await await vs Unwrap()

Question

Given a method such as

public async Task<Task> ActionAsync() {     ... } 

What is the difference between

await await ActionAsync(); 

and

await ActionAsync().Unwrap(); 

if any.

Answer 1

Unwrap() creates a new task instance that represent whole operation on each call. In contrast to await task created in such a way is differ from original inner task. See the Unwrap() docs, and consider the following code:

private async static Task Foo() {     Task<Task<int>> barMarker = Bar();      Task<int> awaitedMarker = await barMarker;     Task<int> unwrappedMarker = barMarker.Unwrap();      Console.WriteLine(Object.ReferenceEquals(originalMarker, awaitedMarker));     Console.WriteLine(Object.ReferenceEquals(originalMarker, unwrappedMarker)); }  private static Task<int> originalMarker; private static Task<Task<int>> Bar() {     originalMarker = Task.Run(() => 1);;     return originalMarker.ContinueWith((m) => m); } 

Output is:

True False 

Update with benchmark for .NET 4.5.1: I tested both versions, and it turns out that version with double await is better in terms of memory usage. I used Visual Studio 2013 memory profiler. Test includes 100000 calls of each version.

x64:

╔══════════════════╦═══════════════════════╦═════════════════╗ ║ Version          ║ Inclusive Allocations ║ Inclusive Bytes ║ ╠══════════════════╬═══════════════════════╬═════════════════╣ ║ await await      ║ 761                   ║ 30568           ║ ║ await + Unwrap() ║ 100633                ║ 8025408         ║ ╚══════════════════╩═══════════════════════╩═════════════════╝ 

x86:

╔══════════════════╦═══════════════════════╦═════════════════╗ ║ Version          ║ Inclusive Allocations ║ Inclusive Bytes ║ ╠══════════════════╬═══════════════════════╬═════════════════╣ ║ await await      ║ 683                   ║ 16943           ║ ║ await + Unwrap() ║ 100481                ║ 4809732         ║ ╚══════════════════╩═══════════════════════╩═════════════════╝ 

Answer 2

There won't be any functional difference.