從函數返回一個通用函數類型的值

Question

我遇到了返回類型「T」值的麻煩。這是更容易顯示，而不是解釋所以這裏是方法：

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 

任何想法或建議？

謝謝。

最後的回報是我遇到問題的地方。它說「的nodeValue不能被轉換爲T類型」

我曾嘗試下面的場景，我得到的錯誤：「整型的值不能coverted鍵入T」：

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

Answer 1

如果存儲作爲一個對象之前的價值，它的工作原理：

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