Behaviour of CType

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Bert Tuijl (thuis) - 29 Apr 2006 23:54 GMT

Can someone explain this new-to-visualbasic programmer why my statement
throws an exception?

I have defined a class:
Class myDataSet
Inherits System.Data.DataSet
Public Sub fun(ByVal someparm As String)
dosomething()
End Sub
End Class

And I created a webservice function that returns a System.Data.DataSet
I try to stuff this dataset into my own dataset-type
dim myDataSet mds as myDataSet = new myDataSet
'mds = dswebsvc(arg) ' vbcompiler does not allow this but rather:
mds = CType( dswebsvc(arg), myDataSet) ' compiler allows this

But at runtime, this statement throws an exception with the message:
Unable to cast object of type 'System.Data.DataSet' to type 'myDataSet'

To overcome this problem I tried to overload the CType() function in the
myDataSet class but that is not allowed since apparently the compiler
automatically generates this overload!

What can I do to get rid of this nasty exception?

Bert

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Steve Gerrard - 30 Apr 2006 00:40 GMT

> Can someone explain this new-to-visualbasic programmer why my statement throws
> an exception?
[quoted text clipped - 10 lines]
> But at runtime, this statement throws an exception with the message:
> Unable to cast object of type 'System.Data.DataSet' to type 'myDataSet'

First, you should ask your .Net questions in a dotnet group, not a VB6 group.

Second, since myDataSet inherits from DataSet, that means that:
All instance of myDataSet are also an instance of DataSet, but
Not all instances of DataSet are also an instance of myDataSet.

You can't "get rid of this nasty exception"; you actually can't cast a generic
DataSet to myDataSet, since it isn't one!

Perhaps you meant for your web service function dswebsvc() to return an instance
of myDataSet?

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VB Forum / General 2 / April 2006

Behaviour of CType