Can C# start a C# COM server as external process?

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Hello

I'm toying with the idea of using COM as inter-process communication between C# applications/modules. In the second step as a COM service.

BIG advantage: Calls to methods and callbacks (events) can directly checked by compiler at compile time.

Disadvantages: Old technology, not really build for C#

In the internet you can find a few samples of a C# implemented COM server. A runtime creation works like

C#

var obj = Activator.CreateInstance(Type.GetTypeFromProgID("Server.MyClass", true));
Console.WriteLine(obj.GetType().InvokeMember("SomeMethod", BindingFlags.InvokeMethod, null, obj, null));

So far, so good. The executable file can find COM via the registry and thus start it.

But I don't like method names as strings. This is very dangerous when methods are deleted or renamed. A better solution would be to use the IDL/TLB information directly by adding a reference to the server's interop assembly in the client. After adding, the compiler now knows the interfaces and classes, but the server is now loaded into the client's process like a inproc dll.

C#

MyClass obj = new MyClass();
Console.WriteLine(obj.SomeMethod());

This seems only to work, if the server is written in C++. Then you can refer to the generated interop assembly through tlbimp. But you can't create a additional interop assembly from a COM assemply. A tlbexp and a afterward tlbimp is not possible. Tlbimp says, that the tlb is already created from a COM assembly.

Two questions:
a) Is it technical possible to start a C# server out of process?
b) Is it (still) appropriate to use COM for this?

Your expertise and opinion is needed!

What I have tried:

Playing with sample projects..

Posted 6-Jun-24 21:45pm

Michael Sabo

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Richard Deeming 7-Jun-24 4:19am

If you're using .NET Framework, there's no need to use COM for IPC; there is built-in support for IPC channels to enable efficient communication between processes on the same machine:
System.Runtime.Remoting.Channels.Ipc Namespace | Microsoft Learn[^]

There are also classes for named and anonymous pipes, which still work in the latest version of .NET:
Pipe Operations in .NET - .NET | Microsoft Learn[^]

Or you could build something using gRPC[^].

Michael Sabo 7-Jun-24 8:16am

Regrettable that the classes/technology no longer exist in .NET Core.

Richard Deeming 7-Jun-24 8:17am

The IPC channels don't. But the named/anonymous pipes still exist, and gRPC supports .NET Core as well.

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Pete O'Hanlon · Answer 1 · 2024-06-07T01:49:00

You can - but that's going to depend on having registered the COM object as out-of-process. Suppose you have a COM class in bob.exe. Register it with regsvr32 /u /RegServer bob.exe. At that point, the COM classes in the assembly are registered to run out-of-process.

If you know the progid, you should be able to get a runtime instance of the object using:

C#

Type bobType = Type.GetTyprFromProgId(bobProgId, true);
object bobObject = Activator.CreateInstance(bobType);

Then, to call the methods you should be able to do something like this:

C#

string whoIsCodeProject = (string)bobObject.InvokeMember("WhatIsChrisReallyCalled", BindingFlags.InvokeMethod, null, bobObject, new object[] { 42 });

You can see that I'm calling a method and passing in a value of 42 because the COM method expects the magic H2G2 number.

Note, I've entered the code directly here from memory (to the technique), so there may be a minor error here or there.

Now, the question is, would I recommend doing this? Well no. There are so many other technologies that you could use. For strongly named connections, I like Protobuf and that would be my goto technology there. The only reason I would be using COM is if I were communicating with something that absolutely had to be COM.

Can C# start a C# COM server as external process?

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Can C# start a C# COM server as external process?

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