What is asynchrony in C#? What are async/await? Must an async method run in another thread?

Asynchronous programming in C# is the ability to perform operations without blocking the main thread, allowing the program to continue working while a long-running operation (such as input/output or network requests) is performed in the background.

async and await are keywords that simplify writing asynchronous code. async marks a method as asynchronous, and await pauses the execution of the method until the asynchronous operation completes, without blocking the thread.

Important: An async method does not necessarily run on a different thread. It can run on the same thread, but when await is used, control returns to the calling code, allowing the UI or server to remain responsive. If within an async method there are operations that truly run on a different thread (for example, Task.Run), then another thread is involved.

Example:

public async Task<string> DownloadContentAsync(string url)
{
    using var client = new HttpClient();
    string content = await client.GetStringAsync(url); // does not block the thread
    return content;
}

Here, GetStringAsync is an asynchronous operation that does not block the thread but does not necessarily run on a separate thread.