Interlocked.CompareExchange Interlocked.CompareExchange Interlocked.CompareExchange Interlocked.CompareExchange Method

定義

比較兩個值是否相等,如果相等,則取代第一個值。Compares two values for equality and, if they are equal, replaces the first value.

多載

CompareExchange(Double, Double, Double) CompareExchange(Double, Double, Double) CompareExchange(Double, Double, Double) CompareExchange(Double, Double, Double)

比較兩個雙精確度浮點數是否相等;如果相等,則取代第一個值。Compares two double-precision floating point numbers for equality and, if they are equal, replaces the first value.

CompareExchange(Int32, Int32, Int32) CompareExchange(Int32, Int32, Int32) CompareExchange(Int32, Int32, Int32) CompareExchange(Int32, Int32, Int32)

比較兩個 32 位元帶正負號的整數是否相等,如果相等,則取代第一個值。Compares two 32-bit signed integers for equality and, if they are equal, replaces the first value.

CompareExchange(Int64, Int64, Int64) CompareExchange(Int64, Int64, Int64) CompareExchange(Int64, Int64, Int64) CompareExchange(Int64, Int64, Int64)

比較兩個 64 位元帶正負號的整數是否相等,如果相等,則取代第一個值。Compares two 64-bit signed integers for equality and, if they are equal, replaces the first value.

CompareExchange(IntPtr, IntPtr, IntPtr) CompareExchange(IntPtr, IntPtr, IntPtr) CompareExchange(IntPtr, IntPtr, IntPtr) CompareExchange(IntPtr, IntPtr, IntPtr)

比較兩個平台特定的控制代碼或指標是否相等;如果相等,則取代第一個。Compares two platform-specific handles or pointers for equality and, if they are equal, replaces the first one.

CompareExchange(Object, Object, Object) CompareExchange(Object, Object, Object) CompareExchange(Object, Object, Object) CompareExchange(Object, Object, Object)

比較兩個物件的參考是否相等;如果相等,則取代第一個物件。Compares two objects for reference equality and, if they are equal, replaces the first object.

CompareExchange(Single, Single, Single) CompareExchange(Single, Single, Single) CompareExchange(Single, Single, Single) CompareExchange(Single, Single, Single)

比較兩個單精確度浮點數是否相等;如果相等,則取代第一個值。Compares two single-precision floating point numbers for equality and, if they are equal, replaces the first value.

CompareExchange<T>(T, T, T) CompareExchange<T>(T, T, T) CompareExchange<T>(T, T, T) CompareExchange<T>(T, T, T)

比較指定參考型別 T 的兩個執行個體是否參考相等;如果相等,則取代第一個。Compares two instances of the specified reference type T for reference equality and, if they are equal, replaces the first one.

CompareExchange(Double, Double, Double) CompareExchange(Double, Double, Double) CompareExchange(Double, Double, Double) CompareExchange(Double, Double, Double)

比較兩個雙精確度浮點數是否相等;如果相等,則取代第一個值。Compares two double-precision floating point numbers for equality and, if they are equal, replaces the first value.

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

參數

location1
Double Double Double Double

目的端,其值會與 comparand 進行比較且可能會被取代。The destination, whose value is compared with comparand and possibly replaced.

value
Double Double Double Double

當比較的結果相等時,會取代目的端值的值。The value that replaces the destination value if the comparison results in equality.

comparand
Double Double Double Double

location1 的值比較的值。The value that is compared to the value at location1.

傳回

location1 中的原始值。The original value in location1.

例外狀況

location1 的位址為 null 指標。The address of location1 is a null pointer.

範例

下列程式碼範例示範會累積的累計值的安全執行緒方法Double值。The following code example demonstrates a thread-safe method that accumulates a running total of Double values. 兩個執行緒新增一系列的Double值 使用安全執行緒方法和一般的加法,和當執行緒完成時,會比較總計。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. 雙處理器的電腦上,沒有顯著的差異總計。On a dual-processor computer, there is a significant difference in the totals.

