How to cancel parallel LINQ queries in .NET C#

We saw in several posts on TPL on this blog how the CancellationToken object can be used to cancel Tasks. They can be used to cancel parallel queries as well. An instance of the token must be supplied to the WithCancellation extension method.

Define the cancellation token and the data source:

CancellationTokenSource cancellationTokenSource
	= new CancellationTokenSource();

int[] integerArray = new int[10000000];
for (int i = 0; i < integerArray.Length; i++)
{
	integerArray[i] = i;
}

Define the query. Notice how the token is provided to the query:

IEnumerable<double> query = integerArray
	.AsParallel()
	.WithCancellation(cancellationTokenSource.Token)
	.Select(item =>
	{
		return Math.Sqrt(item);
	});

Start a separate task that will cancel the token after 5 seconds:

Task.Factory.StartNew(() =>
{
	Thread.Sleep(5000);
	cancellationTokenSource.Cancel();
	Console.WriteLine("Token source cancelled");
});

Loop through the query results and catch the OperationCancelledException:

try
{
	foreach (double d in query)
	{
		Console.WriteLine("Result: {0}", d);
	}
}
catch (OperationCanceledException)
{
	Console.WriteLine("Caught cancellation exception");
}

Do not assume that cancelling the token will cause the item processing to stop immediately. Items that were already being processed when the token was cancelled will be completed.

View the list of posts on the Task Parallel Library here.

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About Andras Nemes
I'm a .NET/Java developer living and working in Stockholm, Sweden.

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