Problem: InvalidCastException When Casting Object As Generic Type

  • Thread starter Thread starter Guest
  • Start date Start date
G

Guest

I have written a Generic method that takes an object, a type, T, and returns
a System.Nullable (Of T) depending on whether or not the object IsDBNull.

It was working fine until I tried to pass a string as the object and my own
custom Structure as the type, T. Then I received an InvalidCastException.
This is despite the fact that a CType from a String to my Structure works
fine. I assume I probably need to implement some interface that
VisualBasic.CompilerServices.Conversions.ToGenericParameter can call. But I
don't know what that interface might be.

I have narrowed everything down here to reproduce the error I am
experiencing. Please note that this code is simplified and doesn't do the
DBNull checking or the conversion to a System.Nullable(Of T). If the code
below can be solved, I can work on fixing the more complex code I actually
have.

Here is the code to reproduce the error. Just drop into a console application:

' no error checking right now, just plain jane simple
Public Structure FancyInt

Public Value As Integer

Public Overloads Shared Narrowing Operator CType(ByVal source As String) As
FancyInt
Dim fi As FancyInt
fi.Value = CInt(source)
Return fi
End Operator

Public Overloads Shared Widening Operator CType(ByVal source As FancyInt)
As String
Return source.ToString
End Operator

Public Overrides Function ToString() As String
Return Value.ToString
End Function

Sub New(ByVal s As String)
Value = CInt(s)
End Sub

End Structure

Module Module1

Sub Main()
Console.WriteLine(New FancyInt("061091977")) 'good
Console.WriteLine(CType("061091977", Integer)) 'good
Console.WriteLine(CType("061091977", FancyInt)) 'good
Console.WriteLine(FancyConvert(Of Integer)("061091977"))' good
Console.WriteLine(FancyConvert(Of FancyInt)("061091977")) 'bad line 18
End Sub

Function FancyConvert(Of T As Structure)(ByVal o As Object) As T
Return CType(o, T) ' line 21
End Function

End Module

Here is the console output of the above Sub Main:

C:\>Test.exe

61091977
61091977
61091977
61091977
System.InvalidCastException: Specified cast is not valid.
at
Microsoft.VisualBasic.CompilerServices.Conversions.ToGenericParameter[T](Object Value)
at Test.Module1.FancyConvert[T](Object o) in
C:\TestProject\Module1.vb:line 21
at Test.Module1.Main() in C:\TestProject\Module1.vb:line 18

C:\>

So, what do I need to do to FancyInt so it is successfully cast just like
the Integer is successfully cast?

FYI - I also already tried implementing IConvertible on FancyInt as well as
tried implementing a TypeConverter along with applyng the TypeConverter
attribute on FancyInt, but no luck with either.

I must be missing something simple.

Any help?
 
BJS said:
I have written a Generic method that takes an object, a type, T, and returns
a System.Nullable (Of T) depending on whether or not the object IsDBNull.

It was working fine until I tried to pass a string as the object and my own
custom Structure as the type, T. Then I received an InvalidCastException.
This is despite the fact that a CType from a String to my Structure works
fine. I assume I probably need to implement some interface that
VisualBasic.CompilerServices.Conversions.ToGenericParameter can call. But I
don't know what that interface might be.

I have narrowed everything down here to reproduce the error I am
experiencing. Please note that this code is simplified and doesn't do the
DBNull checking or the conversion to a System.Nullable(Of T). If the code
below can be solved, I can work on fixing the more complex code I actually
have.

Here is the code to reproduce the error. Just drop into a console application:

' no error checking right now, just plain jane simple
Public Structure FancyInt

Public Value As Integer

Public Overloads Shared Narrowing Operator CType(ByVal source As String) As
FancyInt
Dim fi As FancyInt
fi.Value = CInt(source)
Return fi
End Operator

Public Overloads Shared Widening Operator CType(ByVal source As FancyInt)
As String
Return source.ToString
End Operator

Public Overrides Function ToString() As String
Return Value.ToString
End Function

Sub New(ByVal s As String)
Value = CInt(s)
End Sub

End Structure

Module Module1

Sub Main()
Console.WriteLine(New FancyInt("061091977")) 'good
Console.WriteLine(CType("061091977", Integer)) 'good
Console.WriteLine(CType("061091977", FancyInt)) 'good
Console.WriteLine(FancyConvert(Of Integer)("061091977"))' good
Console.WriteLine(FancyConvert(Of FancyInt)("061091977")) 'bad line 18
End Sub

Function FancyConvert(Of T As Structure)(ByVal o As Object) As T
Return CType(o, T) ' line 21
End Function

End Module

Here is the console output of the above Sub Main:

C:\>Test.exe

61091977
61091977
61091977
61091977
System.InvalidCastException: Specified cast is not valid.
at
Microsoft.VisualBasic.CompilerServices.Conversions.ToGenericParameter[T](Object Value)
at Test.Module1.FancyConvert[T](Object o) in
C:\TestProject\Module1.vb:line 21
at Test.Module1.Main() in C:\TestProject\Module1.vb:line 18

C:\>

So, what do I need to do to FancyInt so it is successfully cast just like
the Integer is successfully cast?

FYI - I also already tried implementing IConvertible on FancyInt as well as
tried implementing a TypeConverter along with applyng the TypeConverter
attribute on FancyInt, but no luck with either.

I must be missing something simple.

Any help?
Click to expand...

Since your FancyConvert method uses Object as the parameter there is no
conversion routine from type Object to type FancyInt. And from what I
know, you aren't allowed to define a CType to convert from type Object.
You would get the same failure using CType directly as well.

Dim s As Object = "061091977" ' use type Object
Console.WriteLine(CType(s, FancyInt)) ' fails here
 
Back
Top