Interlocked.CompareExchange Method (Int32, Int32, Int32)
Compares two 32-bit signed integers for equality and, if they are equal, replaces the first value.
Assembly: mscorlib (in mscorlib.dll)
Public Shared Function CompareExchange ( ByRef location1 As Integer, value As Integer, comparand As Integer ) As Integer
The destination, whose value is compared with comparand and possibly replaced.
The value that replaces the destination value if the comparison results in equality.
The value that is compared to the value at location1.
Return ValueType: System.Int32
The original value in location1.
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 themethod 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. 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
Available since 8
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Available since 8.1