1.

Returns

UInt32

The original value in location1.

Attributes

CLSCompliantAttribute

Exceptions

NullReferenceException

The address of location1 is a null pointer.

CompareExchange(Single, Single, Single)

Source:

Interlocked.cs

Compares two single-precision floating point numbers for equality and, if they are equal, replaces the first value, as an atomic operation.

public:
 static float CompareExchange(float % location1, float value, float comparand);

public static float CompareExchange (ref float location1, float value, float comparand);

static member CompareExchange : single * single * single -> single

Public Shared Function CompareExchange (ByRef location1 As Single, value As Single, comparand As Single) As Single

Parameters

location1

Single

The destination, whose value is compared with comparand and possibly replaced.

value

Single

The value that replaces the destination value if the comparison results in equality.

comparand

Single

The value that is compared to the value at location1.

Returns

Single

The original value in location1.

Exceptions

NullReferenceException

The address of location1 is a null pointer.

Examples

The following code example demonstrates a thread-safe method that accumulates a running total of Single values. Two threads add a series of Single values using the thread-safe method and ordinary addition, and when the threads complete the totals are compared. On a dual-processor computer, there is a significant difference in the totals.

In the thread-safe method, the initial value of the running total is saved, and then the CompareExchange method is used to exchange the newly computed total with the old total. If the return value is not equal to the saved value of the running total, then another thread has updated the total in the meantime. In that case, the attempt to update the running total must be repeated.

// This example demonstrates a thread-safe method that adds to a
// running total.  
using System;
using System.Threading;

public class ThreadSafe
{
    // Field totalValue contains a running total that can be updated
    // by multiple threads. It must be protected from unsynchronized 
    // access.
    private float totalValue = 0.0F;

    // The Total property returns the running total.
    public float Total { get { return totalValue; }}

    // AddToTotal safely adds a value to the running total.
    public float AddToTotal(float addend)
    {
        float initialValue, computedValue;
        do
        {
            // Save the current running total in a local variable.
            initialValue = totalValue;

            // Add the new value to the running total.
            computedValue = initialValue + addend;

            // CompareExchange compares totalValue to initialValue. If
            // they are not equal, then another thread has updated the
            // running total since this loop started. CompareExchange
            // does not update totalValue. CompareExchange returns the
            // contents of totalValue, which do not equal initialValue,
            // so the loop executes again.
        }
        while (initialValue != Interlocked.CompareExchange(ref totalValue, 
            computedValue, initialValue));
        // If no other thread updated the running total, then 
        // totalValue and initialValue are equal when CompareExchange
        // compares them, and computedValue is stored in totalValue.
        // CompareExchange returns the value that was in totalValue
        // before the update, which is equal to initialValue, so the 
        // loop ends.

        // The function returns computedValue, not totalValue, because
        // totalValue could be changed by another thread between
        // the time the loop ends and the function returns.
        return computedValue;
    }
}

public class Test
{
    // Create an instance of the ThreadSafe class to test.
    private static ThreadSafe ts = new ThreadSafe();
    private static float control;

    private static Random r = new Random();
    private static ManualResetEvent mre = new ManualResetEvent(false);

    public static void Main()
    {
        // Create two threads, name them, and start them. The
        // thread will block on mre.
        Thread t1 = new Thread(TestThread);
        t1.Name = "Thread 1";
        t1.Start();
        Thread t2 = new Thread(TestThread);
        t2.Name = "Thread 2";
        t2.Start();

        // Now let the threads begin adding random numbers to 
        // the total.
        mre.Set();
        
        // Wait until all the threads are done.
        t1.Join();
        t2.Join();

        Console.WriteLine("Thread safe: {0}  Ordinary float: {1}", 
            ts.Total, control);
    }

    private static void TestThread()
    {
        // Wait until the signal.
        mre.WaitOne();

        for(int i = 1; i <= 1000000; i++)
        {
            // Add to the running total in the ThreadSafe instance, and
            // to an ordinary float.
            //
            float testValue = (float) r.NextDouble();
            control += testValue;
            ts.AddToTotal(testValue);
        }
    }
}

/* On a dual-processor computer, this code example produces output 
   similar to the following:

Thread safe: 17039.57  Ordinary float: 15706.44
 */

' This example demonstrates a thread-safe method that adds to a
' running total.  
Imports System.Threading

Public Class ThreadSafe
    ' Field totalValue contains a running total that can be updated
    ' by multiple threads. It must be protected from unsynchronized 
    ' access.
    Private totalValue As Single = 0.0

