为什么不能推断出二元运算符中的常见类型？

Question

我收到编译器警告“无法推断出常见类型;假设'对象'。”来自以下代码：

Dim occurrence As CacheableDocumentOccurrence = _
    If(DirectCast(IdentityMap.GetItem(id), CacheableDocumentOccurrence),
        Function() As CacheableDocumentOccurrence
            Dim x = New CacheableDocumentOccurrence()
            IdentityMap.Add(x)
            Return x
        End Function)

为什么？

Answer 1

如果If不返回CacheableDocumentOccurrence，则GetItem(id)语句会返回Nothing，但会返回Function（lambda表达式）！ 在这两种情况下，您都必须返回CacheableDocumentOccurrence。

Dim occurrence As CacheableDocumentOccurrence = DirectCast(IdentityMap.GetItem(id)
If occurrence  Is Nothing Then
    occurrence = New CacheableDocumentOccurrence()
    IdentityMap.Add(x)
End If 

请注意，您的函数不仅仅是某些代码的括号。它无论如何都不会被执行;相反，结果将是AddressOf函数。