What is the purpose of maxNumberOfServerInstances in the NamedPipeServerStream class?

Question

Currently writing a Windows service in .NET and I'm using named pipes to have other processes communicate with my service. In the more complicated constructors of NamedPipeServerStream, there's a parameter with a descriptive name of maxNumberOfServerInstances. Awesome. But what does that mean?

MSDN's documentation is also helpful at explaining:

The maximum number of server instances that share the same name.

Okay. That still doesn't really tell me what this does for me, or how I utilize it. It would make sense to be if NamedPipeServerStream also accepted some delegate for "run this code when I receive a connection", so then each "Server instance" would then run that code. But that's not the case.

Answer 1

In practice it limits the number of NamedPipeServerStream instances you can create for the same pipe name. This will throw System.IO.IOException: All pipe instances are busy.:

var pipeStreamA = new NamedPipeServerStream("mypipe1", PipeDirection.InOut, 1);
var pipeStreamB = new NamedPipeServerStream("mypipe1", PipeDirection.InOut, 1);

If you change to -1 or 2, then it won't throw an exception. Also, it only respects the value on the first instance you create. If you specify 1 on the first call and 2 on the second, it will still throw an exception.

It's probably a good idea to set this to the maximum number of simultaneous servers that you expect to try to run, because as Hans mentioned, it may be using the number as a hint for the amount of resources to allocate.

Answer 2

It is an odd argument, you'll find a bit more info about it in the documentation for the underlying Windows API function (CreateNamedPipe). Pipes use a very precious resource for the pipe buffers, they are allocated from the non-paged kernel memory pool. I think this argument helps Windows optimize the usage of the pool. Exactly how that's done is hopelessly undocumented.

The perfect number for a single service that accepts multiple client connections is 1. You'd only increase it if you want to run multiple services that all do the same job. That's pretty rare.