Hi,
I've got a function that returns an object of type: system.web.ui.control - Now I've created a very simple class, derived from system.web.ui.control: - Public Class MyTest
-
Inherits System.Web.UI.Control
-
-
End Class
However, when I try to cast the result of the function into an instance of MyTest - it throws an 'Unable to cast' exception. I frankly can't understand why - the only thing I think it might be is that 'MyTest' class exists in a different namespace: Testframework. A GetType on MyTest reveals that it is of type: Testframework.MyTest - but I can't see why this causes casting to fail. FYI: I've tried the following ways to cast: -
Dim blat As MyTest
-
Dim voon As System.Web.UI.Control
-
-
voon = MyFunctionThatReturnsSystemWebUIControl()
-
-
'Method 1
-
blat = DirectCast(voon, Testframework.MyTest)
-
-
'Method 2
-
blat = CType(voon, Testframework.MyTest)
-
-
'Method 3
-
blat = voon
- I've also tried declaring 'blat' and 'voon' as New instances, prior to assignment - e.g. just omitting the function call, and seeing if I can assign a new object of type System.Web.UI.Control to a new object of type Testframework.MyTest.
Any help appericated,
Mat.
4  1803 
Hi,
I've got a function that returns an object of type: system.web.ui.control - Now I've created a very simple class, derived from system.web.ui.control:
- Public Class MyTest
-
Inherits System.Web.UI.Control
-
-
End Class
However, when I try to cast the result of the function into an instance of MyTest - it throws an 'Unable to cast' exception. I frankly can't understand why - the only thing I think it might be is that 'MyTest' class exists in a different namespace: Testframework. A GetType on MyTest reveals that it is of type: Testframework.MyTest - but I can't see why this causes casting to fail. FYI: I've tried the following ways to cast:
-
Dim blat As MyTest
-
Dim voon As System.Web.UI.Control
-
-
voon = MyFunctionThatReturnsSystemWebUIControl()
-
-
'Method 1
-
blat = DirectCast(voon, Testframework.MyTest)
-
-
'Method 2
-
blat = CType(voon, Testframework.MyTest)
-
-
'Method 3
-
blat = voon
- I've also tried declaring 'blat' and 'voon' as New instances, prior to assignment - e.g. just omitting the function call, and seeing if I can assign a new object of type System.Web.UI.Control to a new object of type Testframework.MyTest.
Any help appericated,
Mat.
An object of type MyTest is a System.Web.UI.Control but not all System.Web.UI.Controls are MyTest type objects.
An object of type MyTest is a System.Web.UI.Control but not all System.Web.UI.Controls are MyTest type objects.
I'm not sure I quite understand - I mean, I see what you're saying - I just can't see how a class that seems logically identical to System.Web.UI.Control (bar the name), cannot cast objects that are of type System.Web.UI.Control?
You are trying to cast in the wrong direction.
You can only cast "up" the heirarchy (to a base class).
Your class can be cast as a System.Web.UI.Control, but not the other way around.
To extend the previous example:
Even though a "Red Toyota Camry" is a car, not all cars are red. Nor are they all Toyota's, etc.
You can refer to your "Red Toyota Camry" as a car.
You could also refer to a "Blue Chevy Malibu" as a car.
But, even though you call your "Red Toyota Camry" a "car", you could not then go and call it a "Blue Chevy Malibu"
You are trying to cast in the wrong direction.
You can only cast "up" the heirarchy (to a base class).
Your class can be cast as a System.Web.UI.Control, but not the other way around.
To extend the previous example:
Even though a "Red Toyota Camry" is a car, not all cars are red. Nor are they all Toyota's, etc.
You can refer to your "Red Toyota Camry" as a car.
You could also refer to a "Blue Chevy Malibu" as a car.
But you would not refer to the "Red Toyota Camry" as a "Blue Chevy Malibu"
Aaaah. Yep, clear now. Many Thanks
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