Parallel.For<TLocal> Method (Int32, Int32, ParallelOptions, Func<TLocal>, Func<Int32, ParallelLoopState, TLocal, TLocal>, Action<TLocal>)

.NET Framework (current version)

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Executes a for (For in Visual Basic) loop with thread-local data in which iterations may run in parallel, loop options can be configured, and the state of the loop can be monitored and manipulated.

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

public static ParallelLoopResult For<TLocal>(
	int fromInclusive,
	int toExclusive,
	ParallelOptions parallelOptions,
	Func<TLocal> localInit,
	Func<int, ParallelLoopState, TLocal, TLocal> body,
	Action<TLocal> localFinally


Type: System.Int32

The start index, inclusive.

Type: System.Int32

The end index, exclusive.

Type: System.Threading.Tasks.ParallelOptions

An object that configures the behavior of this operation.

Type: System.Func<TLocal>

The function delegate that returns the initial state of the local data for each task.

Type: System.Func<Int32, ParallelLoopState, TLocal, TLocal>

The delegate that is invoked once per iteration.

Type: System.Action<TLocal>

The delegate that performs a final action on the local state of each task.

Return Value

Type: System.Threading.Tasks.ParallelLoopResult

A structure that contains information about which portion of the loop completed.

Type Parameters


The type of the thread-local data.

Exception Condition

The body argument is null.


The localInit argument is null.


The localFinally argument is null.


The parallelOptions argument is null.


The CancellationToken in the parallelOptions argument is canceled.


The CancellationTokenSource associated with the CancellationToken in the parallelOptions has been disposed.


The exception that contains all the individual exceptions thrown on all threads.

The body delegate is invoked once for each value in the iteration range (fromInclusive, toExclusive). It is provided with the following parameters: the iteration count (Int32), a ParallelLoopState instance that may be used to break out of the loop prematurely, and some local state that may be shared amongst iterations that execute on the same task.

The localInit delegate is invoked once for each task that participates in the loop's execution and returns the initial local state for each of those tasks. These initial states are passed to the first body invocations on each task. Then, every subsequent body invocation returns a possibly modified state value that is passed to the next body invocation. Finally, the last body invocation on each task returns a state value that is passed to the localFinally delegate. The localFinally delegate is invoked once per task to perform a final action on each task's local state. This delegate might be invoked concurrently on multiple threads; therefore, you must synchronize access to any shared variables.

The Parallel.For method may use more tasks than threads over the lifetime of its execution, as existing tasks complete and are replaced by new tasks. This gives the underlying TaskScheduler object the chance to add, change, or remove threads that service the loop.

If fromInclusive is greater than or equal to toExclusive, then the method returns immediately without performing any iterations.

The following example uses thread-local variables to compute the sum of the results of many lengthy operations. This example limits the degree of parallelism to four.

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

class ThreadLocalForWithOptions
   // The number of parallel iterations to perform.
   const int N = 1000000;

   static void Main()
      // The result of all thread-local computations.
      int result = 0;

      // This example limits the degree of parallelism to four.
      // You might limit the degree of parallelism when your algorithm
      // does not scale beyond a certain number of cores or when you 
      // enforce a particular quality of service in your application.

      Parallel.For(0, N, new ParallelOptions { MaxDegreeOfParallelism = 4 },
         // Initialize the local states
         () => 0,
         // Accumulate the thread-local computations in the loop body
         (i, loop, localState) =>
            return localState + Compute(i);
         // Combine all local states
         localState => Interlocked.Add(ref result, localState)

      // Print the actual and expected results.
      Console.WriteLine("Actual result: {0}. Expected 1000000.", result);

   // Simulates a lengthy operation.
   private static int Compute(int n)
      for (int i = 0; i < 10000; i++) ;
      return 1;

Universal Windows Platform
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Windows Phone
Available since 8.1
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