如何：處理 PLINQ 查詢中的例外狀況

此主題中的第一個範例顯示如何處理當執行 PLINQ 查詢時可從它擲回的 System.AggregateException。 第二個範例顯示如何將 try-catch 區塊放在委派內，且盡可能地接近將擲回例外狀況的位置。 如此一來，您可以在發生例外狀況時立即攔截它們，並可能繼續執行查詢。 當系統允許例外狀況反昇至聯結的執行緒時，查詢可能就可以在引發例外狀況之後，繼續處理某些項目。

在某些 PLINQ 回復為循序執行且發生例外狀況的情況中，例外狀況可能會直接傳播，而不會包裝在 AggregateException 中。 此外，ThreadAbortException 一律會直接傳播。

範例 1

此範例會示範如何在執行查詢的程式碼周圍放置 try-catch 區塊，以攔截任何擲回的 System.AggregateException。

// 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 corrupt 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.");
        }
    }
}

' 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 corrupt 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

在此範例中，擲回例外狀況之後查詢無法繼續。 應用程式程式碼攔截到例外狀況時，PLINQ 已經停止所有執行緒上的查詢。

範例 2

下列範例會示範如何將 try-catch 區塊放在委派中，以使其可攔截例外狀況並繼續執行查詢。

// Paste into PLINQDataSample class.
static void PLINQExceptions_2()
{
    var customers = GetCustomersAsStrings().ToArray();
    // Using the raw string array here.
    // First, we must simulate some corrupt input
    customers[54] = "###";

    // Assume that in this app, we expect malformed data
    // occasionally and by design we just report it and continue.
    static bool IsTrue(string[] f, string c)
    {
        try
        {
            string s = f[3];
            return s.StartsWith(c);
        }
        catch (IndexOutOfRangeException)
        {
            Console.WriteLine($"Malformed cust: {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);
        }
    }
}

' Paste into PLINQDataSample class
Shared Sub PLINQExceptions_2()

    Dim customers() = GetCustomersAsStrings().ToArray()
    ' Using the raw string array here.
    ' First, we must simulate some corrupt 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

編譯程式碼

• 若要編譯並執行這些範例，請將它們複製到 [PLINQ 資料範] 範例中，並從 Main 呼叫方法。

穩固程式設計

除非您知道如何處理，否則請勿攔截例外狀況，這樣您才不會損毀您的程式狀態。

另請參閱

• ParallelEnumerable
• 平行 LINQ (PLINQ)