从泛型函数类型的函数返回值

Question

我无法返回“T”类型的值。展示比解释容易，方法如下：

Protected Function GetElementValue(Of T)(ByVal nodeName As String, Optional missingIfNotExists As Boolean = True,
                                                        Optional missingIfEmpty As Boolean = True,
                                                        Optional ByRef defaultVal As T = Nothing,
                                                        Optional maxLength As Integer = Nothing) As T

    'Set up the node to get the value from
    Dim node = xmlRoot.SelectSingleNode(nodeName)

    Select Case True

        Case IsNothing(node)                'If the node is missing from the xml document

            'Add the node to the misssing elements array if missingIfNotExists = True
            If missingIfNotExists Then missingElements.Add(nodeName)

            'Return the default value
            Return defaultVal

        Case node.InnerText.Trim.Length = 0 'The node exists in the xml document but has no value

            'Add it to missing elements if missingIfEmpty = True
            If missingIfEmpty Then missingElements.Add(nodeName)

            'If there is a default value passed in, return that value
            Return defaultVal

        Case Else                           'The node exists and contains data

    End Select

    'The element exists and contains data
    Dim nodeValue = node.InnerText.Trim

    'If a size constraint was passed in, ensure the element data is not too long. Shorten the string if it is
    If Not IsNothing(maxLength) AndAlso nodeValue.Length > maxLength Then nodeValue = nodeValue.Substring(0, maxLength)

    Return CType(nodeValue, T)

End Function

有什么想法或建议吗？

谢谢。

最后一次返回是我遇到问题的地方。它说“nodeValue 不能转换为 T 类型”

我尝试了以下场景，但出现错误：“整数类型的值不能转换为 T 类型”：

Select Case defaultVal.GetType()
    Case GetType(Integer)
        Return CType(nodeValue, Integer)
End Select

Answer 1

如果你之前将值存储为对象，它可以工作：

Sub Main()
    Dim myInt = Foo(Of Integer)("123")
End Sub

Function Foo(Of T)(input As String) As T
    Dim bar As Object = input
    Return CType(bar, T)
End Function