    ' The Total property returns the running total.
    Public ReadOnly Property Total As Single
        Get
            Return totalValue
        End Get
    End Property

    ' AddToTotal safely adds a value to the running total.
    Public Function AddToTotal(ByVal addend As Single) As Single
        Dim initialValue, computedValue As Single
        Do
            ' Save the current running total in a local variable.
            initialValue = totalValue

            ' Add the new value to the running total.
            computedValue = initialValue + addend

            ' CompareExchange compares totalValue to initialValue. If
            ' they are not equal, then another thread has updated the
            ' running total since this loop started. CompareExchange
            ' does not update totalValue. CompareExchange returns the
            ' contents of totalValue, which do not equal initialValue,
            ' so the loop executes again.
        Loop While initialValue <> Interlocked.CompareExchange( _
            totalValue, computedValue, initialValue)
        ' If no other thread updated the running total, then 
        ' totalValue and initialValue are equal when CompareExchange
        ' compares them, and computedValue is stored in totalValue.
        ' CompareExchange returns the value that was in totalValue
        ' before the update, which is equal to initialValue, so the 
        ' loop ends.

        ' The function returns computedValue, not totalValue, because
        ' totalValue could be changed by another thread between
        ' the time the loop ends and the function returns.
        Return computedValue
    End Function
End Class

Public Class Test
    ' Create an instance of the ThreadSafe class to test.
    Private Shared ts As New ThreadSafe()
    Private Shared control As Single

    Private Shared r As New Random()
    Private Shared mre As New ManualResetEvent(false)

    <MTAThread> _
    Public Shared Sub Main()
        ' Create two threads, name them, and start them. The
        ' threads will block on mre.
        Dim t1 As New Thread(AddressOf TestThread)
        t1.Name = "Thread 1"
        t1.Start()
        Dim t2 As New Thread(AddressOf TestThread)
        t2.Name = "Thread 2"
        t2.Start()

        ' Now let the threads begin adding random numbers to 
        ' the total.
        mre.Set()
        
        ' Wait until all the threads are done.
        t1.Join()
        t2.Join()

        Console.WriteLine("Thread safe: {0}  Ordinary Single: {1}", ts.Total, control)
    End Sub

    Private Shared Sub TestThread()
        ' Wait until the signal.
        mre.WaitOne()

        For i As Integer = 1 to 1000000
            ' Add to the running total in the ThreadSafe instance, and
            ' to an ordinary Single.
            '
            Dim testValue As Single = r.NextDouble()
            control += testValue
            ts.AddToTotal(testValue)
        Next
    End Sub
End Class

' On a dual-processor computer, this code example produces output 
' similar to the following:
'
'Thread safe: 17039.57  Ordinary Single: 15706.44

Remarks

If comparand and the value in location1 are equal, then value is stored in location1. Otherwise, no operation is performed. The compare and exchange operations are performed as an atomic operation. The return value of CompareExchange is the original value in location1, whether or not the exchange takes place.

See also

• Managed Threading
• Overview of synchronization primitives

CompareExchange(Object, Object, Object)

Source:

Interlocked.cs

Compares two objects for reference equality and, if they are equal, replaces the first object, as an atomic operation.

public:
 static System::Object ^ CompareExchange(System::Object ^ % location1, System::Object ^ value, System::Object ^ comparand);

public static object? CompareExchange (ref object? location1, object? value, object? comparand);

static member CompareExchange : obj * obj * obj -> obj

Public Shared Function CompareExchange (ByRef location1 As Object, value As Object, comparand As Object) As Object

Parameters

location1

Object

The destination object that is compared by reference with comparand and possibly replaced.

value

Object

The object that replaces the destination object if the reference comparison results in equality.

comparand

Object

The object that is compared by reference to the object at location1.

Returns

Object

The original value in location1.

Exceptions

NullReferenceException

The address of location1 is a null pointer.

Remarks

Important

Beginning with .NET Framework 2.0, the CompareExchange<T>(T, T, T) method overload provides a type-safe alternative for reference types. We recommend that you call it instead of this overload.

If comparand and the object in location1 are equal by reference, then value is stored in location1. Otherwise, no operation is performed. The compare and exchange operations are performed as an atomic operation. The return value of CompareExchange is the original value in location1, whether or not the exchange takes place.

Note

The objects are compared for reference equality rather than value equality. As a result, two boxed instances of the same value type (for example, the integer 3) always appear to be unequal and no operation is performed. Do not use this overload with value types.

