Problem with multiple inheritance

Hello,

I work on an application with multiple inheritance. I work with Visual
C++ 2005 & Windows CE.

I have a class named A.

I have a class named AB which inherits from A.

I have a class named C.
Class C has a function I_Am_From_C().

I have a class named ABC which inherits from AB and from C.

Class AB has a function InheritsFromC() which gives false;
Class ABC has a function InheritsFromC() which gives true;

I am trying to write generic C++ code to be able to deal with my
classes.

In my code, O is an object coming from another function. It can be an
AB or an ABC object.
I write the following :

void MyFunction(A *myObject)
{
   C *mySecondObject;

   if (myObject->InheritsFromC() == true)
    {
     mySecondObject = (C *)myObject;
     mySecondObject->I_Am_From_C();
    }

}

but when I make this code run, the calling of the I_Am_From_C()
function crashes, and in debug I can see that the C object named
mySecondObject is not correctly initialized (all his parameters have
random values).

Isn't it a good way to manage myObject when it inherits from C ? Can
you help me with a better way to be able to call the I_Am_From_C()
function from myObject where it effectively inherits from C ?

Thank you very much !