Interlocked.Increment Method (Int32)

 
System_CAPS_noteNote

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

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

Public Shared Function Increment (
	ByRef location As Integer
) As Integer

Parameters

location
Type: System.Int32

The variable whose value is to be incremented.

Return Value

Type: System.Int32

The incremented value.

Exception Condition
NullReferenceException

The address of location is a null pointer.

This method handles an overflow condition by wrapping: if location = Int32.MaxValue, location + 1 = Int32.MinValue. 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 0 and incremented each time the random number generator returns a midpoint value until it reaches 10,000. Because three threads generate the random numbers, the Increment(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.

Imports System.Threading

Module Example
   Const LOWERBOUND As Integer = 0
   Const UPPERBOUND As Integer = 1001

   Dim lockObj As New Object()
   Dim rnd As New Random()
   Dim cte As CountdownEvent

   Dim totalCount As Integer = 0
   Dim totalMidpoint As Integer = 0
   Dim midpointCount As Integer = 0

   Public Sub Main()
      cte = New CountdownEvent(1)
      ' Start three threads. 
      For ctr As Integer = 0 To 2
         cte.AddCount()
         Dim th As New Thread(AddressOf GenerateNumbers)
         th.Name = "Thread" + ctr.ToString()
         th.Start()
      Next
      cte.Signal()
      cte.Wait()
      Console.WriteLine()
      Console.WriteLine("Total midpoint values:  {0,10:N0} ({1:P3})",
                        totalMidpoint, totalMidpoint/totalCount)
      Console.WriteLine("Total number of values: {0,10:N0}", 
                        totalCount)                  
   End Sub

   Private Sub GenerateNumbers()
      Dim midpoint As Integer = (upperBound - lowerBound) \ 2
      Dim value As Integer = 0
      Dim total As Integer = 0
      Dim midpt As Integer = 0
      Do
         SyncLock lockObj
            value = rnd.Next(lowerBound, upperBound)
         End SyncLock
         If value = midpoint Then 
            Interlocked.Increment(midpointCount)
            midpt += 1
         End If
         total += 1    
      Loop While midpointCount < 10000

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

      Dim s As String = String.Format("Thread {0}:", Thread.CurrentThread.Name) + vbCrLf +
                        String.Format("   Random Numbers: {0:N0}", total) + vbCrLf +
                        String.Format("   Midpoint values: {0:N0} ({1:P3})", midpt, midpt/total)
      Console.WriteLine(s)
      cte.Signal()
   End Sub
End Module
' The example displays output like the following:
'       Thread Thread2:
'          Random Numbers: 2,776,674
'          Midpoint values: 2,773 (0.100 %)
'       Thread Thread1:
'          Random Numbers: 4,876,100
'          Midpoint values: 4,873 (0.100 %)
'       Thread Thread0:
'          Random Numbers: 2,312,310
'          Midpoint values: 2,354 (0.102 %)
'       
'       Total midpoint values:      10,000 (0.100 %)
'       Total number of values:  9,965,084

The following example is similar to the previous one, except that it uses the 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.

Imports System.Collections.Generic
Imports System.Threading
Imports System.Threading.Tasks

Module Example
   Const LOWERBOUND As Integer = 0
   Const UPPERBOUND As Integer = 1001

   Dim lockObj As New Object()
   Dim rnd As New Random()

   Dim totalCount As Integer = 0
   Dim totalMidpoint As Integer = 0
   Dim midpointCount As Integer = 0

   Public Sub Main()
      Dim tasks As New List(Of Task)()
      ' Start three tasks. 
      For ctr As Integer = 0 To 2
         tasks.Add(Task.Run( Sub()
                                Dim midpoint As Integer = (upperBound - lowerBound) \ 2
                                Dim value As Integer = 0
                                Dim total As Integer = 0
                                Dim midpt As Integer = 0
                                Do
                                   SyncLock lockObj
                                      value = rnd.Next(lowerBound, upperBound)
                                   End SyncLock
                                   If value = midpoint Then 
                                      Interlocked.Increment(midpointCount)
                                      midpt += 1
                                   End If
                                   total += 1    
                                Loop While midpointCount < 50000

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

                                Dim s As String = String.Format("Task {0}:", Task.CurrentId) + vbCrLf +
                                                  String.Format("   Random Numbers: {0:N0}", total) + vbCrLf +
                                                  String.Format("   Midpoint values: {0:N0} ({1:P3})", midpt, midpt/total)
                                Console.WriteLine(s)
                             End Sub ))
      Next

      Task.WaitAll(tasks.ToArray())
      Console.WriteLine()
      Console.WriteLine("Total midpoint values:  {0,10:N0} ({1:P3})",
                        totalMidpoint, totalMidpoint/totalCount)
      Console.WriteLine("Total number of values: {0,10:N0}", 
                        totalCount)                  
   End Sub
End Module
' The example displays output like the following:
'       Task 3:
'          Random Numbers: 10,855,250
'          Midpoint values: 10,823 (0.100 %)
'       Task 1:
'          Random Numbers: 15,243,703
'          Midpoint values: 15,110 (0.099 %)
'       Task 2:
'          Random Numbers: 24,107,425
'          Midpoint values: 24,067 (0.100 %)
'       
'       Total midpoint values:      50,000 (0.100 %)
'       Total number of values: 50,206,378

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