Close form problem

Hallo,

I am creating a "ClassLibrary" project in C#. I am using VS 2008. My dll is calling from AutoCad for creation of some AutoCad entities. Now,

In my dll two forms are there.

Form A
Form B

From Form A, Form B is opening. I am using the following lines to open the Form B

Form B.Show(this);

I am actually hiding the Form B at close event by using the following lines

       private void frmAssignMilestone_FormClosing(object sender, FormClosingEventArgs e)
        {
            this.Hide();
            e.Cancel = true;
        }

Now When I am trying to close Form A after closing of Form B using the following line

this.close

The Form A is not closing. This is the problem. Please Help

Thanx