.NET Framework - Class boxed in an object
Asked By Nio
21-Nov-09 03:49 AM
Hello, I have a class implementing comparison operators and cast operators
(CType) .
This class called MyOpTest
Now look at this code:
Dim MyOp As New MyOpTest
Dim data As String = MyOp 'OK no error, MyOpTest
implements "Shared narrowing operator CType(byval a As MyOpTest) As String"
Dim obj As Object = New MyOpTest 'now instance is boxed into an
object type
Dim Result as Boolean = Cbool(obj < 2) 'OK no error MyOpTest implements
Dim data2 As String = obj 'This failed!! casting
exception from MyOpTest!! to String
So why when instance is boxed into object, casting operations are not called
and comparison operators are called ?
Is there any solution ? (a way to implicit unbox object type) ?
Thanks for your answers.
Otherwise
(1)
MyOpTest
(1)
Result
(1)
DirectCast
(1)
CType
(1)
MyOp
(1)
Cbool
(1)
Implements
(1)
Armin Zingler replied to Nio
Nio schrieb:
No, it does not work. Option Strict Off? Switch is it on, first. Otherwise
you can throw away all type awareness. it is a _narrowing_ operator, which means,
it could fail at runtime.
CBool is an explicit cast. The compiler trusts you.
Whereas "Dim data2 As String = obj" requires an implicit cast/conversion. Casting
from Object to String is not defined/allowed.
I do not think so.
--
Armin
Armin Zingler replied to Armin Zingler
Armin Zingler schrieb:
When I wrote the 2nd part of my answer, I was not aware of Option Strict Off, yet.
I misread it and assumed a compile time error but you mentioned an exception. Sorry,
my fault.
Well, with Option Strict On, you do not get an exception, so I think this changed
the situation. But, you can also not write
Dim data2 As String = CType(obj, String) 'failure
because it is the compiler that is responsible for making use of your own operator.
it is not used in this case because the expression type is Object, not MyOpTest.
So you would have to write
Dim data2 As String = CType(DirectCast(obj, myoptest), String)
--
Armin
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