C#到VB.Net:为什么转换为VB时无法编译？

Question

我有这个C#扩展方法,它将扩展Value类型为IList的任何字典.当我在VB.Net中编写等效代码时,我得到以下编译错误:

"扩展方法'添加'有一些永远无法满足的类型约束".

我觉得这实在令人费解的相同类型的限制可以在C#中得到满足.

所以我的问题是:为什么这在VB中不起作用？有没有办法让这些相同的类型约束在VB中工作？转换代码时出错了吗？我希望有人可以对此有所了解,因为我已经在这个问题上摸了一会儿.:)

(如果您感到好奇,扩展方法旨在简化在单个键下将多个值添加到字典中(例如一个客户下的多个订单).但这并不重要,我只关心我的令人费解的行为观察VB).

这是适用的C#版本:

/// <summary>
/// Adds the specified value to the multi value dictionary.
/// </summary>
/// <param name="key">The key of the element to add.</param>
/// <param name="value">The value of the element to add. The value can be null for reference types.</param>
public static void Add<KeyType, ListType, ValueType>(this Dictionary<KeyType, ListType> thisDictionary, 
                                                     KeyType key, ValueType value)
where ListType : IList<ValueType>, new()
{
    //if the dictionary doesn't contain the key, make a new list under the key
    if (!thisDictionary.ContainsKey(key))
    {
        thisDictionary.Add(key, new ListType());
    }

    //add the value to the list at the key index
    thisDictionary[key].Add(value);
}

这是不能编译的VB版本:

''' <summary> 
''' Adds the specified value to the multi value dictionary. 
''' </summary> 
''' <param name="key">The key of the element to add.</param> 
''' <param name="value">The value of the element to add. The value can be null for reference types.</param> 
<System.Runtime.CompilerServices.Extension()> _
Public Sub Add(Of KeyType, ListType As {IList(Of ValueType), New}, ValueType) _
              (ByVal thisDictionary As Dictionary(Of KeyType, ListType), ByVal key As KeyType, ByVal value As ValueType)
    'if the dictionary doesn't contain the key, make a new list under the key 
    If Not thisDictionary.ContainsKey(key) Then
        thisDictionary.Add(key, New ListType())
    End If

    'add the value to the list at the key index 
    thisDictionary(key).Add(value)
End Sub

Answer 1

只有<System.Runtime.CompilerServices.Extension()>存在时才会出现问题.VB编译器强制要求约束必须仅使用第一个参数进行验证.由于扩展方法(Dictionary(Of KeyType, ListType))的第一个参数依赖于第三个参数(ValueType)通过IList(Of TValue)约束,因此无法在VB中编译.