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

.NET Framework (current version)

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Executes a for (For in Visual Basic) loop with 64-bit indexes in which iterations may run in parallel and the state of the loop can be monitored and manipulated.

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

public static ParallelLoopResult For(
	long fromInclusive,
	long toExclusive,
	Action<long, ParallelLoopState> body


Type: System.Int64

The start index, inclusive.

Type: System.Int64

The end index, exclusive.

Type: System.Action<Int64, ParallelLoopState>

The delegate that is invoked once per iteration.

Return Value

Type: System.Threading.Tasks.ParallelLoopResult

A ParallelLoopResult structure that contains information on what portion of the loop completed.

Exception Condition

The body argument is null.


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), and a ParallelLoopState instance that may be used to break out of the loop prematurely.

Calling the Break method informs the for operation that iterations after the current one don’t have to be executed, but all iterations before the current one do.

Therefore, calling Break is similar to using a break operation within a conventional for loop in a language like C#, but it is not a perfect substitute: For example, there is no guarantee that iterations after the current one will definitely not execute.

If executing all iterations before the current one is not necessary, use the Stop method instead of using Break. Calling Stop informs the for loop that it may abandon all remaining iterations, regardless of whether they're before or after the current iteration, because all required work will have already been completed. However, as with Break, there are no guarantees regarding which other iterations will not execute.

If a loop is ended prematurely, the ParallelLoopResult structure that is returned will contain relevant information about the loop's completion.

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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