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how-to-cancel-a-plinq-query.md

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Learn more about: How to: Cancel a PLINQ Query
How to: Cancel a PLINQ Query
03/30/2017
csharp
vb
PLINQ queries, how to cancel
cancellation, PLINQ
80b14640-edfa-4153-be1b-3e003d3e9c1a

How to: Cancel a PLINQ Query

The following examples show two ways to cancel a PLINQ query. The first example shows how to cancel a query that consists mostly of data traversal. The second example shows how to cancel a query that contains a user function that is computationally expensive.

Note

When "Just My Code" is enabled, Visual Studio will break on the line that throws the exception and display an error message that says "exception not handled by user code." This error is benign. You can press F5 to continue from it, and see the exception-handling behavior that is demonstrated in the examples below. To prevent Visual Studio from breaking on the first error, just uncheck the "Just My Code" checkbox under Tools, Options, Debugging, General.

This example is intended to demonstrate usage, and might not run faster than the equivalent sequential LINQ to Objects query. For more information about speedup, see Understanding Speedup in PLINQ.

Example 1

[!code-csharpPLINQ#16] [!code-vbPLINQ#16]

The PLINQ framework does not roll a single xref:System.OperationCanceledException into an xref:System.AggregateException?displayProperty=nameWithType; the xref:System.OperationCanceledException must be handled in a separate catch block. If one or more user delegates throws an OperationCanceledException(externalCT) (by using an external xref:System.Threading.CancellationToken?displayProperty=nameWithType) but no other exception, and the query was defined as AsParallel().WithCancellation(externalCT), then PLINQ will issue a single xref:System.OperationCanceledException (externalCT) rather than an xref:System.AggregateException?displayProperty=nameWithType. However, if one user delegate throws an xref:System.OperationCanceledException, and another delegate throws another exception type, then both exceptions will be rolled into an xref:System.AggregateException.

The general guidance on cancellation is as follows:

  1. If you perform user-delegate cancellation, you should inform PLINQ about the external xref:System.Threading.CancellationToken and throw an xref:System.OperationCanceledException(externalCT).

  2. If cancellation occurs and no other exceptions are thrown, then handle an xref:System.OperationCanceledException rather than an xref:System.AggregateException.

Example 2

The following example shows how to handle cancellation when you have a computationally expensive function in user code.

[!code-csharpPLINQ#17] [!code-vbPLINQ#17]

When you handle the cancellation in user code, you do not have to use xref:System.Linq.ParallelEnumerable.WithCancellation%2A in the query definition. However, we recommend that you do use xref:System.Linq.ParallelEnumerable.WithCancellation%2A, because xref:System.Linq.ParallelEnumerable.WithCancellation%2A has no effect on query performance and it enables the cancellation to be handled by query operators and your user code.

In order to ensure system responsiveness, we recommend that you check for cancellation around once per millisecond; however, any period up to 10 milliseconds is considered acceptable. This frequency should not have a negative impact on your code's performance.

When an enumerator is disposed, for example when code breaks out of a foreach (For Each in Visual Basic) loop that is iterating over query results, then the query is canceled, but no exception is thrown.

See also