See also

• Managed Threading
• Overview of synchronization primitives

CompareExchange(Int64, Int64, Int64)

Source:

Interlocked.cs

Compares two 64-bit signed integers for equality and, if they are equal, replaces the first value, as an atomic operation.

public:
 static long CompareExchange(long % location1, long value, long comparand);

public static long CompareExchange (ref long location1, long value, long comparand);

static member CompareExchange : int64 * int64 * int64 -> int64

Public Shared Function CompareExchange (ByRef location1 As Long, value As Long, comparand As Long) As Long

Parameters

location1

Int64

The destination, whose value is compared with comparand and possibly replaced.

value

Int64

The value that replaces the destination value if the comparison results in equality.

comparand

Int64

The value that is compared to the value at location1.

Returns

Int64

The original value in location1.

Exceptions

NullReferenceException

The address of location1 is a null pointer.

Remarks

If comparand and the value in location1 are equal, then value is stored in location1. Otherwise, no operation is performed. The compare and exchange operations are performed as an atomic operation. The return value of CompareExchange is the original value in location1, whether or not the exchange takes place.

See also

• Managed Threading
• Overview of synchronization primitives

CompareExchange(Int32, Int32, Int32)

Source:

Interlocked.cs

Compares two 32-bit signed integers for equality and, if they are equal, replaces the first value, as an atomic operation.

public:
 static int CompareExchange(int % location1, int value, int comparand);

public static int CompareExchange (ref int location1, int value, int comparand);

static member CompareExchange : int * int * int -> int

Public Shared Function CompareExchange (ByRef location1 As Integer, value As Integer, comparand As Integer) As Integer

Parameters

location1

Int32

The destination, whose value is compared with comparand and possibly replaced.

value

Int32

The value that replaces the destination value if the comparison results in equality.

comparand

Int32

The value that is compared to the value at location1.

Returns

Int32

The original value in location1.

Exceptions

NullReferenceException

The address of location1 is a null pointer.

Examples

The following code example demonstrates a thread-safe method that accumulates a running total. The initial value of the running total is saved, and then the CompareExchange method is used to exchange the newly computed total with the old total. If the return value is not equal to the saved value of the running total, then another thread has updated the total in the meantime. In that case, the attempt to update the running total must be repeated.

Note

The Add method, introduced in version 2.0 of the .NET Framework, provides a more convenient way to accumulate thread-safe running totals for integers.

// This example demonstrates a thread-safe method that adds to a
// running total.  It cannot be run directly.  You can compile it
// as a library, or add the class to a project.
#using <system.dll>

using namespace System::Threading;
public ref class ThreadSafe
{
private:

   // totalValue contains a running total that can be updated
   // by multiple threads. It must be protected from unsynchronized 
   // access.
   int totalValue;

public:

   property int Total 
   {

      // The Total property returns the running total.
      int get()
      {
         return totalValue;
      }

   }

   // AddToTotal safely adds a value to the running total.
   int AddToTotal( int addend )
   {
      int initialValue;
      int computedValue;
      do
      {
         
         // Save the current running total in a local variable.
         initialValue = totalValue;
         
         // Add the new value to the running total.
         computedValue = initialValue + addend;
         
         // CompareExchange compares totalValue to initialValue. If
         // they are not equal, then another thread has updated the
         // running total since this loop started. CompareExchange
         // does not update totalValue. CompareExchange returns the
         // contents of totalValue, which do not equal initialValue,
         // so the loop executes again.
      }
      while ( initialValue != Interlocked::CompareExchange( totalValue, computedValue, initialValue ) );

      
      // If no other thread updated the running total, then 
      // totalValue and initialValue are equal when CompareExchange
      // compares them, and computedValue is stored in totalValue.
      // CompareExchange returns the value that was in totalValue
      // before the update, which is equal to initialValue, so the 
      // loop ends.
      // The function returns computedValue, not totalValue, because
      // totalValue could be changed by another thread between
      // the time the loop ends and the function returns.
      return computedValue;
   }

};

// This example demonstrates a thread-safe method that adds to a
// running total.  It cannot be run directly.  You can compile it
// as a library, or add the class to a project.
using System.Threading;

public class ThreadSafe {
    // totalValue contains a running total that can be updated
    // by multiple threads. It must be protected from unsynchronized 
    // access.
    private int totalValue = 0;

    // The Total property returns the running total.
    public int Total {
        get { return totalValue; }
    }

