Declaring and using COM interfaces in C#

[Peter Zwosta]

I have two C# projects:
- In project1 TestClass implements a COM interface ITest and builds a
class library (Register for COM interop = true)
- In project2 I create TestClass and want to typecast it to ITest to
call Init. The cast leads to an InvalidCastException.

Does anyone know what I have to do to get this to work?
I also don't understand why the explicit implementation of Init in
TestClass, "public void ITest.Init(){}" leads to the compiler error
"The modifier public is not valid for this item"

Project1:
--------
file ITest.cs:
....
[ComImport]
[Guid("89BCB740-6119-101A-BCB7-00DD010655AF")]
public interface ITest
{ void Init(); }

file Main.cs:
....
[Guid("520D5B3E-96A7-4b9a-B165-439B4E1FCC92")]
[ClassInterface(ClassInterfaceType.None)]
public class TestClass : ITest
{ public void Init(){} }

Project2:
---------
file ITest.cs: see Project 1

file Main.cs
....
string classKey = "{520D5B3E-96A7-4b9a-B165-439B4E1FCC92}";
Type objType = Type.GetTypeFromCLSID(new Guid(classKey));
ITest itest = (ITest) Activator.CreateInstance(objType);
->> System.InvalidCastException

I have tried several attributes for ClassInterfaceType and also for
the ComInterfaceType of ITest but with always the same result.

Any help would be appreciated.
Thank you
Peter

[Mattias Sjögren]

Peter,

I have two C# projects:
- In project1 TestClass implements a COM interface ITest and builds a
class library (Register for COM interop = true)
- In project2 I create TestClass and want to typecast it to ITest to
call Init. The cast leads to an InvalidCastException.
For communication between .NET objects, type identity is scoped by the
assembly that defines the type. So ITest in your first assembly is
different from ITest in the second, even if they happen to be defined
identically and exposed to COM with a single IID.

Why are you using COM here at all?

I also don't understand why the explicit implementation of Init in
TestClass, "public void ITest.Init(){}" leads to the compiler error
"The modifier public is not valid for this item"

Because explicitly implemented interface members are always private,
and you're not allowed to apply any access modifier.


Mattias

--
Mattias Sjögren [MVP] mattias @ mvps.org
http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
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