C#: What is async/await in C#?

async/await — syntactic sugar for asynchronous programming.

Basic usage:

1async Task<string> GetDataAsync()
2{
3    using var client = new HttpClient();
4    return await client.GetStringAsync("https://api.example.com");
5}

How it works:

• async — marks method as asynchronous.
• await — suspends method until task completes.
• Thread is NOT blocked — freed for other work.
• State machine generated by compiler.

Return types:

• Task — async method with no return value.
• Task<T> — async method returning T.
• ValueTask<T> — for hot paths (avoids allocation).
• void — only for event handlers (avoid otherwise).

Common mistakes:

• async void (fire and forget, exceptions lost).
• .Result or .Wait() (deadlock risk).
• Task.Run in ASP.NET (defeats purpose).