    // AddToTotal safely adds a value to the running total.
    public int AddToTotal(int addend) {
        int initialValue, computedValue;
        do {
            // Save the current running total in a local variable.
            initialValue = totalValue;

            // Add the new value to the running total.
            computedValue = initialValue + addend;

            // CompareExchange compares totalValue to initialValue. If
            // they are not equal, then another thread has updated the
            // running total since this loop started. CompareExchange
            // does not update totalValue. CompareExchange returns the
            // contents of totalValue, which do not equal initialValue,
            // so the loop executes again.
        } while (initialValue != Interlocked.CompareExchange(
            ref totalValue, computedValue, initialValue));
        // If no other thread updated the running total, then 
        // totalValue and initialValue are equal when CompareExchange
        // compares them, and computedValue is stored in totalValue.
        // CompareExchange returns the value that was in totalValue
        // before the update, which is equal to initialValue, so the 
        // loop ends.

        // The function returns computedValue, not totalValue, because
        // totalValue could be changed by another thread between
        // the time the loop ends and the function returns.
        return computedValue;
    }
}

' This example demonstrates a thread-safe method that adds to a
' running total.  It cannot be run directly.  You can compile it
' as a library, or add the class to a project.
Imports System.Threading

Public Class ThreadSafe
    ' Field totalValue contains a running total that can be updated
    ' by multiple threads. It must be protected from unsynchronized 
    ' access.
    Private totalValue As Integer = 0

    ' The Total property returns the running total.
    Public ReadOnly Property Total As Integer
        Get
            Return totalValue
        End Get
    End Property

    ' AddToTotal safely adds a value to the running total.
    Public Function AddToTotal(ByVal addend As Integer) As Integer
        Dim initialValue, computedValue As Integer
        Do
            ' Save the current running total in a local variable.
            initialValue = totalValue

            ' Add the new value to the running total.
            computedValue = initialValue + addend

            ' CompareExchange compares totalValue to initialValue. If
            ' they are not equal, then another thread has updated the
            ' running total since this loop started. CompareExchange
            ' does not update totalValue. CompareExchange returns the
            ' contents of totalValue, which do not equal initialValue,
            ' so the loop executes again.
        Loop While initialValue <> Interlocked.CompareExchange( _
            totalValue, computedValue, initialValue)
        ' If no other thread updated the running total, then 
        ' totalValue and initialValue are equal when CompareExchange
        ' compares them, and computedValue is stored in totalValue.
        ' CompareExchange returns the value that was in totalValue
        ' before the update, which is equal to initialValue, so the 
        ' loop ends.

        ' The function returns computedValue, not totalValue, because
        ' totalValue could be changed by another thread between
        ' the time the loop ends and the function returns.
        Return computedValue
    End Function
End Class

Remarks

If comparand and the value in location1 are equal, then value is stored in location1. Otherwise, no operation is performed. The compare and exchange operations are performed as an atomic operation. The return value of CompareExchange is the original value in location1, whether or not the exchange takes place.

See also

• Managed Threading
• Overview of synchronization primitives

CompareExchange(Double, Double, Double)

Source:

Interlocked.cs

Compares two double-precision floating point numbers for equality and, if they are equal, replaces the first value, as an atomic operation.

public:
 static double CompareExchange(double % location1, double value, double comparand);

public static double CompareExchange (ref double location1, double value, double comparand);

static member CompareExchange : double * double * double -> double

Public Shared Function CompareExchange (ByRef location1 As Double, value As Double, comparand As Double) As Double

Parameters

location1

Double

The destination, whose value is compared with comparand and possibly replaced.

value

Double

The value that replaces the destination value if the comparison results in equality.

comparand

Double

The value that is compared to the value at location1.

Returns

Double

The original value in location1.

Exceptions

NullReferenceException

The address of location1 is a null pointer.

Examples

The following code example demonstrates a thread-safe method that accumulates a running total of Double values. Two threads add a series of Double values using the thread-safe method and ordinary addition, and when the threads complete the totals are compared. On a dual-processor computer, there is a significant difference in the totals.

In the thread-safe method, the initial value of the running total is saved, and then the CompareExchange method is used to exchange the newly computed total with the old total. If the return value is not equal to the saved value of the running total, then another thread has updated the total in the meantime. In that case, the attempt to update the running total must be repeated.

