All instances busy exception on named pipe creation

Question

I have a windows service which is communicating with a gui application via named pipes. Therefor i have a thread running waiting for the app to connect which is running fine if i do it once. But if the thread is creating a new instance of the named pipe stream server the already established connection breaks down and i get the all instances busy exception. The code fragment where the exception is thrown is this:

class PipeStreamWriter : TextWriter
{

    static NamedPipeServerStream _output = null;
    static StreamWriter _writer = null;
    static Thread myThread = null;

        public PipeStreamWriter()
        {
            if (myThread == null)
            {
                ThreadStart newThread = new ThreadStart(delegate{WaitForPipeClient();});
                myThread = new Thread(newThread);
                myThread.Start();
            }
        }

        public static void WaitForPipeClient()
        {
            Thread.Sleep(25000);
            while (true)
            {
                NamedPipeServerStream ps = new NamedPipeServerStream("mytestp");
                ps.WaitForConnection();
                _output = ps;
                _writer = new StreamWriter(_output);

            }
        }

The exception is thrown when creating the new pipe server stream NamedPipeServerStream ps = new NamedPipeServerStream("mytestp") the second time.

EDIT:

I found the answer and it works when the max number of server instances is specified NamedPipeServerStream ps = new NamedPipeServerStream("mytestp",PipeDirection.Out,10);

The default value for this seems to be -1. Which leads to another but not that important question: Someone knows why it is -1 and not 1 when it behaves like beeing 1?

Answer 1

There are two overloads of the NamedPipeServerStream constructor that assign a default to the maxNumberOfServerInstances variable, namely:

public NamedPipeServerStream(String pipeName)

and

public NamedPipeServerStream(String pipeName, PipeDirection direction)

Looking at the reference source proves that this default is 1 and not -1. This explains the behavior you observed.

Possible solutions are:

1. use a constructor that allows you to specify the limit, and pass a value larger than 1

2. same as 1, and use the built-in constant NamedPipeServerStream.MaxAllowedServerInstances to ask for the maximum number of handles the operating system will be able to allocate.