為什么此功能與此代理具有不兼容的簽名？

Question

編譯器告訴我這些簽名是不兼容的，但它們對我來說似乎很好。 我想念什么嗎？ [VS 2008]

Public MustOverride Function OverridePickUpLocationFromDeliveryAddress(ByVal objDeliveryAddress As DeliveryLocationWS.DeliveryAddress, _
                                                                       ByRef lstProcessingMessages As List(Of String), _
                                                                       ByVal objProcessorHelperLists As ProcessorHelperLists) As Integer

Public Sub New()    
    Dim fncTest As Func(Of DeliveryLocationWS.DeliveryAddress, List(Of String), ProcessorHelperLists, Integer) = AddressOf OverridePickUpLocationFromDeliveryAddress
End Sub

Answer 1

您的函數通過引用（ lstProcessingMessages ）獲取參數，這對於您嘗試將其分配給的Func(Of T1, T2, T3, T4)是不可接受的。

據我所知， Func委托不支持“by ref”作為類型參數。

static void Main()
{
    Func<int, int> myDoItFunc1 = DoIt; // Doesn't work because of ref param
    Func<int, int> myDoItFunc2 = DoItFunc; // Does work
}

public static int DoItFunc(int i)
{
    return DoIt(ref i);
}

public static int DoIt(ref int i)
{
    return 0;
}

但是，為什么您lstProcessingMessages通過ref傳遞lstProcessingMessages ？ 這是參考類型。 您是否希望完全替換列表的引用，或者您只能清除並填充？

static void Main()
{
    var myList = new List<int>();

    AddToMyList(myList);

    // myList now contains 3 integers.

    ReplaceList(myList);

    // myList STILL contains 3 integers.

    ReplaceList(ref myList);

    // myList is now an entirely new list.
}

public static void AddToMyList(List<int> myList)
{
    myList.Add(1); // Works because you're calling "add"
    myList.Add(2); // on the *same instance* of my list
    myList.Add(3); // (object is a reference type, and List extends object)
}

public static void ReplaceList(List<int> myList)
{
    myList = new List<int>();
}

public static void ReplaceList(ref List<int> myList)
{
    myList = new List<int>();
}

Answer 2

Func不起作用，因為參數聲明為ByVal 。 但是，您可以聲明自己的委托類型：

Delegate Function PickUpLocationDelegate(ByVal objDeliveryAddress As DeliveryLocationWS.DeliveryAddress, _
  ByRef lstProcessingMessages As List(Of String), _
  ByVal objProcessorHelperLists As ProcessorHelperLists) As Integer

Public Sub New()
    Dim fncTest As PickUpLocationDelegate = AddressOf OverridePickUpLocationFromDeliveryAddress
End Sub