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C# COM object for VB6 that needs a Variant property - Property Set is failing
I'm having to support an ancient VB6 application. Currently, I'm writing a COM object class for it. Everything works except for assigning a value to a property. This particular property needs to behave like a VB6 variant data type when used in VB6 code.
I have the simplest little class in C#
I have the simplest little class in C#
[ComVisible(true)]
[Guid("1EB35E9B-60EE-4A26-BAEB-6A2D7AC32FF0")]
[ClassInterface(ClassInterfaceType.AutoDual)]
public class FooThingy
{
private object _value;
public object Value
{
get
{
return _value;
}
set
{
_value = value;
}
}
public FooThingy()
{
}
}
Public Sub Main()
1 Dim foo As FooThingy
2 Set foo = New FooThingy
3 If (foo.value <> "hello") Then
4 foo.value = "Testing"
5 End If
End Sub
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That produces the same error but at compile time instead of runtime. You can't use the set statement to assign primitives.
Put a breakpoint on line 3 and check the value of foo.value using shift -f9
see if it has a valid value
see if it has a valid value
There's no reason why it cannot be empty. This is a Variant data type. Empty is absolutely a valid value and there is other VB6 code that performs checks for empty variant types. Blank is not an acceptable substitute for empty. "" does not equal NULL.
This isn't an object problem. It's a COM marshaling problem.
This isn't an object problem. It's a COM marshaling problem.
Update
This works:
This works:
[MarshalAs(UnmanagedType.Struct)]
public object Value;
[MarshalAs(UnmanagedType.Struct)]
private object _value;
[DispId(0)]
public object Value
{
[return: MarshalAs(UnmanagedType.Struct)]
get
{
return _value;
}
[param: MarshalAs(UnmanagedType.Struct)]
set
{
_value = value;
}
}
See if this article helps. It explains how to use managed code from unmanaged code
https://www.codeproject.com/Articles/154144/Using-Unmanaged-VB6-Code-in-NET
https://www.codeproject.com/Articles/154144/Using-Unmanaged-VB6-Code-in-NET
Sorry. That doesn't address my issue of how to make the VB6 code in my original question function. It does give me an idea though. I'll need to check a few things and this could take some time. I'll get back to you.
So this got me most of the way there. It turns out that if I create the interface as a VB6 class and compile it as an ActiveX DLL I can use that interface to create my C# class. The only downside is there's no constructor but in this case I don't need one since the class is created entirely in C# and never instantiated directly in VB6 code.
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