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Interlocked.CompareExchange Method (Int32, Int32, Int32)

Compares two 32-bit signed integers for equality and, if they are equal, replaces one of the values.

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

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

Parameters

location1
Type: System.Int32

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

value
Type: System.Int32

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

comparand
Type: System.Int32

The value that is compared to the value at location1.

Return Value

Type: System.Int32
The original value in location1.

ExceptionCondition
NullReferenceException

The address of location1 is a null pointer.

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.

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.

NoteNote

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.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;
    }
}

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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