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

.NET Flow Control: A Deep Dive into yield and await

yield and await are fundamental .NET keywords enabling sophisticated control flow, simulating concurrency. Understanding their underlying mechanisms, however, requires careful examination. This article clarifies their functionality.

await's Control Flow Mechanism

Encountering await triggers these runtime actions:

• For incomplete tasks, await attaches a continuation delegate, resuming execution later.
• Control returns to the caller, enabling parallel execution.
• Upon task completion, the continuation executes, resuming the method from its paused point.

Managing Continuation Data

The await-generated continuation delegate includes:

• The instruction pointer (next instruction).
• Local variable and temporary values.

This data resides on the heap, ensuring accessibility even outside the typical stack activation sequence.

Call Stack Preservation

Unlike standard call stacks, crucial activation record information is initially heap-allocated. Caller activation records aren't stored, as they're accessed post-await.

yield and Iterator State Management

yield, within iterator blocks, functions similarly to await. Upon encountering yield, the local state is moved to the heap, along with an instruction pointer for the next MoveNext execution. Exception handling is integrated within the iterator block's internal management.

Exception Handling Strategies

Uncaught exceptions are captured within the task and re-thrown when the task's result is accessed.

Summary

The complexity of yield and await arises from their reliance on continuation-passing style (CPS). The runtime strategically offloads activation information to the heap, preserving control flow and iterator state. While understanding lambda expressions with await is crucial, exploring CPS further, as detailed in Eric Lippert's blog posts on yield, iterators, and asynchronous programming, provides deeper comprehension.

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