在使用安全執行緒方法時,會儲存計算加總的初始值,然後CompareExchange方法用來交換新計算的總和,最舊的總計。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. 如果傳回的值不等於儲存的值的加總,然後另一個執行緒已經更新總同時。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. 在此情況下,您必須重複嘗試更新計算加總。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

備註

如果comparand中的值location1相等,則value會儲存在location1If comparand and the value in location1 are equal, then value is stored in location1. 否則,不會執行任何作業。Otherwise, no operation is performed. 比較和交換作業成為不可部分完成的作業。The compare and exchange operations are performed as an atomic operation. 傳回值CompareExchange是中的原始值location1無論交換發生。The return value of CompareExchange is the original value in location1, whether or not the exchange takes place.

另請參閱

CompareExchange(Int32, Int32, Int32) CompareExchange(Int32, Int32, Int32) CompareExchange(Int32, Int32, Int32) CompareExchange(Int32, Int32, Int32)

比較兩個 32 位元帶正負號的整數是否相等,如果相等,則取代第一個值。Compares two 32-bit signed integers for equality and, if they are equal, replaces the first value.

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

參數

location1
Int32 Int32 Int32 Int32

目的端,其值會與 comparand 進行比較且可能會被取代。The destination, whose value is compared with comparand and possibly replaced.

value
Int32 Int32 Int32 Int32

當比較的結果相等時,會取代目的端值的值。The value that replaces the destination value if the comparison results in equality.

comparand
Int32 Int32 Int32 Int32

location1 的值比較的值。The value that is compared to the value at location1.

傳回

location1 中的原始值。The original value in location1.

例外狀況

location1 的位址為 null 指標。The address of location1 is a null pointer.

範例

下列程式碼範例示範會加總累積的安全執行緒的方法。The following code example demonstrates a thread-safe method that accumulates a running total. 計算加總的初始值會儲存,然後CompareExchange方法用來交換新計算的總和,最舊的總計。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. 如果傳回的值不等於儲存的值的加總,然後另一個執行緒已經更新總同時。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. 在此情況下,您必須重複嘗試更新計算加總。In that case, the attempt to update the running total must be repeated.

注意

Add的.NET Framework 2.0 版中引入的方法提供更便利的方式,累積安全執行緒累計總數的整數。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

備註

如果comparand中的值location1相等,則value會儲存在location1If comparand and the value in location1 are equal, then value is stored in location1. 否則,不會執行任何作業。Otherwise, no operation is performed. 比較和交換作業成為不可部分完成的作業。The compare and exchange operations are performed as an atomic operation. 傳回值CompareExchange是中的原始值location1無論交換發生。The return value of CompareExchange is the original value in location1, whether or not the exchange takes place.

另請參閱

CompareExchange(Int64, Int64, Int64) CompareExchange(Int64, Int64, Int64) CompareExchange(Int64, Int64, Int64) CompareExchange(Int64, Int64, Int64)

比較兩個 64 位元帶正負號的整數是否相等,如果相等,則取代第一個值。Compares two 64-bit signed integers for equality and, if they are equal, replaces the first value.

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

參數

location1
Int64 Int64 Int64 Int64

目的端,其值會與 comparand 進行比較且可能會被取代。The destination, whose value is compared with comparand and possibly replaced.

value
Int64 Int64 Int64 Int64

當比較的結果相等時,會取代目的端值的值。The value that replaces the destination value if the comparison results in equality.

comparand
Int64 Int64 Int64 Int64

location1 的值比較的值。The value that is compared to the value at location1.

傳回

location1 中的原始值。The original value in location1.

例外狀況

location1 的位址為 null 指標。The address of location1 is a null pointer.

備註

如果comparand中的值location1相等,則value會儲存在location1If comparand and the value in location1 are equal, then value is stored in location1. 否則,不會執行任何作業。Otherwise, no operation is performed. 比較和交換作業成為不可部分完成的作業。The compare and exchange operations are performed as an atomic operation. 傳回值CompareExchange是中的原始值location1無論交換發生。The return value of CompareExchange is the original value in location1, whether or not the exchange takes place.

