What is the simplest method of inter-process communication between 2 C# processes?

Anonymous pipes.

Use Asynchronous operations with BeginRead/BeginWrite and AsyncCallback.

I would suggest using the Windows Communication Foundation:

http://en.wikipedia.org/wiki/Windows_Communication_Foundation

You can pass objects back and forth, use a variety of different protocols. I would suggest using the binary tcp protocol.

Named pipes on WCF.

http://msdn.microsoft.com/en-us/library/ms733769.aspx

Following are the ways to do Inter Process Communication asynchronously:

1. Anonymous Pipes https://docs.microsoft.com/en-us/dotnet/api/system.io.pipes.anonymouspipeserverstream?view=net-5.0
2. Named Pipes https://www.codeproject.com/Articles/1179195/Full-Duplex-Asynchronous-Read-Write-with-Named-Pip
3. Sockets https://docs.microsoft.com/en-us/dotnet/framework/network-programming/asynchronous-server-socket-example
4. Shared memory

There's also MSMQ (Microsoft Message Queueing) which can operate across networks as well as on a local computer. Although there are better ways to communicate it's worth looking into: https://msdn.microsoft.com/en-us/library/ms711472(v=vs.85).aspx

The easiest solution in C# for inter-process communication when security is not a concern and given your constraints (two C# processes on the same machine) is the Remoting API. Now Remoting is a legacy technology (not the same as deprecated) and not encouraged for use in new projects, but it does work well and does not require a lot of pomp and circumstance to get working.

There is an excellent article on MSDN for using the class IpcChannel from the Remoting framework (credit to Greg Beech for the find here) for setting up a simple remoting server and client.

I Would suggest trying this approach first, and then try to port your code to WCF (Windows Communication Framework). Which has several advantages (better security, cross-platform), but is necessarily more complex. Luckily MSDN has a very good article for porting code from Remoting to WCF.

If you want to dive in right away with WCF there is a great tutorial here.

necromancing

it's just way more easy to make a shared file if possible!

//out
File.AppendAllText("sharedFile.txt", "payload text here");
// in
File.ReadAllText("sharedFile.txt");