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Interlocked.Decrement Method (Int32)

Decrements a specified variable and stores the result, as an atomic operation.

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

public static int Decrement(
	ref int location
)

Parameters

location
Type: System.Int32

The variable whose value is to be decremented.

Return Value

Type: System.Int32
The decremented value.

ExceptionCondition
ArgumentNullException

The address of location is a null pointer.

This method handles an overflow condition by wrapping: If location = Int32.MinValue, location - 1 = Int32.MaxValue. No exception is thrown.

The following example determines how many random numbers that range from 0 to 1,000 are required to generate 1,000 random numbers with a midpoint value. To keep track of the number of midpoint values, a variable, midpointCount, is set equal to 1,000 and decremented each time the random number generator returns a midpoint value. Because three threads generate the random numbers, the Decrement(Int32) method is called to ensure that multiple threads don't update midpointCount concurrently. Note that a lock is also used to protect the random number generator, and that a CountdownEvent object is used to ensure that the Main method doesn't finish execution before the three threads.

using System;
using System.Threading;

public class Example
{
   const int LOWERBOUND = 0;
   const int UPPERBOUND = 1001;

   static Object lockObj = new Object();
   static Random rnd = new Random();
   static CountdownEvent cte;

   static int totalCount = 0;
   static int totalMidpoint = 0;
   static int midpointCount = 10000;

   public static void Main()
   {
      cte = new CountdownEvent(1);
      // Start three threads.  
      for (int ctr = 0; ctr <= 2; ctr++) {
         cte.AddCount();
         Thread th = new Thread(GenerateNumbers);
         th.Name = "Thread" + ctr.ToString();
         th.Start();
      }
      cte.Signal();
      cte.Wait();
      Console.WriteLine();
      Console.WriteLine("Total midpoint values:  {0,10:N0} ({1:P3})",
                        totalMidpoint, totalMidpoint/((double)totalCount));
      Console.WriteLine("Total number of values: {0,10:N0}", 
                        totalCount);                  
   }

   private static void GenerateNumbers()
   {
      int midpoint = (UPPERBOUND - LOWERBOUND) / 2;
      int value = 0;
      int total = 0;
      int midpt = 0;

      do {
         lock (lockObj) {
            value = rnd.Next(LOWERBOUND, UPPERBOUND);
         }
         if (value == midpoint) { 
            Interlocked.Decrement(ref midpointCount);
            midpt++;
         }
         total++;    
      } while (midpointCount > 0);

      Interlocked.Add(ref totalCount, total);
      Interlocked.Add(ref totalMidpoint, midpt);

      string s = String.Format("Thread {0}:\n", Thread.CurrentThread.Name) +
                 String.Format("   Random Numbers: {0:N0}\n", total) + 
                 String.Format("   Midpoint values: {0:N0} ({1:P3})", midpt, 
                               ((double) midpt)/total);
      Console.WriteLine(s);
      cte.Signal();
   }
}
// The example displays output like the following: 
//       Thread Thread2: 
//          Random Numbers: 3,204,021 
//          Midpoint values: 3,156 (0.099 %) 
//       Thread Thread0: 
//          Random Numbers: 4,073,592 
//          Midpoint values: 4,015 (0.099 %) 
//       Thread Thread1: 
//          Random Numbers: 2,828,192 
//          Midpoint values: 2,829 (0.100 %) 
//        
//       Total midpoint values:      10,000 (0.099 %) 
//       Total number of values: 10,105,805

The following example is similar to the previous one, except that it uses the task-based asynchronous pattern instead of a thread procedure to generate 50,000 random midpoint integers. In this example, a lambda expression replaces the GenerateNumbers thread procedure, and the call to the Task.WaitAll method eliminates the need for the CountdownEvent object.

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

public class Example
{
   const int LOWERBOUND = 0;
   const int UPPERBOUND = 1001;

   static Object lockObj = new Object();
   static Random rnd = new Random();

   static int totalCount = 0;
   static int totalMidpoint = 0;
   static int midpointCount = 50000;

   public static void Main()
   {
      List<Task> tasks = new List<Task>();

      // Start three tasks.  
      for (int ctr = 0; ctr <= 2; ctr++) 
         tasks.Add(Task.Run( () => { int midpoint = (UPPERBOUND - LOWERBOUND) / 2;
                                     int value = 0;
                                     int total = 0;
                                     int midpt = 0;

                                     do {
                                        lock (lockObj) {
                                           value = rnd.Next(LOWERBOUND, UPPERBOUND);
                                        }
                                        if (value == midpoint) { 
                                           Interlocked.Decrement(ref midpointCount);
                                           midpt++;
                                        }
                                        total++;    
                                     } while (midpointCount > 0 );

                                     Interlocked.Add(ref totalCount, total);
                                     Interlocked.Add(ref totalMidpoint, midpt);

                                     string s = String.Format("Task {0}:\n", Task.CurrentId) +
                                                String.Format("   Random Numbers: {0:N0}\n", total) + 
                                                String.Format("   Midpoint values: {0:N0} ({1:P3})", midpt, 
                                                              ((double) midpt)/total);
                                     Console.WriteLine(s); 
                                   } ));

      Task.WaitAll(tasks.ToArray());

      Console.WriteLine();
      Console.WriteLine("Total midpoint values:  {0,10:N0} ({1:P3})",
                        totalMidpoint, totalMidpoint/((double)totalCount));
      Console.WriteLine("Total number of values: {0,10:N0}", 
                        totalCount);                  
   }
}
// The example displays output like the following: 
//       Task 1: 
//          Random Numbers: 24,530,624 
//          Midpoint values: 24,675 (0.101 %) 
//       Task 2: 
//          Random Numbers: 7,079,718 
//          Midpoint values: 7,093 (0.100 %) 
//       Task 3: 
//          Random Numbers: 18,284,617 
//          Midpoint values: 18,232 (0.100 %) 
//        
//       Total midpoint values:      50,000 (0.100 %) 
//       Total number of values: 49,894,959

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
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