I took this example directly out of the book "Programmin g MS Visual
Basic.net" - they say this code works:
Class Person
Public FirstName As String
Public LastName As String
End Class
Class Employee
Inherits Person
Public Salary As Integer
End Class
Private Sub Page_Load(ByVal sender As Object, ByVal e As
System.EventArg s) Handles MyBase.Load
Dim p As New Person
p.FirstName = "Ralph"
p.LastName = "X"
Dim emp As Employee = DirectCast(p, Employee) ' Exception When This
Line Executes
emp.Salary = 1000
End Sub
When i try to cast the base to the inherited class i get a
"System.Invalid CastException: Specified cast is not valid."
The book says specifically that this code should execute fine so i think i
am missing something small - any ideas?
Thanks in advance,
J. Shane Kunkle jk*****@vt.edu 3 855
"J. Shane Kunkle" <sh***@caudillw eb.com> wrote in message
news:en******** ******@TK2MSFTN GP12.phx.gbl... I took this example directly out of the book "Programmin g MS Visual Basic.net" - they say this code works:
Class Person Public FirstName As String Public LastName As String End Class
Class Employee Inherits Person
Public Salary As Integer End Class
Private Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArg s) Handles MyBase.Load Dim p As New Person p.FirstName = "Ralph" p.LastName = "X"
Dim emp As Employee = DirectCast(p, Employee) ' Exception When This Line Executes emp.Salary = 1000 End Sub
When i try to cast the base to the inherited class i get a "System.Invalid CastException: Specified cast is not valid."
The book says specifically that this code should execute fine so i think i am missing something small - any ideas?
You are not missing anything. This code should not execute.
If you changed the line
DIm p as New Person
to
Dim p as Person = new Employee
it will work.
The line
Dim p as New Person
creates a non-Employee Person. You cannot downcast to Employee unless the
object really _is_ an Employee. You can always treat an Employee as a
Person, but not vice versa.
David
J. Shane Kunkle,
Either you typed the sample incorrectly or the book is flat out wrong.
I don't have the book, so I don't know what the book stated.
DirectCast requires the actual object to either Inherit from or Implement
the type that is listed.
Your p variable is instantiated as a Person object, Seeing as Employee
inherits from Person, it cannot be an Employee object, hence the
InvalidCastExce ption.
To get your sample to work, try: Dim p As Person = New Employee p.FirstName = "Ralph" p.LastName = "X"
Dim emp As Employee = DirectCast(p, Employee)
Notice that we are initializing the p variable with an Employee object,
seeing as p now contains an Employee object, the DirectCast will work.
Hope this helps
Jay
"J. Shane Kunkle" <sh***@caudillw eb.com> wrote in message
news:en******** ******@TK2MSFTN GP12.phx.gbl...I took this example directly out of the book "Programmin g MS Visual Basic.net" - they say this code works:
Class Person Public FirstName As String Public LastName As String End Class
Class Employee Inherits Person
Public Salary As Integer End Class
Private Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArg s) Handles MyBase.Load Dim p As New Person p.FirstName = "Ralph" p.LastName = "X"
Dim emp As Employee = DirectCast(p, Employee) ' Exception When This Line Executes emp.Salary = 1000 End Sub
When i try to cast the base to the inherited class i get a "System.Invalid CastException: Specified cast is not valid."
The book says specifically that this code should execute fine so i think i am missing something small - any ideas?
Thanks in advance, J. Shane Kunkle jk*****@vt.edu
DirectCast comes in handy when you are looping through a group of objects
that all inherit from a common type:
dim e as Person = New employee
....
dim c as Person = New customer
....
....Put employees and customers in a Collection
Dim p as Person
For Each p in PersonGroupColl ection
Select Case p.gettype.name
case "Person"
...do this
case "Customer"
...do that
Next
"J. Shane Kunkle" wrote: I took this example directly out of the book "Programmin g MS Visual Basic.net" - they say this code works:
Class Person Public FirstName As String Public LastName As String End Class
Class Employee Inherits Person
Public Salary As Integer End Class
Private Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArg s) Handles MyBase.Load Dim p As New Person p.FirstName = "Ralph" p.LastName = "X"
Dim emp As Employee = DirectCast(p, Employee) ' Exception When This Line Executes emp.Salary = 1000 End Sub
When i try to cast the base to the inherited class i get a "System.Invalid CastException: Specified cast is not valid."
The book says specifically that this code should execute fine so i think i am missing something small - any ideas?
Thanks in advance, J. Shane Kunkle jk*****@vt.edu This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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