另請參閱

CompareExchange(IntPtr, IntPtr, IntPtr) CompareExchange(IntPtr, IntPtr, IntPtr) CompareExchange(IntPtr, IntPtr, IntPtr) CompareExchange(IntPtr, IntPtr, IntPtr)

比較兩個平台特定的控制代碼或指標是否相等;如果相等,則取代第一個。Compares two platform-specific handles or pointers for equality and, if they are equal, replaces the first one.

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

參數

location1
IntPtr IntPtr IntPtr IntPtr

目的地IntPtr,其值會與具有值comparand且可能被取代的valueThe destination IntPtr, whose value is compared with the value of comparand and possibly replaced by value.

value
IntPtr IntPtr IntPtr IntPtr

IntPtr當比較結果相等時會取代目的端值。The IntPtr that replaces the destination value if the comparison results in equality.

comparand
IntPtr IntPtr IntPtr IntPtr

IntPtr的值,比較location1The IntPtr that is compared to the value at location1.

傳回

location1 中的原始值。The original value in location1.

例外狀況

location1 的位址為 null 指標。The address of location1 is a null pointer.

備註

如果comparand中的值location1相等,則value會儲存在location1If comparand and the value in location1 are equal, then value is stored in location1. 否則,不會執行任何作業。Otherwise, no operation is performed. 比較和交換作業成為不可部分完成的作業。The compare and exchange operations are performed as an atomic operation. 這個方法的傳回值是中的原始值location1無論交換發生。The return value of this method is the original value in location1, whether or not the exchange takes place.

注意

IntPtr 是特定平台類型。IntPtr is a platform-specific type.

另請參閱

CompareExchange(Object, Object, Object) CompareExchange(Object, Object, Object) CompareExchange(Object, Object, Object) CompareExchange(Object, Object, Object)

比較兩個物件的參考是否相等;如果相等,則取代第一個物件。Compares two objects for reference equality and, if they are equal, replaces the first object.

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

參數

location1
Object Object Object Object

以傳址方式與 comparand 比較的目的地物件,且可能被取代。The destination object that is compared by reference with comparand and possibly replaced.

value
Object Object Object Object

若參考比較結果相等,此物件會取代目的地物件。The object that replaces the destination object if the reference comparison results in equality.

comparand
Object Object Object Object

以傳址方式與位於location1 之物件比較的物件。The object that is compared by reference to the object at location1.

傳回

location1 中的原始值。The original value in location1.

例外狀況

location1 的位址為 null 指標。The address of location1 is a null pointer.

備註

重要

從 .NET Framework 2.0 開始,CompareExchange<T>(T, T, T) 方法多載會為參考型別提供型別安全的替代方法。Beginning with .NET Framework 2.0, the CompareExchange<T>(T, T, T) method overload provides a type-safe alternative for reference types. 我們建議您呼叫它而不是這個多載。We recommend that you call it instead of this overload.

如果comparand和中的物件location1上是相等的參考,然後value會儲存在location1If comparand and the object in location1 are equal by reference, then value is stored in location1. 否則,不會執行任何作業。Otherwise, no operation is performed. 比較和交換作業成為不可部分完成的作業。The compare and exchange operations are performed as an atomic operation. 傳回值CompareExchange是中的原始值location1無論交換發生。The return value of CompareExchange is the original value in location1, whether or not the exchange takes place.

注意

參考相等,而不是實值相等比較的物件。The objects are compared for reference equality rather than value equality. 如此一來,兩個經過 boxing 處理的執行個體相同的實值型別 (例如,整數值 3) 永遠看起來是不相等,而且會執行任何作業。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. 請勿使用這個多載具有實值類型。Do not use this overload with value types.

另請參閱

CompareExchange(Single, Single, Single) CompareExchange(Single, Single, Single) CompareExchange(Single, Single, Single) CompareExchange(Single, Single, Single)

比較兩個單精確度浮點數是否相等;如果相等,則取代第一個值。Compares two single-precision floating point numbers for equality and, if they are equal, replaces the first value.

