What is the difference between Task and Thread, and how does async/await work in C#?
Quick answer
A Thread is an operating-system execution unit, while a Task represents an asynchronous operation scheduled on the thread pool; await releases the current thread while the operation is pending and resumes the method when it completes.
Creating threads is expensive and you must manage them yourself. A Task is a higher-level abstraction: it can represent CPU work queued to the thread pool (Task.Run) or I/O that does not need a thread at all while waiting, such as an HTTP call or database query.
When the compiler sees await, it turns the method into a state machine. If the awaited task is not finished, the method returns control to its caller and the thread is free to serve other requests. When the task completes, the rest of the method continues, possibly on another thread. This is why async I/O lets a web server handle many more concurrent requests with the same number of threads. async/await is about scalability for I/O-bound work; for CPU-bound work it does not make anything faster by itself.
public async Task<string> GetPageAsync(HttpClient client, string url)
{
// thread is released while waiting on the network
string html = await client.GetStringAsync(url);
return html.ToUpperInvariant();
}
// run independent calls concurrently
var results = await Task.WhenAll(GetPageAsync(c, a), GetPageAsync(c, b));Key points
- Task = promise of work, not a thread
- await frees the thread during I/O
- Use Task.WhenAll for independent operations