abstract class CBase
{
protected int _i;

public abstract void DoIt();
}

class C1 : CBase
{
public virtual void DoIt()
{
_i = 1;
}
}

class C2 : CBase
{
public virtual void DoIt()
{
_i = 2;
}
}

class C3: C1, C2
{
public void Do()
{
DoIt();
Console.Write(i); // ???
}
}