Will Socket.SendAsync send everything in 1 call?

[xqrzd]

Socket.SendAsync() returns an int, does this mean we have to loop in case it doesn't send everything in one call, or is it guaranteed to send the entire buffer?

Is this safe?

ReadOnlyMemory<byte> buffer = ...
await socket.SendAsync(buffer);

or is this required?

ReadOnlyMemory<byte> buffer = ...
while (buffer.Length > 0)
{
    var written = await socket.SendAsync(buffer);
    buffer = buffer[written..];
}

[Clockwork-Muse]

Per the documentation:

Returns
ValueTask<Int32>
An asynchronous task that completes with the number of bytes sent.

Yes, the loop will be required.

[xqrzd]

Thanks. I thought maybe it wouldn't be since NetworkStream is a thin wrapper on top and didn't return a byte count.

[Clockwork-Muse]

NetworkStream does some funny things under the covers, it doesn't actually (directly) wrap this method. Keep in mind there's a different async API for sockets as well....

[stephentoub]

We should update the docs. As long as the Socket.Blocking is true, for a connection-oriented protocol (e.g. TCP), Socket.SendAsync will ensure all the data has been sent (from the perspective of the application) and you do not need a loop. This is why NetworkStream.WriteAsync doesn't have or need a loop.

On Windows, the implementation uses WSASend, which makes this guarantee. And on Unix, the implementation has its own loop to provide a similar guarantee.

[xqrzd]

Thank you!

[kerryjiang]

The doc still doesn't indicate this information:
https://learn.microsoft.com/en-us/dotnet/api/system.net.sockets.socket.sendasync?view=net-8.0

[kasperk81]

https://learn.microsoft.com/en-us/dotnet/api/system.net.sockets.socket.blocking?view=net-8.0#remarks says

The Blocking property has no effect on asynchronous methods. If you are sending and receiving data asynchronously and want to block execution, use the ManualResetEvent class.

is this correct?