How Do `yield` and `await` Manage Control Flow in .NET?

Control flow mechanism of yield and await in .NET

Your understanding of the yield and await keywords in .NET is generally correct. yield Allows the iterator block to return control to the calling code and resume execution where it left off when the iterator is called again. await Not only waits for the callee to complete, but also returns control to the caller. When the caller await the method again, execution will resume where it left off.

The key to understanding these structures lies in their implementation mechanisms. For await, it is essentially equivalent to a "high-level return". When a await expression is encountered, the runtime checks whether the task has been completed. If not completed, it allocates a delegate, executes the remainder of the method as a continuation of the task, and returns it to the caller. If the task is completed, the remainder of the method is executed immediately.

The continuation of

await contains a number that identifies the next instruction pointer to be executed in a lookup table. This ensures that when the task resumes, it continues execution from the correct location. Continuations also store the values ​​of all local and temporary variables, allowing a method to resume execution as if it had never been interrupted.

As for the question of how the runtime handles multiple method calls before await , the answer is that the activation record of the caller and the activation record of the method it calls are not stored in the same stack. Instead, await the relevant information for the recovery is stored on the heap. This ensures that the stack is not overwritten by subsequent method calls.

If an unhandled exception occurs, it is caught and stored in the task. This exception will be re-thrown when the results of the task are obtained. This behavior ensures that exceptions are handled correctly even in the presence of asynchronous operations.

A similar principle applies to the implementation of yield. When the iterator blocks yield a value, the state of the local variables is copied to the heap, along with a number indicating the MoveNext instruction that should resume execution the next time the iterator is called. This allows the iterator to pause execution at yield point and resume execution later without losing any state.

By leveraging these complex mechanisms, the yield and await keywords provide a powerful way to implement asynchronous and iterative operations in .NET. They allow simulating concurrency without the need for threads, simplifying code and improving performance.

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