Understanding the Restriction: await
and lock
in C#
C#'s await
keyword is crucial for asynchronous programming, enabling non-blocking operations. However, using await
inside a lock
statement is strictly prohibited. This restriction is a key design choice to prevent a common source of errors: deadlocks.
Microsoft's documentation explains that an await
expression within a lock
creates a risk. The code's execution can pause after the await
yields control, resuming later on a potentially different thread. This timing gap can lead to situations where other threads obtain locks, reversing the lock order and resulting in a deadlock.
Let's examine a hypothetical example:
class Async { public static async Task<IDisposable> Lock(object obj) { while (!Monitor.TryEnter(obj)) await Task.Yield(); return new ExitDisposable(obj); } private class ExitDisposable : IDisposable { private readonly object obj; public ExitDisposable(object obj) { this.obj = obj; } public void Dispose() { Monitor.Exit(this.obj); } } }
This code attempts to mimic asynchronous locking, but as the compiler indicates, it's prone to indefinite blocking within ExitDisposable.Dispose()
, potentially leading to deadlocks. This happens because Monitor.Exit
might execute on a different thread than the one that acquired the lock, potentially violating lock order.
In essence, the ban on await
within lock
is a proactive measure to avoid deadlocks in multithreaded applications. It's best to avoid combining asynchronous operations with lock
statements and consider alternative synchronization methods such as reader-writer locks or synchronization primitives from System.Threading
.
The above is the detailed content of Why is `await` Prohibited Inside a `lock` Statement in C#?. For more information, please follow other related articles on the PHP Chinese website!

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