Parallel.For<TLocal> Method (Int64, Int64, ParallelOptions, Func<TLocal>, Func<Int64, ParallelLoopState, TLocal, TLocal>, Action<TLocal>)

.NET Framework (current version)

Executes a for (For in Visual Basic) loop with 64-bit indexes and 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>(
	long fromInclusive,
	long toExclusive,
	ParallelOptions parallelOptions,
	Func<TLocal> localInit,
	Func<long, ParallelLoopState, TLocal, TLocal> body,
	Action<TLocal> localFinally


Type: System.Int64

The start index, inclusive.

Type: System.Int64

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 thread.

Type: System.Func<Int64, 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 thread.

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 (Int64), 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 thread.

The localInit delegate is invoked once for each thread that participates in the loop's execution and returns the initial local state for each of those threads. These initial states are passed to the first body invocations on each thread. 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 thread returns a state value that is passed to the localFinally delegate. The localFinally delegate is invoked once per thread to perform a final action on each thread'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.

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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