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如何：处理 PLINQ 查询中的异常

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更新：2010 年 5 月

本主题中的第一个示例演示如何处理在 PLINQ 查询执行时可能从中引发的 System.AggregateException。 第二个示例演示如何将 try-catch 块放在委托内，尽可能靠近将会引发异常的位置。 通过这种方式，一旦发生异常，您就可以捕获它们并且可能继续执行查询。 如果允许异常向上冒泡回到联接线程，则查询操作也许可以在引发异常后继续处理一些项目。

在某些情况下，当 PLINQ 回退到顺序执行并发生异常时，可能会直接传播该异常，而不是包装到 AggregateException 中。 此外，始终可以直接传播 ThreadAbortException。

注意注意

当启用“仅我的代码”时，Visual Studio 将在引发异常的行上中断运行，并显示错误消息“异常未由用户代码处理”。此错误是良性的。可以按 F5 从中断处继续运行，并查看在以下示例中演示的异常处理行为。若要阻止 Visual Studio 在出现第一个错误时中断运行，只需在“工具”->“选项”->“调试”->“常规”下取消选中“仅我的代码”复选框即可。

本示例旨在演示用法，运行速度可能不如等效的顺序 LINQ to Objects 查询快。有关加速的更多信息，请参见了解 PLINQ 中的加速。

示例

此示例演示如何将 try-catch 块放在执行查询以捕获任何引发的 System.AggregateException 的代码的周围。

' 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

// Paste into PLINQDataSample class.
static void PLINQExceptions_1()
{
    // Using the raw string array here. See PLINQ Data Sample.
    string[] customers = GetCustomersAsStrings().ToArray();


    // First, we must simulate some currupt input.
    customers[54] = "###";

    var parallelQuery = from cust in customers.AsParallel()
                        let fields = cust.Split(',')
                        where fields[3].StartsWith("C") //throw indexoutofrange
                        select new { city = fields[3], thread = Thread.CurrentThread.ManagedThreadId };
    try
    {
        // We use ForAll although it doesn't really improve performance
        // since all output is serialized through the Console.
        parallelQuery.ForAll(e => Console.WriteLine("City: {0}, Thread:{1}", e.city, e.thread));
    }

    // In this design, we stop query processing when the exception occurs.
    catch (AggregateException e)
    {
        foreach (var ex in e.InnerExceptions)
        {
            Console.WriteLine(ex.Message);
            if (ex is IndexOutOfRangeException)
                Console.WriteLine("The data source is corrupt. Query stopped.");
        }
    }
}

在此示例中，查询在引发异常后无法继续。 到应用程序代码捕获异常时，PLINQ 已经在所有线程上停止了查询。

下面的示例演示如何将 try-catch 块放在委托中，以便能够捕获异常并继续执行查询。

' 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

// Paste into PLINQDataSample class.
static void PLINQExceptions_2()
{

    var customers = GetCustomersAsStrings().ToArray();
    // Using the raw string array here.
    // First, we must simulate some currupt input
    customers[54] = "###";

    // 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.
    Func<string[], string, bool> isTrue = (f, c) =>
    {
        try
        {
            string s = f[3];
            return s.StartsWith(c);
        }
        catch (IndexOutOfRangeException e)
        {
            Console.WriteLine("Malformed cust: {0}", f);
            return false;
        }
    };

    // Using the raw string array here
    var parallelQuery = from cust in customers.AsParallel()
                        let fields = cust.Split(',')
                        where isTrue(fields, "C") //use a named delegate with a try-catch
                        select new { city = fields[3] };
    try
    {
        // We use ForAll although it doesn't really improve performance
        // since all output must be serialized through the Console.
        parallelQuery.ForAll(e => Console.WriteLine(e.city));
    }

    // IndexOutOfRangeException will not bubble up      
    // because we handle it where it is thrown.
    catch (AggregateException e)
    {
        foreach (var ex in e.InnerExceptions)
            Console.WriteLine(ex.Message);
    }
}

编译代码

  • 若要编译并运行这些示例，请将它们复制到“PLINQ 数据示例”示例中，并从 Main 中调用方法。

可靠编程

除非您知道如何处理异常因而不会损坏程序的状态，否则请不要捕获异常。

请参见

参考

ParallelEnumerable

概念

并行 LINQ (PLINQ)

修订记录

日期

修订记录

原因

2010 年 5 月

添加了有关用法与 加速的注释。

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