Don't feel bad. I always find that failing to spot the obvious is actually incredibly easy :-}

>I feel like a total idiot. Thank you for catching it.
>
>>You haven't instantiated an instance of MyDerivedClass (MyDerivedClass m_DerivedClass = new MyDerivedClass();)
>>>Hi,
>>>
>>>I am having a problem that I will try to present here is a simplified manner:
>>>
>>>

>>>public class MyBaseClass
>>>{
>>>    public DataSet myDataSet;
>>>    
>>>    public void MyMethod( string cStr )
>>>    {
>>>          // do something with the string cStr
>>>   }
>>>}
>>>
>>>public class MyDerivedClass : MyBaseClass
>>>{
>>>               // Nothing is this class, so far
>>> }
>>>
>>>public partial class frmMyForm : Form
>>>{
>>>   private string cStr
>>>   public MyDefirvedClass m_DerivedClass;
>>>
>>>   public frmMyForm()
>>>   {
>>>         InitializeComponent();
>>>         m_DerivedClass.MyMethod( cStr );                        //  !!! This where I get error Null Reference
>>>   }
>>>
>>>}
>>>

>>>
>>>Note that the when I type m_DerivedClass in the frmMyForm constructor the intellisense shows the methods (MyMethod among them) of the class MyBaseClass so I know that I have declared "using" correctly. Why do I get the Null Reference on call MyMethod() then?
>>>
>>>TIA