Question

我可以在盒装内置vb.net类型上成功使用CStr()。但是如何用盒装自定义类型实现同样的目标呢？我得到了

Exception thrown:  'System.InvalidCastException' in Microsoft.VisualBasic.dll

代码：

' sample custom type
Structure Record
    Public Property Value As Integer
    Overloads Function TOSTRING() As String  ' capitalizaition intentional to reveal usage 
        Return ">>" & Value.ToString() & "<<"
    End Function
    Shared Operator &(left As String, right As Record) As String
        Return left & right.TOSTRING()
    End Operator
    Shared Widening Operator CType(left As Record) As String
        Return left.TOSTRING()
    End Operator
End Structure

' both use cases
Sub Main()
    ' demo with built-in type
    Dim i As Integer = 3
    Dim ib As Object = i ' boxed into Object
    Debug.Print("i:" & CStr(i))
    Debug.Print("ib:" & CStr(ib)) ' works OK

    ' demo with custom type
    Dim r As New Record With {.Value = 3}
    Dim rb As Object = r ' boxed into Object
    Debug.Print("r:" & CStr(r))
    Debug.Print("rb:" & CStr(rb)) ' Exception thrown: 
                                  ' 'System.InvalidCastException' in Microsoft.VisualBasic.dll
End Sub

Answer 1

您可以这样覆盖ToString：

Public Overrides Function ToString() As String
    'Put the logic here
    Return ">>" & Value.ToString() & "<<"
End

然后使用Convert.ToString()方法将对象转换为字符串。

示例：

Dim r As New Record With {.Value = 3}
Dim rb As Object = r ' boxed into Object
MessageBox.Show("rb:" & Convert.ToString(rb)) 'Shows >>3<<

Answer 2

最好的方法是覆盖用户类型中的.ToString。我相信Cstr()，CBool()还有更多，以便向后兼容。（VB6）

在类型中覆盖.ToString的另一个好处是，在调试中，当您查看类型的引用变量时，您拥有的对象信息比对象类型更有意义。

如何让CStr（）处理盒装自定义数据类型？

2 个答案: