Why does my async NamedPipe freeze when enabling writer.AutoFlush?

Below is the explanation of a common pitfall when using duplex named pipes with StreamReader and StreamWriter in asynchronous mode.

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The Problem

In your code you’re creating a single stream (the NamedPipe stream) and then wrapping it twice—once with a StreamReader and once with a StreamWriter. When you set

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writer.AutoFlush = true;

the StreamWriter immediately flushes its internal buffers on each write. In the duplex (reading and writing on the same pipe) asynchronous scenario, this can cause a deadlock situation because the underlying NamedPipe stream isn’t designed to support two independent wrappers simultaneously performing asynchronous operations.

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The Mistake

Using separate reader and writer wrappers on the same NamedPipe stream with AutoFlush enabled
This practice can cause each side (the reader and writer) to block waiting on the other. The AutoFlush setter itself isn’t doing any heavy lifting, but when it causes writes to be flushed (and a flush on a duplex pipe is not a “fire‐and‐forget” operation) the internal coordination between the two wrappers leads to a deadlock.

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The Solution

There are a couple of ways to resolve this:

1. Disable AutoFlush and flush manually.
   Remove the writer.AutoFlush = true setting and after each write call, explicitly call await writer.FlushAsync(); to flush the writer when you know that the other end is ready to read.

2. Avoid using two separate wrappers on the same stream.
   While duplex pipes are supported (using PipeDirection.InOut), you need to be careful with using StreamReader and StreamWriter concurrently. You may consider either:

   • Creating a single wrapper class that handles the read and write coordination.
   • Using the stream directly for one side of the operation.

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Adjusted Code Example

Here’s one way to modify your code by removing AutoFlush and calling FlushAsync explicitly:

Server code:

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var server = new NamedPipeServerStream(
    "MyPipe", 
    PipeDirection.InOut, 
    NamedPipeServerStream.MaxAllowedServerInstances, 
    PipeTransmissionMode.Byte, 
    PipeOptions.Asynchronous);

Console.WriteLine("Waiting for client connection...");
await server.WaitForConnectionAsync();
Console.WriteLine("Client connected.");

using var reader = new StreamReader(server, Encoding.UTF8);
await using var writer = new StreamWriter(server, Encoding.UTF8);

// NOTE: Removed writer.AutoFlush = true;

while (true)
{
    var message = await reader.ReadLineAsync();
    Console.WriteLine($"Received: {message}");
    await writer.WriteLineAsync($"Echo: {message}");
    await writer.FlushAsync(); // Explicit flush instead of AutoFlush
}

Client code:

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await using var client = new NamedPipeClientStream(
    ".", 
    "MyPipe", 
    PipeDirection.InOut, 
    PipeOptions.Asynchronous);

Console.WriteLine("Connecting to server...");
await client.ConnectAsync();
Console.WriteLine("Connected to server.");

using var reader = new StreamReader(client, Encoding.UTF8);
await using var writer = new StreamWriter(client, Encoding.UTF8);

// NOTE: Removed writer.AutoFlush = true;
Console.WriteLine("Writer initialized.");

await writer.WriteLineAsync("Hello, server!");
await writer.FlushAsync(); // Ensure the data is sent
Console.WriteLine("Message sent.");

var response = await reader.ReadLineAsync();
Console.WriteLine($"Server response: {response}");

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Recap

• Cause: Wrapping the same duplex pipe stream with separate StreamReader and StreamWriter instances (and enabling AutoFlush) can cause deadlock.
• Fix: Remove AutoFlush and use manual flushes via FlushAsync or refactor your design so that you are not wrapping the pipe stream twice.

This should resolve the freeze issue you encountered.