Interlocked::CompareExchange Method (Int32%, Int32, Int32)


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

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

static int CompareExchange(
	int% location1,
	int value,
	int comparand


Type: System::Int32%

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

Type: System::Int32

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

Type: System::Int32

The value that is compared to the value at location1.

Return Value

Type: System::Int32

The original value in location1.

Exception Condition

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.


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

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


   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;

         // 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;


Universal Windows Platform
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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