Method 'set_Description' in type 'myAssembly.NetProduct' from assembly 'myAssembly' does not have an implementation - vb.net

Question

I have a DLL file created in VB6. It contains a class named Product and that contains the following simple code:
Option Explicit
Private sDescription As String
Public Property Get Description() As String
Description = sDescription
End Property
Public Property Let Description(Value As String)
sDescription = Value
End Property
I want to use this DLL in VB.NET, which is nothing more than registering the DLL on my system and including the DLL file in the references. Visual Studio automatically generates an interop DLL to consume the COM DLL. This interop DLL generates interfaces for all classes. In VB.NET I want to create a new class that implements the Product interface from the interop DLL. So I code:
Imports myAssembly
Public Class NetProduct
Implements myAssembly.Product
Public Property Description As String Implements _Product.Description
Get
Throw New NotImplementedException()
End Get
Set(value As String)
Throw New NotImplementedException()
End Set
End Property
End Class
The property is auto-generated because I implemented the Product interface. But here comes the problem because when I start using the NetProduct class I get an error telling me this:
Method 'set_Description' in type 'myProject.NetProduct' from
assembly 'myProject, Version=1.0.0.0, Culture=neutral,
PublicKeyToken=null' does not have an implementation.
The problem is that there is no method set_Description in the interface. When I view the definition of the Product interface it shows me the following:
Imports System.Runtime.InteropServices
Namespace myAssembly
<CoClass(GetType(ProductClass))> <Guid("49CE2F98-931C-441B-B322-9F39B6D6F212")>
Public Interface Product
Implements _Product
End Interface
End Namespace
The definition of the _Product interface is:
Imports System.Runtime.InteropServices
Namespace myAssembly
<Guid("49CE2F98-931C-441B-B322-9F39B6D6F212")> <TypeLibTypeAttribute(4304)>
Public Interface _Product <DispId(1745027072)>
Property Description As String
End Interface
End Namespace
When I use the interface myAssembly.Product directly to create a new object then everything works as you would expect. The property does not pose a problem there. But when I implement the interface in a .NET class the problem arises.
How do I solve this?
[update 1] After creating a method Set_Description I see the following error appear:
property 'Description' implicitly defines 'set_Description', which
conflicts with a member of the same name in class 'NetProduct'.
This must have something to do with my problem, although I don't know what it is. I already tried completing the property to make sure the Throw New NotImplementedException() wouldn't be in the way but that didn't make the error go away. My code builds just fine by the way. The error I gave earlier is a runtime error. Not a build error.
Private myDescription As String
Public Property Description As String Implements Product.Description
Get
Return myDescription
End Get
Set(value As String)
myDescription = value
End Set
End Property
[update 2] I have used JetBrains DotPeek to disassemble the interop.dll that Visual Studio generates. Disassembly is coded in C#. It contains 2 interfaces and 1 class for the single Product class from VB6. Here are all details.
I'll start with the Product class itself.
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
namespace myAssembly
{
[ClassInterface(0)]
[Guid("C54B96A8-1499-4B76-8508-0B732E551326")]
[TypeLibType(2)]
[ComImport]
public class ProductClass : _Product, Product
{
[MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
public extern ProductClass();
[DispId(1745027072)]
public virtual extern string Description { [DispId(1745027072), MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)] [return: MarshalAs(UnmanagedType.BStr)] get; [DispId(1745027072), MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)] [param: MarshalAs(UnmanagedType.BStr), In, Out] set; }
}
}
The ProductClass uses 2 interfaces. I don't understand why because one of those is just an implementation of the other. This is the Product interface.
using System.Runtime.InteropServices;
namespace myAssembly
{
[CoClass(typeof (ProductClass))]
[Guid("49CE2F98-931C-441B-B322-9F39B6D6F212")]
[ComImport]
public interface Product : _Product
{
}
}
And then we have the _Product interface. They even share the same Guid. It might have something to do with backwards compatibility.
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
namespace myAssembly
{
[Guid("49CE2F98-931C-441B-B322-9F39B6D6F212")]
[TypeLibType(4304)]
[ComImport]
public interface _Product
{
[DispId(1745027072)]
string Description { [DispId(1745027072), MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)] [return: MarshalAs(UnmanagedType.BStr)] get; [DispId(1745027072), MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)] [param: MarshalAs(UnmanagedType.BStr), In, Out] set; }
}
}
This is all I could find. Still no clue where the error for Set_Description comes from.
[Update 3] Example code
The code for the VB6 class is on top of this question. Nothing fancy there. The code for testing implementation in .NET is like this:
Imports myAssembly
Public Class NetProduct
Implements myAssembly.Product
Private myDescription As String
Public Property Description As String Implements Product.Description
Get
Return myDescription
End Get
Set(value As String)
myDescription = value
End Set
End Property
End Class
To test the NetProduct class I dropped a Button on a Form and create an instance of the class when the button is being clicked.
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click '<- Error happens here, so on loading the datatype!
Dim Product As New NetProduct 'Error does NOT happen here.
End Sub
The whole project compiles without errors. The project even runs without errors UNTIL you click the button. Probably because the NetProduct type is first loaded on that point.

Answer 1

I used a console app to do my test. Other than that, my VB.NET code is basically identical to yours in update 3. The VB.NET properties were auto-generated by VS with the stub Throw New NotImplementedException() after using the Implements statement :
Imports OurCOMDll
Class TestClass
Implements OurCOMDll.ClassInCOMDll
Dim msStringProperty As String = String.Empty
Public Property StringProperty As String Implements _ClassInCOMDll.StringProperty
Get
StringProperty= msStringProperty
End Get
Set(value As String)
msStringProperty = value
End Set
End Property
End Class
Module Module1
Sub Main()
Dim o As New OurCOMDll.ClassInCOMDll
o.StringProperty = "Hello World!"
Console.WriteLine(o.StringProperty) ' Outputs 'Hello World!' as expected
Console.ReadLine()
End Sub
End Module
Same is true for the VB6 code. The string property is implemented like yours.
Distinguishing factors so far:
VS 2019 vs. VS 2017
(Consuming) GUI vs. Console application
Different property names