delegates and callbacks

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I am having this scenario which needs your help.
I have a C# app that interops with a C DLL. My C# app has to pass two fuction pointers to the DLL for it callback. The DLL starts a timer and that fires a timer callback every second from which the C# callback be fired.
My code snippets
In C# I have this class.

namespace
{
public delegate void Callback1(int num);
public delegate void Callback2(bool isUpdated);
public class TView
{
        [DllImport("xx.dll", CallingConvention = CallingConvention.Cdecl)]
        public static extern void StartUpdate(Callback1 func1, Callback2 func2);
	
	public void UpdateTview()
	{
        
		Callback1 a_CallBack = new Callback1(UpdateFunction1); 
		Callback2 b_CallBack = new Callback2(UpdateFunction2);  
		StartUpdate(a_Callback, b_Callback);  
	}
	public void UpdateFunction1(int n)
	{
	}
 	public void UpdateFunction2(bool isUpdated)
	{
	}
}

In my DLL I have the StartUpdate function as follows

__declspec(dllexport) void __cdecl StartUpdate( void (*Callback1)(int), void (*Callback2)(bool)
{
	/*start the timer here*/
	bool success = ::CreateTimerQueueTimer(
		&m_timerHandle,
		NULL,
		(WAITORTIMERCALLBACK)TimerFire,
		NULL,
		0,
		1000,
		WT_EXECUTEINTIMERTHREAD);
	
}
void TimerFire()
{
	printf("Timer Callback called");
	if(condition1)
	//(*Callback1)(10); //should call UpdateFunction1
	if(condition2)
	//(*Callback2)(true); //should call UpdateFunction2
}

How should I achieve this. I can pass one function pointer as lpparam of TimerFire, but how do I pass other function pointer to TimerFire()? Is there any other approach?
Your help is greatly appreciated.
Thanks in advance.

Posted 17-Mar-11 15:08pm

Balaji Ganesan1

Updated 17-Mar-11 15:14pm

v2

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2 solutions

Solution 2

Niklas is right. You should change the type of your timer function.

C++

//defines typedefs to make declarations easier
typedef void (__stdcall *CALLBACK1)(int);
typedef void (__stdcall *CALLBACK2)(bool);

//type of the pointer passed to you callback
struct TIMER_PARAM
{
    CALLBACK1 callback1;
    CALLBACK2 callback2;
};

void CALLBACK WaitOrTimerCallback(PVOID lpParameter, BOOLEAN TimerOrWaitFired)
{
    //get your params
    TIMER_PARAM* pParam = (TIMER_PARAM*)lpParameter;

    if(condition1)
        pParam->callback1(10);
    if(condition2)
        pParam->callback2(true);
}

__declspec(dllexport) void __cdecl StartUpdate(
    CALLBACK1 callback1, CALLBACK2 callback2)
{
    //this pointer will be passed to your callback
    //you will need to delete this pointer somewhere in your code
    //to prevent a memory leak
    TIMER_PARAM* parameters = new TIMER_PARAM;
    parameters->Callback1 = callback1;
    parameters->Callback1 = callback2;

    /*start the timer here*/
    bool success = ::CreateTimerQueueTimer(
        &m_timerHandle,
        NULL,
        TimerFire,
        // will be given as an argument to your timer function
        parameters,
        0,
        1000,
        WT_EXECUTEINTIMERTHREAD);
}

Posted 17-Mar-11 22:59pm

Olivier Levrey

Comments

Espen Harlinn 18-Mar-11 5:51am

Good effort, instructive piece of code, my 5

Olivier Levrey 18-Mar-11 5:55am

Thank you Espen.

Balaji Ganesan1 18-Mar-11 13:30pm

Thanks much to both Olivier and Niklas. That did it. Thanks!!!!

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Niklas L · Accepted Answer · 2011-03-17T21:12:00

Solution 1

Use the Parameter argument in the call. If you want to send more than one pointer, you will have to wrap it up in a std::pair, std::tuple, or a custom struct/class, who's pointer you supply to the function.

Btw, your TimerFire function should look like:

VOID CALLBACK WaitOrTimerCallback(
  __in  PVOID lpParameter,
  __in  BOOLEAN TimerOrWaitFired
);

Your type-cast fools the compiler.

Posted 17-Mar-11 21:12pm

Niklas L

Updated 17-Mar-11 21:13pm

v2

Comments

Olivier Levrey 18-Mar-11 4:59am

Voted 5.

Dalek Dave 18-Mar-11 5:22am

Good Call.

Espen Harlinn 18-Mar-11 5:50am

Nice one, my 5