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How to: Handle Exceptions in a PLINQ Query

The first example in this topic shows how to handle the System.AggregateException that can be thrown from a PLINQ query when it executes. The second example shows how to put try-catch blocks within delegates, as close as possible to where the exception will be thrown. In this way, you can catch them as soon as they occur and possibly continue query execution. When exceptions are allowed to bubble up back to the joining thread, then it is possible that a query may continue to process some items after the exception is raised.

In some cases when PLINQ falls back to sequential execution, and an exception occurs, the exception may be propagated directly, and not wrapped in an AggregateException. Also, ThreadAbortExceptions are always propagated directly.

NoteNote

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.

This example shows how to put the try-catch blocks around the code that executes the query to catch any System.AggregateExceptions that are thrown.


' Paste into PLINQDataSample class
Shared Sub PLINQExceptions_1()

    ' Using the raw string array here. See PLINQ Data Sample.
    Dim customers As String() = GetCustomersAsStrings().ToArray()

    ' First, we must simulate some currupt input.
    customers(20) = "###"

    'throws indexoutofrange
    Dim query = From cust In customers.AsParallel() _
        Let fields = cust.Split(","c) _
        Where fields(3).StartsWith("C") _
        Select fields
    Try
        ' We use ForAll although it doesn't really improve performance
        ' since all output is serialized through the Console.
        query.ForAll(Sub(e)
                         Console.WriteLine("City: {0}, Thread:{1}")
                     End Sub)
    Catch e As AggregateException

        ' In this design, we stop query processing when the exception occurs.
        For Each ex In e.InnerExceptions
            Console.WriteLine(ex.Message)
            If TypeOf ex Is IndexOutOfRangeException Then
                Console.WriteLine("The data source is corrupt. Query stopped.")
            End If
        Next
    End Try
End Sub


In this example, the query cannot continue after the exception is thrown. By the time your application code catches the exception, PLINQ has already stopped the query on all threads.

The following example shows how to put a try-catch block in a delegate to make it possible to catch an exception and continue with the query execution.


' Paste into PLINQDataSample class
Shared Sub PLINQExceptions_2()

    Dim customers() = GetCustomersAsStrings().ToArray()
    ' Using the raw string array here.
    ' First, we must simulate some currupt input
    customers(20) = "###"

    ' Create a delegate with a lambda expression.
    ' Assume that in this app, we expect malformed data
    ' occasionally and by design we just report it and continue.
    Dim isTrue As Func(Of String(), String, Boolean) = Function(f, c)

                                                           Try

                                                               Dim s As String = f(3)
                                                               Return s.StartsWith(c)

                                                           Catch e As IndexOutOfRangeException

                                                               Console.WriteLine("Malformed cust: {0}", f)
                                                               Return False
                                                           End Try
                                                       End Function

    ' Using the raw string array here
    Dim query = From cust In customers.AsParallel()
                        Let fields = cust.Split(","c)
                        Where isTrue(fields, "C")
                       Select New With {.City = fields(3)}
    Try
        ' We use ForAll although it doesn't really improve performance
        ' since all output must be serialized through the Console.
        query.ForAll(Sub(e) Console.WriteLine(e.city))


        ' IndexOutOfRangeException will not bubble up      
        ' because we handle it where it is thrown.
    Catch e As AggregateException
        For Each ex In e.InnerExceptions
            Console.WriteLine(ex.Message)
        Next
    End Try
End Sub


  • To compile and run these examples, copy them into the PLINQ Data Sample example and call the method from Main.

Do not catch an exception unless you know how to handle it so that you do not corrupt the state of your program.

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