C# Language Specification

Classification of await expressions

The expression await t is classified the same way as the expression (t).GetAwaiter().GetResult(). Thus, if the return type of GetResult is void, the await-expression is classified as nothing. If it has a non-void return type T, the await-expression is classified as a value of type T.

3. Runtime evaluation of await expressions

At runtime, the expression await t is evaluated as follows:

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  An awaiter a is obtained by evaluating the expression (t).GetAwaiter().
  
• 
  A bool b is obtained by evaluating the expression (a).IsCompleted.
  
• 
  If b is false then evaluation depends on whether a implements the interface System.Runtime.CompilerServices.ICriticalNotifyCompletion (hereafter known as ICriticalNotifyCompletion for brevity). This check is done at binding time; i.e. at runtime if a has the compile time type dynamic, and at compile time otherwise. Let r denote the resumption delegate (§Error: Reference source not found):
  

• 
  If a does not implement ICriticalNotifyCompletion, then the expression
  (a as (INotifyCompletion)).OnCompleted(r) is evaluated.
  
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  If a does implement ICriticalNotifyCompletion, then the expression
  (a as (ICriticalNotifyCompletion)).UnsafeOnCompleted(r) is evaluated.
  
• 
  Evaluation is then suspended, and control is returned to the current caller of the async function.
  

• 
  Either immediately after (if b was true), or upon later invocation of the resumption delegate (if b was false), the expression (a).GetResult() is evaluated. If it returns a value, that value is the result of the await-expression. Otherwise the result is nothing.
  

An awaiter’s implementation of the interface methods INotifyCompletion.OnCompleted and ICriticalNotifyCompletion.UnsafeOnCompleted should cause the delegate r to be invoked at most once. Otherwise, the behavior of the enclosing async function is undefined.

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