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For－Each问题
Question:
我从DictionaryBase中继承了一个类，但For－each就出问题了，错误信息是"Specified cast
is not valid"， 请问是怎么回事？
is not valid"， 请问是怎么回事？
Public Class Customer
Private m_CustName As String
Public ReadOnly Property CustName()
Get
Return m_CustName
End Get
End Property
Public Sub New(ByVal NewName As String)
m_CustName = NewName
End Sub
End Class
Public Class Customers
Inherits DictionaryBase
Public Sub Add(ByVal NewEntry As Customer, ByVal NewKey As String)
Dictionary.Add(NewKey, NewEntry)
End Sub
Default Public ReadOnly Property Item(ByVal Key As String) As Customer
Get
Return CType(Dictionary.Item(Key), Customer)
End Get
End Property
End Class
Module Module1
Sub Main()
Dim colCust As New Customers()
Dim cust As Customer
colCust.Add(New Customer("Joe"), "Joe")
colCust.Add(New Customer("Ann"), "Ann")
Try
'this does work!
Console.WriteLine(colCust("Joe").CustName)
'this does not work!
For Each cust In colCust
Console.WriteLine(cust.CustName)
Next
Catch ex As Exception
Console.WriteLine(ex.ToString)
End Try
Console.ReadLine()
End Sub
End Module
|
Answer:
上面的代码肯定会导致invalid cast异常。因为对于DictoinaryBase类来说，你每次都会从For Each语句提取DictionaryEntry类。该异常就是说你不能将一个DictionaryEntry转换成Customer类。所以可以试试下面的代码：
Dim de As DictionaryEntry For Each de In colCust cust = CType(de.Value, Customer) Console.WriteLine(cust.CustName) Next |