Question

I have a C dll that I’m wrapping so that I can call it from C#. One function uses events to notify you when a state has changed, and figuring out to deal with it took some digging. It seems to be working ok, but I’m curious if anyone has more experience with this than I do and can offer any advice.

The function is defined in the dll’s .h file as:

int NotifyStateChange(OVERLAPPED *overlapped);  
typedef int (*NOTIFY_STATE_CHANGE_PROC)(OVERLAPPED *);  

Sample C code calling it:

OVERLAPPED overlapped;  
overlapped.hEvent = CreateEvent(NULL, FALSE, FALSE, NULL);  
fns.NotifyStateChange(&overlapped);  
WaitForSingleObject(overlapped.hEvent, INFINITE);  
// ...query for the new state or whatever...

This is how I approached it in C#:

[DllImport("myfuncs.dll")]
unsafe public static extern int NotifyStateChange(NativeOverlapped* lpOverlapped);

static private ManualResetEvent m_stateChangeEvent = new ManualResetEvent(false);

public static DeviceState WaitForStateChange()
{
    unsafe
    {
        Overlapped overlapped = new Overlapped(0, 0, 
            m_stateChangeEvent.SafeWaitHandle.DangerousGetHandle(), null);
        IOCompletionCallback callback = StateChangeCallback;
        byte[] userData = new byte[100];
        NativeOverlapped* nativeOverlapped = overlapped.Pack(callback, userData);
        NotifyStateChange(nativeOverlapped);
        m_stateChangeEvent.WaitOne();
        Overlapped.Unpack(nativeOverlapped);
        Overlapped.Free(nativeOverlapped);
    }
    return GetCurrentState();
}

[ComVisibleAttribute(true)]
unsafe static public void StateChangeCallback (
    uint errorCode, 
    uint bytesTransferred, 
    NativeOverlapped* overlapped)
{
    m_stateChangeEvent.Set();
}

One thing I’m unclear on is the need for userData. NotifyStateChange just fires an event, it doesn’t return any data. Passing a null userData to Pack() seemed to work fine, but I’m concerned something may be going on with userData under the covers that I’m not aware of.

I appreciate any advice on if there’s a more proper way to do this.

Eric

Answer 1

You are already blocking on the event handle, there is no need to use the Overlapped class and the callback. All you have to do is to set the NativeOverlapped.EventHandle member to the DangerousGetHandle() return value. The DLL will set the event.

Also, declare the argument for NotifyStateChange as ref instead of a pointer, saves you from having to use the unsafe keyword.