Why is `await` Prohibited Inside a `lock` Statement in C#?

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.

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