How does a .NET app redirect its own Standard Input stream?

Question

I am attempting bidirectional communication (IPC) between 2 C# Apps using only Standard Input streams. The parent app launches the child app with a Process, where RedirectStandardInput = true. So the parent process is able to send commands to the child process using childProc.StandardInput.WriteLine(). I am capturing these messages asynchronously using BeginRead() and EndRead() of the stream acquired with Console.OpenStandardInput(). Parent to child communication is working great. I am able to send and receive messages asynchronously.

But when the child attempts to write to the parent, using the same code, .NET throws this error:

StandardInput has not been redirected.

So, in simple terms, how does a .NET app redirect its own standard input stream? I will then redirect the standard input stream of my parent process, so the child can send messages to it.

My parent process is a WinForms C# App built with .NET 4.0 x86.

Edit: Here is the code of the IPC Server

internal class IPCServer {

    private Stream cmdStream;
    private byte[] cmdBuffer = new byte[4096];

    public IPCServer() {

        cmdStream = Console.OpenStandardInput(4096);

        GetNextCmd();
    }

    private void GetNextCmd() {

        // wait for next
        cmdStream.BeginRead(cmdBuffer, 0, cmdBuffer.Length, CmdRecived, null);
    }

    private void CmdRecived(IAsyncResult ar) {

        // input read asynchronously completed
        int bytesRead = 0;
        try {
            bytesRead = cmdStream.EndRead(ar);
        } catch (Exception) { }
        if (bytesRead > 0) {

            // accept cmd
            .........

            // wait for next
            GetNextCmd();
        }
    }


}

And here is the code of the IPC Client

internal class IPCClient {

    private Process appProc;

    public IPCClient(Process appProcess) {

        if (!appProcess.StartInfo.RedirectStandardInput) {
            //MessageBox.Show("IPCClient : StandardInput for process '" + appProcess.ProcessName + "' has not been 'redirected'!");
            return;
        }

        appProc = appProcess;

    }

    public void SendCmd(string cmd) {

        if (appProc != null) {
            appProc.StandardInput.WriteLine(cmd);
        }

    }

}

In the parent process:

// open child app
ProcessStartInfo info = new ProcessStartInfo(childProcPath, args);
info.WorkingDirectory = childProcDir;
info.LoadUserProfile = true;
info.UseShellExecute = false;
info.RedirectStandardError = true;
info.RedirectStandardOutput = true;
info.RedirectStandardInput = true;

childProc = Process.Start(info);

// connect to app for IPC
Client = new IPCClient();
Client.Init(childProc);

// recieve cmds from app
Server = new IPCServer();
Server.OnCmdRecieved = GotCmd;
Server.Init();

In the child process:

ownerProcess = ....

Server = new IPCServer();
Server.OnCmdRecieved = GotCmd;
Server.Init();

Client = new IPCClient();
Client.Init(ownerProcess);

Show some code instead of describing its principles in words. — Yeldar Kurmangaliyev
– Yeldar Kurmangaliyev, Commented Dec 3, 2015 at 11:17
I/O redirection is a very ho-hum way to do IPC, having to use text and battling the encoding limitations and giving up on normal console usage is very little fun. Use a named pipe instead. — Hans Passant
– Hans Passant, Commented Dec 3, 2015 at 13:45

Yeldar Kurmangaliyev · Accepted Answer · 2015-12-03 11:39:52Z

2

You can do it in a more convenient way. Just make your client application know nothing about input \ output redirection. Server should bother about redirection.

I have implemented a simplest client-server application using your technique, which demonstrates how it should be used.

Client

Client is a simplest application, which simply echoes the message back and closes when you enter exit:

static void Main(string[] args)
{
    string str;

    do
    {
        str = Console.ReadLine();

        Console.WriteLine("Executed: " + str);
    } while (str != "exit");
}

The client knows nothing about redirection, it simply works like a console application and reads \ writes into a console.

Server

Server runs client processes and reads \ writes in its streams.

Process p = new Process();

p.StartInfo.FileName = "ConsoleApplication1.exe";
p.StartInfo.UseShellExecute = false;
p.StartInfo.RedirectStandardInput = true;
p.StartInfo.RedirectStandardOutput = true;

p.Start();

p.StandardInput.WriteLine("a");
p.StandardInput.WriteLine("b");
p.StandardInput.WriteLine("c");
p.StandardInput.WriteLine("d");
p.StandardInput.WriteLine("exit");
p.StandardInput.WriteLine("f");

p.StandardInput.Close();

string res = p.StandardOutput.ReadToEnd();

After execution of this code, res contains the following text:

Executed: a
Executed: b
Executed: c
Executed: d
Executed: exit

answered Dec 3, 2015 at 11:39

Yeldar Kurmangaliyev

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1 Comment

Robin Rodricks Over a year ago

Not possible this way, because the client app will be communicating asynchronously with the server. Its not a request/response system.

Robin Rodricks · Accepted Answer · 2016-03-19 12:20:09Z

1

What if the child process simply uses BeginRead() and EndRead() on the child process' StandardOutput stream. That should work!

edited Mar 19, 2016 at 12:20

answered Dec 3, 2015 at 12:09

Robin Rodricks

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