一：背景
1. 讲故事
前段时间训练营里的一位朋友提了一个问题，我用ReadAsync做文件异步读取时，我知道在Win32层面会传 lpOverlapped 到内核层，那在内核层回头时，它是如何通过这个 lpOverlapped 寻找到 ReadAsync 这个异步的Task的呢？
这是一个好问题，这需要回答人对异步完整的运转流程有一个清晰的认识，即使有清晰的认识也不能很好的口头表述出来，就算表述出来对方也不一定能听懂，所以干脆开两篇文章来尝试解读一下吧。
二：lpOverlapped 如何映射
1. 测试案例
为了能够讲清楚，我们先用 fileStream.ReadAsync 方法来写一段异步读取来产生Overlapped，参考代码如下：
static void Main(string[] args){UseAwaitAsync();Console.ReadLine();}static async Task<string> UseAwaitAsync(){string filePath = "D:\\dumps\\trace-1\\GenHome.DMP";Console.WriteLine($"{DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss:fff")} 请求发起...");FileStream fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read, bufferSize: 16, useAsync: true);{byte[] buffer = new byte[fileStream.Length];int bytesRead = await fileStream.ReadAsync(buffer, 0, buffer.Length);string content = Encoding.UTF8.GetString(buffer, 0, bytesRead);var query = $"{DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss:fff")} 获取到结果:{content.Length}";Console.WriteLine(query);return query;}}很显然上面的方法会调用 Win32 中的 ReadFile，接下来上一下它的签名和 _OVERLAPPED 结构体。
BOOL ReadFile([in] HANDLE hFile,[out] LPVOID lpBuffer,[in] DWORD nNumberOfBytesToRead,[out, optional] LPDWORD lpNumberOfBytesRead,[in, out, optional] LPOVERLAPPED lpOverlapped
);typedef struct _OVERLAPPED {ULONG_PTR Internal;ULONG_PTR InternalHigh;union {struct {DWORD Offset;DWORD OffsetHigh;} DUMMYSTRUCTNAME;PVOID Pointer;} DUMMYUNIONNAME;HANDLE hEvent;
} OVERLAPPED, *LPOVERLAPPED;2. 寻找映射的两端
既然是映射嘛，肯定要找到两个端口，即非托管层的 NativeOverlapped 和 托管层的 ThreadPoolBoundHandleOverlapped。
- 非托管 _OVERLAPPED
在 C# 中用 NativeOverlapped 结构体表示 Win32 的 _OVERLAPPED 结构，参考如下：
public struct NativeOverlapped
{public nint InternalLow;public nint InternalHigh;public int OffsetLow;public int OffsetHigh;public nint EventHandle;
}- 托管 ThreadPoolBoundHandleOverlapped
ReadAsync 所产生的 Task<int> 在底层是经过ValueTask, OverlappedValueTaskSource 一阵痉挛后弄出来的，最后会藏匿在 Overlapped 子类的 ThreadPoolBoundHandleOverlapped 中，参考代码和模型图如下：
public override Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken){ValueTask<int> valueTask = this.ReadAsync(new Memory<byte>(buffer, offset, count), cancellationToken);if (!valueTask.IsCompletedSuccessfully){return valueTask.AsTask();}return this._lastSyncCompletedReadTask.GetTask(valueTask.Result);}private unsafe static ValueTuple<SafeFileHandle.OverlappedValueTaskSource, int> QueueAsyncReadFile(SafeFileHandle handle, Memory<byte> buffer, long fileOffset, CancellationToken cancellationToken, OSFileStreamStrategy strategy){SafeFileHandle.OverlappedValueTaskSource overlappedValueTaskSource = handle.GetOverlappedValueTaskSource();NativeOverlapped* ptr = overlappedValueTaskSource.PrepareForOperation(buffer, fileOffset, strategy);if (Interop.Kernel32.ReadFile(handle, (byte*)overlappedValueTaskSource._memoryHandle.Pointer, buffer.Length, IntPtr.Zero, ptr) == 0){overlappedValueTaskSource.RegisterForCancellation(cancellationToken);}overlappedValueTaskSource.FinishedScheduling();return new ValueTuple<SafeFileHandle.OverlappedValueTaskSource, int>(overlappedValueTaskSource, -1);}
最后就是两端的映射关系了，先通过 malloc 分配了一块私有内存，中间隔了一个refcount 的 8byte大小，模型图如下：
3. 眼见为实
要想眼见为实，可以从C#源码中的Overlapped.AllocateNativeOverlapped方法寻找答案。
