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?