How does dotnet understand that an IO operation has completed?

In .NET, the mechanism used for asynchronous IO operations is based on the operating system and the completion port model (I/O Completion Ports on Windows). When you call an asynchronous method, such as ReadAsync or WriteAsync, .NET registers a callback or a continuation that will be invoked upon completion of the operation.

Under the hood, the OS notifies .NET about the completion of the operation through special signals or events. In .NET, this is handled via IOCompletionCallback and related structures. Thus, .NET understands that the IO operation has finished when it receives a notification from the OS about data readiness or write completion, and it triggers the corresponding callback or task continuation.

Example of using asynchronous file reading:

using var stream = new FileStream("file.txt", FileMode.Open);
byte[] buffer = new byte[1024];
int bytesRead = await stream.ReadAsync(buffer, 0, buffer.Length);
// After await, .NET understands that the operation is complete and returns control

Here, await pauses the method execution until the OS reports the completion of the read operation.