// This example demonstrates a thread-safe method that adds to a
// running total.  
using System;
using System.Threading;

public class ThreadSafe
{
    // Field totalValue contains a running total that can be updated
    // by multiple threads. It must be protected from unsynchronized 
    // access.
    private double totalValue = 0.0;

    // The Total property returns the running total.
    public double Total { get { return totalValue; }}

    // AddToTotal safely adds a value to the running total.
    public double AddToTotal(double addend)
    {
        double initialValue, computedValue;
        do
        {
            // Save the current running total in a local variable.
            initialValue = totalValue;

            // Add the new value to the running total.
            computedValue = initialValue + addend;

            // CompareExchange compares totalValue to initialValue. If
            // they are not equal, then another thread has updated the
            // running total since this loop started. CompareExchange
            // does not update totalValue. CompareExchange returns the
            // contents of totalValue, which do not equal initialValue,
            // so the loop executes again.
        }
        while (initialValue != Interlocked.CompareExchange(ref totalValue, 
            computedValue, initialValue));
        // If no other thread updated the running total, then 
        // totalValue and initialValue are equal when CompareExchange
        // compares them, and computedValue is stored in totalValue.
        // CompareExchange returns the value that was in totalValue
        // before the update, which is equal to initialValue, so the 
        // loop ends.

        // The function returns computedValue, not totalValue, because
        // totalValue could be changed by another thread between
        // the time the loop ends and the function returns.
        return computedValue;
    }
}

public class Test
{
    // Create an instance of the ThreadSafe class to test.
    private static ThreadSafe ts = new ThreadSafe();
    private static double control;

    private static Random r = new Random();
    private static ManualResetEvent mre = new ManualResetEvent(false);

    public static void Main()
    {
        // Create two threads, name them, and start them. The
        // thread will block on mre.
        Thread t1 = new Thread(TestThread);
        t1.Name = "Thread 1";
        t1.Start();
        Thread t2 = new Thread(TestThread);
        t2.Name = "Thread 2";
        t2.Start();

        // Now let the threads begin adding random numbers to 
        // the total.
        mre.Set();
        
        // Wait until all the threads are done.
        t1.Join();
        t2.Join();

        Console.WriteLine("Thread safe: {0}  Ordinary Double: {1}", 
            ts.Total, control);
    }

    private static void TestThread()
    {
        // Wait until the signal.
        mre.WaitOne();

        for(int i = 1; i <= 1000000; i++)
        {
            // Add to the running total in the ThreadSafe instance, and
            // to an ordinary double.
            //
            double testValue = r.NextDouble();
            control += testValue;
            ts.AddToTotal(testValue);
        }
    }
}

/* On a dual-processor computer, this code example produces output 
   similar to the following:

Thread safe: 998068.049623744  Ordinary Double: 759775.417190589
 */

' This example demonstrates a thread-safe method that adds to a
' running total.  
Imports System.Threading

Public Class ThreadSafe
    ' Field totalValue contains a running total that can be updated
    ' by multiple threads. It must be protected from unsynchronized 
    ' access.
    Private totalValue As Double = 0.0

    ' The Total property returns the running total.
    Public ReadOnly Property Total As Double
        Get
            Return totalValue
        End Get
    End Property

    ' AddToTotal safely adds a value to the running total.
    Public Function AddToTotal(ByVal addend As Double) As Double
        Dim initialValue, computedValue As Double
        Do
            ' Save the current running total in a local variable.
            initialValue = totalValue

            ' Add the new value to the running total.
            computedValue = initialValue + addend

            ' CompareExchange compares totalValue to initialValue. If
            ' they are not equal, then another thread has updated the
            ' running total since this loop started. CompareExchange
            ' does not update totalValue. CompareExchange returns the
            ' contents of totalValue, which do not equal initialValue,
            ' so the loop executes again.
        Loop While initialValue <> Interlocked.CompareExchange( _
            totalValue, computedValue, initialValue)
        ' If no other thread updated the running total, then 
        ' totalValue and initialValue are equal when CompareExchange
        ' compares them, and computedValue is stored in totalValue.
        ' CompareExchange returns the value that was in totalValue
        ' before the update, which is equal to initialValue, so the 
        ' loop ends.

        ' The function returns computedValue, not totalValue, because
        ' totalValue could be changed by another thread between
        ' the time the loop ends and the function returns.
        Return computedValue
    End Function
End Class

Public Class Test
    ' Create an instance of the ThreadSafe class to test.
    Private Shared ts As New ThreadSafe()
    Private Shared control As Double

    Private Shared r As New Random()
    Private Shared mre As New ManualResetEvent(false)

    <MTAThread> _
    Public Shared Sub Main()
        ' Create two threads, name them, and start them. The
        ' threads will block on mre.
        Dim t1 As New Thread(AddressOf TestThread)
        t1.Name = "Thread 1"
        t1.Start()
        Dim t2 As New Thread(AddressOf TestThread)
        t2.Name = "Thread 2"
        t2.Start()

        ' Now let the threads begin adding random numbers to 
        ' the total.
        mre.Set()
        
        ' Wait until all the threads are done.
        t1.Join()
        t2.Join()

        Console.WriteLine("Thread safe: {0}  Ordinary Double: {1}", ts.Total, control)
    End Sub

    Private Shared Sub TestThread()
        ' Wait until the signal.
        mre.WaitOne()

        For i As Integer = 1 to 1000000
            ' Add to the running total in the ThreadSafe instance, and
            ' to an ordinary double.
            '
            Dim testValue As Double = r.NextDouble
            control += testValue
            ts.AddToTotal(testValue)
        Next
    End Sub
End Class

' On a dual-processor computer, this code example produces output 
' similar to the following:
'
'Thread safe: 998068.049623744  Ordinary Double: 759775.417190589

Remarks

If comparand and the value in location1 are equal, then value is stored in location1. Otherwise, no operation is performed. The compare and exchange operations are performed as an atomic operation. The return value of CompareExchange is the original value in location1, whether or not the exchange takes place.

See also

• Managed Threading
• Overview of synchronization primitives

CompareExchange(IntPtr, IntPtr, IntPtr)

Source:

Interlocked.cs

Compares two platform-specific handles or pointers for equality and, if they are equal, replaces the first one, as an atomic operation.

public:
 static IntPtr CompareExchange(IntPtr % location1, IntPtr value, IntPtr comparand);

public static IntPtr CompareExchange (ref IntPtr location1, IntPtr value, IntPtr comparand);

static member CompareExchange : nativeint * nativeint * nativeint -> nativeint

Public Shared Function CompareExchange (ByRef location1 As IntPtr, value As IntPtr, comparand As IntPtr) As IntPtr

Parameters

location1

IntPtr

nativeint

The destination IntPtr, whose value is compared with the value of comparand and possibly replaced by value.

value

IntPtr

nativeint

The IntPtr that replaces the destination value if the comparison results in equality.

comparand

IntPtr

nativeint

The IntPtr that is compared to the value at location1.

Returns

IntPtr

nativeint

The original value in location1.

Exceptions

NullReferenceException

The address of location1 is a null pointer.

Remarks

If comparand and the value in location1 are equal, then value is stored in location1. Otherwise, no operation is performed. The compare and exchange operations are performed as an atomic operation. The return value of this method is the original value in location1, whether or not the exchange takes place.

Note

IntPtr is a platform-specific type.

See also

• Managed Threading
• Overview of synchronization primitives

CompareExchange<T>(T, T, T)

Source:

Interlocked.CoreCLR.cs

Compares two instances of the specified reference type T for reference equality and, if they are equal, replaces the first one, as an atomic operation.

public:
generic <typename T>
 where T : class static T CompareExchange(T % location1, T value, T comparand);

public static T CompareExchange<T> (ref T location1, T value, T comparand) where T : class;

static member CompareExchange : 'T * 'T * 'T -> 'T (requires 'T : null)

Public Shared Function CompareExchange(Of T As Class) (ByRef location1 As T, value As T, comparand As T) As T

Type Parameters

T

The type to be used for location1, value, and comparand. This type must be a reference type.

Parameters

location1

T

The destination, whose value is compared by reference with comparand and possibly replaced. This is a reference parameter (ref in C#, ByRef in Visual Basic).

value

T

The value that replaces the destination value if the comparison by reference results in equality.

comparand

T

The value that is compared by reference to the value at location1.

Returns

T

The original value in location1.

Exceptions

NullReferenceException

The address of location1 is a null pointer.

NotSupportedException

An unsupported T is specified.

Remarks

If comparand and the value in location1 are equal by reference, then value is stored in location1. Otherwise, no operation is performed. The comparison and the exchange are performed as an atomic operation. The return value of this method is the original value in location1, whether or not the exchange takes place.

This method only supports reference types. There are overloads of the CompareExchange method for the value types Int32, Int64, IntPtr, Single, and Double, but there is no support for other value types.

Note

This method overload is preferable to the CompareExchange(Object, Object, Object) method overload, because the latter requires the destination object to be accessed late-bound.