GetOverlappedResult

[DllImport("Kernel32.dll")][return: MarshalAs(UnmanagedType.Bool)]
public static extern bool GetOverlappedResult(
   SafeFileHandle hFile,
   ref OVERLAPPED lpOverlapped,
   out uint lpNumberOfBytesTransferred,
   [MarshalAs(UnmanagedType.Bool)] bool bWait
);

Microsoft documentation: Link