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What do the 'async' and 'await' keywords do, and how do they relate to each other? What is the difference between the async and await keywords in C#, and how do they work together? How do you run multiple async operations concurrently? What does ConfigureAwait(false) do? What does ConfigureAwait(false) do, and when is it appropriate to use? How do you run multiple asynchronous operations concurrently and wait for all of them using Task.WhenAll? What are async streams in C#, and how does IAsyncEnumerable<T> differ from a regular IEnumerable<T> for handling large or slow data sources? Why can .Result or .Wait() deadlock, and what's the fix? How do you design for scalability and avoid thread-pool starvation? How does CancellationToken propagate cancellation requests through a chain of async method calls?
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Why does calling .Result or .Wait() on a Task risk causing a deadlock, particularly in ASP.NET (non-Core) or UI applications?

Intermediate

Answer

Synchronously blocking on a Task (.Result/.Wait()) while the current SynchronizationContext only allows one thread to run at a time can deadlock: the blocked thread waits for the async operation to complete, but that operation's continuation is trying to resume on the SAME captured context, which is stuck waiting — neither can proceed.
// Risky: can deadlock in ASP.NET / WinForms / WPF
string result = FetchDataAsync().Result;

// Safe: properly asynchronous all the way up
string result2 = await FetchDataAsync();
Real-world example Debugging a mysterious application hang traced back to a synchronous .Result call inside a legacy ASP.NET MVC controller action.

Common follow-ups: Why doesn't this same deadlock risk apply to ASP.NET Core or console applications by default?

Exception Handling
All 20 Asynchronous Programming questions