Overlapped 类型支持 .NET Framework 结构,因此不适用于直接从代码中使用。Overlapped operations enable a thread to execute a time-consuming I/O operation in the background, leaving the thread free to perform other tasks. To enable overlapped I/O operations on a communications resource, the thread must specify the FILE_FLAG_OVERLAPPED flag in the CreateFile function when the handle is opened. To execute the ReadFile or WriteFile function as an overlapped operation, the calling thread must specify a pointer to an OVERLAPPED structure.The OVERLAPPED structure must contain a handle to a manual-reset (not an auto-reset) event object. The system sets the state of the event object to not-signaled when a call to the I/O function returns before the operation has been completed. The system sets the state of the event object to signaled when the operation has been completed. The thread uses a wait function to check the current state of the event object or to wait for its state to be signaled.
typedef struct _OVERLAPPED {
ULONG_PTR Internal;
ULONG_PTR InternalHigh;
DWORD Offset;
DWORD OffsetHigh;
HANDLE hEvent;
} OVERLAPPED;