What is it?

The Component Object Model (COM) API that underlines .NET and the Windows Runtime supports the concept of Out Of Process (OOP) Servers. This allows for using objects that are in a different process (or even a different machine) as though they were in the local process. This library adds APIs to make the process of creating the "server" in .NET much easier.

Note: COM and Windows Runtime are Windows only.

Why?

Cross Language

Because this uses COM/WinRT for the communication any language that can use COM can use this library. As an example of this, the sample includes a simple C++ console application that talks to the .NET/C# server.

Complex Types

Most RPC/ICP systems are just sending messages between the two processes. At most they can serialize an object graph. COM allows for more complicated objects, where the returned types can have methods, events, and properties. If you could do it with a local object, you can do it with a remote object. The samples include using most of these abilities, including having the remote process load a file as a stream and having the local process use the stream without having to send the whole file across.

Built in Support for Types

Many of the types you are used to using are supported out of the box like collection types, map types, streams, etc. This allows you to not have to worry about how the IPC works.

Security

COM provides various ways to secure usage and creation of objects. For more details, see Security in COM.

Usage

Currently to create an Out-of-Process server requires the C++/WinRT tooling (though no actual C++ code) and a "contract" project. These two limitations will be removed in a future version of the library.

Contract Project

The contract project is a C# project that contains the interfaces of the remote objects. Output is a WinMD that is referenced by the other projects. The interfaces have some rules:

1. The interface must have a GUID assigned using the Windows.Foundation.Metadata.GuidAttribute attribute, not the System.Runtime.InteropServices.GuidAttribute attribute.

2. Asynchronous methods must use the WinRT types (IAsyncAction, IAsyncActionWithProgress<TProgress>, IAsyncOperation<TResult>, IAsyncOperationWithProgress<TResult, TProgress>) instead of Task and Task<T>.

3. Event delegates must be either TypedEventHandler<TSender, TResult> or EventHandler<TResult> instead of EventHandler and EventHandler<T>.

4. Types in method parameters, type parameters, and return types must be:

   • A blittable type.
   • An interface that has a .NET/WinRT Mapping.
   • A WinRT type.
   • Another interface in the project.

5. Methods, properties, and events are all supported.

Metadata Project

The metadata project is a C++/WinRT project that uses MIDL 3.0 to create proxy types in a WinMD that can be referenced by the client of the OOP Server. No actual C++ code is needed, only the IDL.

The IDL is very simple, only needing runtimeclasses that implement the interface from the contract project. Unlike in C#, in MIDL 3.0 the type automatically has the members from the interface so they do not need to be listed again. Importantly the runtimeclass must have an empty constructor, otherwise the proxy type cannot be created.

❗Important: Because of the mix of SDK Style and C++/WinRT, nuget restore is needed to restore for C++/WinRT. In addition <RestoreProjectStyle>Packages.config</RestoreProjectStyle> is needed in the C++ project file.

Server Project

The server project is the only project that references Shmuelie.WinRTServer. It will contain implementations of the interfaces from the contract and when run should register them with an instance of COMServer for COM activation and WinRtServer for WinRT activation. The implementations must have a GUID using the System.Runtime.InteropServices.GuidAttribute attribute.

Because the interfaces must use the WinRT asynchronous types instead of the .NET ones, the implementation will likely need to use AsyncInfo to help adapt between the two systems.

Client Project

A client can be both full trust applications (Win32, WPF, WinForms, etc) or a UWP app.

A UWP client cannot use WinRT activation and must use COM style activation. The UWP sample app shows how to do this. To understand the details behind it, see this blog post.

A full trust client can use WinRT activation, which allows you to create the remote instances simply by new SomeType(), like you would for any other type. The sample WPF application shows this in action (using with WinForms would be similar).

Sample

To help understand usage and show what can be done samples can be found under the tests folder. The sample has:

• .NET 8 Server
• UWP .NET Client App
• C++/WinRT Console Client App
• WPF .NET Framework Client App
• WPF .NET 8 Client App

Note: If Visual Studio fails to build the Metadata project restarting Visual Studio should fix the problem.

Troubleshooting

If you are having issues, check on these things:

• Make sure the client app includes the WinMDs (Interface and Metadata)
• Make sure that the interface project uses the Windows.Foundation.Metadata.GuidAttribute attribute, not the System.Runtime.InteropServices.GuidAttribute attribute.
• Make sure that the server project uses the System.Runtime.InteropServices.GuidAttribute attribute, not the Windows.Foundation.Metadata.GuidAttribute attribute.