Denis,

Perhaps VB is slightly different than C#, but I cannot set this.DialogResult = null; in C#. And another thing, in my testing, I don't need to mess with the DialogResult anyway. Try this and you'll see what I mean:

public class MyForm : Form, IDoExit
{
	public void DoExit()
	{
		MessageBox.Show("Closing Form via DoExit");
		this.Close();
	}
}
public class MyButton : Button
{
	protected override void OnClick(EventArgs e)
	{
	    base.OnClick(e);

	    Form oForm = this.FindForm();
	    if (oForm is IDoExit)
	    {
	        ((IDoExit)oForm).DoExit();
	        MessageBox.Show("Click From Button Class");
	    }
	}
}

And from a button click event handler on your main form, create and open the dialog form:

private void btnOpenForm_Click(object sender, System.EventArgs e)
{
	MyForm oForm = new MyForm();
	MyButton btn = new MyButton();
	oForm.Controls.Add(btn);
	oForm.ShowDialog();
}

Click the button on the dialog form, it closes just fine and I have not done a thing with DialogResult.

Now, just for giggles, in the MyButton class, move the base.OnClick(e) to the end of the OnClick() method, and guess what ... it will no longer close!

public class MyButton : Button
{
	protected override void OnClick(EventArgs e)
	{
	    //base.OnClick(e); comment out, move to end of method

	    Form oForm = this.FindForm();
	    if (oForm is IDoExit)
	    {
	        ((IDoExit)oForm).DoExit();
	        MessageBox.Show("Click From Button Class");
	    }
	    base.OnClick(e); // now form will not close!!!
	}
}

I suspect the problem you were both seeing was where to put the call to the base.OnClick(e)!!

~~Bonnie