public unsafe class Overlapped{private NativeOverlapped* AllocateNativeOverlapped(object? userData){NativeOverlapped* pNativeOverlapped = null;nuint handleCount = 1;pNativeOverlapped = (NativeOverlapped*)NativeMemory.Alloc((nuint)(sizeof(NativeOverlapped) + sizeof(nuint)) + handleCount * (nuint)sizeof(GCHandle));GCHandleCountRef(pNativeOverlapped) = 0;pNativeOverlapped->InternalLow = default;pNativeOverlapped->InternalHigh = default;pNativeOverlapped->OffsetLow = _offsetLow;pNativeOverlapped->OffsetHigh = _offsetHigh;pNativeOverlapped->EventHandle = _eventHandle;GCHandleRef(pNativeOverlapped, 0) = GCHandle.Alloc(this);GCHandleCountRef(pNativeOverlapped)++;return pRet;}private static ref nuint GCHandleCountRef(NativeOverlapped* pNativeOverlapped)=> ref *(nuint*)(pNativeOverlapped + 1);private static ref GCHandle GCHandleRef(NativeOverlapped* pNativeOverlapped, nuint index)=> ref *((GCHandle*)((nuint*)(pNativeOverlapped + 1) + 1) + index);}卦中代码先用 NativeMemory.Alloc 方法分配了一块私有内存，随后还把 Overlapped 给 GCHandle.Alloc 住了，这是防止异步期间对象被移动，有了代码接下来上windbg去眼见为实，在 Kernel32!ReadFile 中下断点观察方法的第五个参数。
0:000> bp Kernel32!ReadFile
0:000> g
Breakpoint 0 hit
KERNEL32!ReadFile:
00007ffd`fa2f56a0 ff25caca0500 jmp qword ptr [KERNEL32!_imp_ReadFile (00007ffd`fa352170)] ds:00007ffd`fa352170={KERNELBASE!ReadFile (00007ffd`f85c5520)}
0:000> k 5# Child-SP RetAddr Call Site
00 000000ff`8837e1c8 00007ffd`96229ce3 KERNEL32!ReadFile
01 000000ff`8837e1d0 00007ffd`96411a4a System_Private_CoreLib!Interop.Kernel32.ReadFile+0xa3 [/_/src/coreclr/System.Private.CoreLib/Microsoft.Interop.LibraryImportGenerator/Microsoft.Interop.LibraryImportGenerator/LibraryImports.g.cs @ 6797]
02 000000ff`8837e2d0 00007ffd`96411942 System_Private_CoreLib!System.IO.RandomAccess.QueueAsyncReadFile+0x8a
03 000000ff`8837e350 00007ffd`96433677 System_Private_CoreLib!System.IO.RandomAccess.ReadAtOffsetAsync+0x112 [/_/src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Windows.cs @ 238]
04 000000ff`8837e3f0 00007ffd`9642d5f8 System_Private_CoreLib!System.IO.Strategies.OSFileStreamStrategy.ReadAsync+0xb7 [/_/src/libraries/System.Private.CoreLib/src/System/IO/Strategies/OSFileStreamStrategy.cs @ 290]
0:000> uf 00007ffd`96229ce3
...6797 00007ffd`96229c98 4c8b7d30 mov r15,qword ptr [rbp+30h]6797 00007ffd`96229c9c 4c897c2420 mov qword ptr [rsp+20h],r156797 00007ffd`96229ca1 498bce mov rcx,r146797 00007ffd`96229ca4 48894dac mov qword ptr [rbp-54h],rcx6797 00007ffd`96229ca8 488bd3 mov rdx,rbx6797 00007ffd`96229cab 488955a4 mov qword ptr [rbp-5Ch],rdx6797 00007ffd`96229caf 448bc6 mov r8d,esi6797 00007ffd`96229cb2 448945b4 mov dword ptr [rbp-4Ch],r8d6797 00007ffd`96229cb6 4c8bcf mov r9,rdi6797 00007ffd`96229cb9 4c894d9c mov qword ptr [rbp-64h],r96797 00007ffd`96229cbd 488d8d40ffffff lea rcx,[rbp-0C0h]6797 00007ffd`96229cc4 ff159e909e00 call qword ptr [System_Private_CoreLib!Interop.CallStringMethod+0x5ab9c8 (00007ffd`96c12d68)]6797 00007ffd`96229cca 488b055708a100 mov rax,qword ptr [System_Private_CoreLib!Interop.CallStringMethod+0x5d3188 (00007ffd`96c3a528)]6797 00007ffd`96229cd1 488b4dac mov rcx,qword ptr [rbp-54h]6797 00007ffd`96229cd5 488b55a4 mov rdx,qword ptr [rbp-5Ch]6797 00007ffd`96229cd9 448b45b4 mov r8d,dword ptr [rbp-4Ch]6797 00007ffd`96229cdd 4c8b4d9c mov r9,qword ptr [rbp-64h]6797 00007ffd`96229ce1 ff10 call qword ptr [rax]6797 00007ffd`96229ce3 8bd8 mov ebx,eax仔细阅读卦中的汇编代码，通过这句 r15,qword ptr [rbp+30h] 可知 pNativeOverlapped 是保存在 r15 寄存器中。
0:000> r r15
r15=00000241ca2d4d70
0:000> dp 00000241ca2d4d70
00000241`ca2d4d70 00000000`00000000 00000000`00000000
00000241`ca2d4d80 00000000`00000000 00000000`00000000
00000241`ca2d4d90 00000000`00000001 00000241`c8761358根据上面的模型图，00000241ca2d4d90 保存的是引用计数，00000241c8761358 就是我们的 ThreadPoolBoundHandleOverlapped ，可以 !do 它一下便知。
最后用 dnspy 在 Overlapped.GetOverlappedFromNative 方法中下一个断点，这个方法会在异步处理完成后，执行NativeOverlapped寻址ThreadPoolBoundHandleOverlapped 的逻辑，截图如下，那个 ReadAsync保存在内部的 _continuationState 字段里。
三：总结
C#的传统做法大多都是采用传参数的方式来建议映射关系，而本篇中用 malloc 开辟一块私有区域来映射两者的关系也真是独一份，实属无奈！