what will the runtime do when await something?

[ljzj2]

I want to ask that what the runtime will do ? waiting or doing something else?

forexample :

await Shell.Current.GoToAsync("../");

when I want to go to some page(or read database or others). what the thread will do?

yes I know it will await the page open and then goto `i+=1` and will it just wait?

so why we use await? if I do not use await and create a delegate then when the page is open then execute the delegate (i+=1) it is same right?

like the MVVM event.

I create a event and when the page is open the event will execute the i+=1

[huoyaoyuan]

what the thread will do?

There isn't the thread during await. The caller thread creates a pending task and immediately return to its caller, and do whatever it wants. For UI applications, the thread will return quickly and go back for listening UI events.

yes I know it will await the page open and then goto i+=1 and will it just wait?

Nothing waits for await. When the inner-most task is completed, it posts the continuation to thread pool, or a UI thread with Dispatcher, which can accept posted code.
Strictly speaking, for UI application, the UI thread is waiting for either a UI event (like mouse) or any posted code.

so why we use await? if I do not use await and create a delegate then when the page is open then execute the delegate (i+=1) it is same right?

This is called event-based asynchronous pattern.

You can check the (very detailed) post about how async/await really works. The posts also explains why async/await replaces other asynchronous patterns to become the major pattern in .NET.

[ljzj2]

看到代码，确实是把代码改成几部分，然后state表示当前执行到哪一部分了，跟我之前的理解是没错的，但是我之前理解错误的地方是runtime调用这个方法，其实按您两位大佬的意思是操作系统调用的，反正不是.net这边调用的。

另外可否问一个问题，为什么TaskAwaiter是struct，不是class？

[huoyaoyuan]

但是我之前理解错误的地方是runtime调用这个方法，其实按您两位大佬的意思是操作系统调用的，反正不是.net这边调用的。

事实上如果最内层Task来源是TaskCompletionSource，而不是通过操作系统的I/O回调，那么会出现由.NET触发调用的情况。
你的理解比较接近于如下代码：

ThreadPool.QueueUserWorkItem(() =>
{
    var result = stream.Read(buffer);
    taskCompletionSource.SetResult(result);
});

这是一个不好的实现（async over sync），通常在讨论async/await的行为时不会默认这种情况。

另外可否问一个问题，为什么TaskAwaiter是struct，不是class？

TaskAwaiter本身不包含任何功能和状态，其存在的主要目的是将标准库提供的API形状（Result、SetContinuationForAwait）转换为C#规定的API形状（GetResult、OnCompleted，还包含一些异常处理的约定）。观察其实现可知道其只包含一个Task字段，并将所有功能转发给Task。
换句话说，TaskAwaiter并不是Task中负责await功能的子模块，只是一个简单的转换包装。

[ljzj2]

令我茅塞顿开

操作系统调用的代码在哪，可否分享一下？

[huoyaoyuan]

整个操作系统调用的入口位于OnNativeIOCompleted：

runtime/src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPoolBoundHandle.WindowsThreadPool.cs

Lines 151 to 168 in 795e21d

	[UnmanagedCallersOnly]
	private static unsafe void OnNativeIOCompleted(IntPtr instance, IntPtr context, IntPtr overlappedPtr, uint ioResult, nuint numberOfBytesTransferred, IntPtr ioPtr)
	{
	var wrapper = ThreadPoolCallbackWrapper.Enter();
	Win32ThreadPoolNativeOverlapped* overlapped = (Win32ThreadPoolNativeOverlapped*)overlappedPtr;
	ThreadPoolBoundHandle boundHandle = overlapped->Data._boundHandle ?? throw new InvalidOperationException(SR.Argument_NativeOverlappedAlreadyFree);
	boundHandle.Release();
	if (NativeRuntimeEventSource.Log.IsEnabled())
	NativeRuntimeEventSource.Log.ThreadPoolIODequeue(Win32ThreadPoolNativeOverlapped.ToNativeOverlapped(overlapped));
	Win32ThreadPoolNativeOverlapped.CompleteWithCallback(ioResult, (uint)numberOfBytesTransferred, overlapped);
	ThreadPool.IncrementCompletedWorkItemCount();
	wrapper.Exit();
	}

其被注册到CreateThreadpoolIo，每当有一个IO完成时，系统都会在线程池中执行。其再从参数中获取对应的OVERLAPPED结构（对应具体的IO操作），查找相应注册的callback并调用。