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

參數

location1
Single Single Single Single

目的端,其值會與 comparand 進行比較且可能會被取代。The destination, whose value is compared with comparand and possibly replaced.

value
Single Single Single Single

當比較的結果相等時,會取代目的端值的值。The value that replaces the destination value if the comparison results in equality.

comparand
Single Single Single Single

location1 的值比較的值。The value that is compared to the value at location1.

傳回

location1 中的原始值。The original value in location1.

例外狀況

location1 的位址為 null 指標。The address of location1 is a null pointer.

範例

下列程式碼範例示範會累積的累計值的安全執行緒方法Single值。The following code example demonstrates a thread-safe method that accumulates a running total of Single values. 兩個執行緒新增一系列的Single值 使用安全執行緒方法和一般的加法,和當執行緒完成時,會比較總計。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. 雙處理器的電腦上,沒有顯著的差異總計。On a dual-processor computer, there is a significant difference in the totals.

在使用安全執行緒方法時,會儲存計算加總的初始值,然後CompareExchange方法用來交換新計算的總和,最舊的總計。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. 如果傳回的值不等於儲存的值的加總,然後另一個執行緒已經更新總同時。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. 在此情況下,您必須重複嘗試更新計算加總。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

備註

如果comparand中的值location1相等,則value會儲存在location1If comparand and the value in location1 are equal, then value is stored in location1. 否則,不會執行任何作業。Otherwise, no operation is performed. 比較和交換作業成為不可部分完成的作業。The compare and exchange operations are performed as an atomic operation. 傳回值CompareExchange是中的原始值location1無論交換發生。The return value of CompareExchange is the original value in location1, whether or not the exchange takes place.

另請參閱

CompareExchange<T>(T, T, T) CompareExchange<T>(T, T, T) CompareExchange<T>(T, T, T) CompareExchange<T>(T, T, T)

比較指定參考型別 T 的兩個執行個體是否參考相等;如果相等,則取代第一個。Compares two instances of the specified reference type T for reference equality and, if they are equal, replaces the first one.

public:
generic <typename T>
 where T : class static T CompareExchange(T % location1, T value, T comparand);
[System.Runtime.InteropServices.ComVisible(false)]
public static T CompareExchange<T> (ref T location1, T value, T comparand) where T : class;
static member CompareExchange :  * '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

類型參數

T

要用於型別location1value,和comparandThe type to be used for location1, value, and comparand. 此類型必須是參考類型。This type must be a reference type.

參數

location1
T T T T

以傳址方式與 comparand 比較其值的目的地,且可能被取代。The destination, whose value is compared by reference with comparand and possibly replaced. 此為參考參數 (在 C# 中為 ref,在 Visual Basic 中為 ByRef)。This is a reference parameter (ref in C#, ByRef in Visual Basic).

value
T T T T

當以傳址方式比較的結果相等時,會取代目的地的值。The value that replaces the destination value if the comparison by reference results in equality.

comparand
T T T T

以傳址方式與位於 location1 之值比較的值。The value that is compared by reference to the value at location1.

傳回

T T T T

location1 中的原始值。The original value in location1.

例外狀況

location1 的位址為 null 指標。The address of location1 is a null pointer.

備註

如果comparand中的值location1上是相等的參考,然後value會儲存在location1If comparand and the value in location1 are equal by reference, then value is stored in location1. 否則,不會執行任何作業。Otherwise, no operation is performed. 比較和交換將會以不可部分完成的作業。The comparison and the exchange are performed as an atomic operation. 這個方法的傳回值是中的原始值location1無論交換發生。The return value of this method is the original value in location1, whether or not the exchange takes place.

這個方法只支援參考型別。This method only supports reference types. 有多載CompareExchange方法,實值型別Int32Int64IntPtrSingle,和Double,但不支援其他實值型別。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.

注意

最好是使用這個方法多載CompareExchange(Object, Object, Object)方法多載,因為後者需要要存取晚期繫結的目的地物件。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.

適用於