Display a child form appears as dialogue only to its parent form

hai,
thanks again, But when i call showdialog it will appear permanently top of my application and prevnet from selection any other forms. I want that child form (B) appear top of only its parent from (A)., so that i could select any other form (C) with out closing the child form (B) and also it  shouldn't appear infront of from (C). and agin when im select the form (A) then the child of should appear as the dialogue window to that from (A0

            Form1 frm = new Form1();
            frm.ShowDialog();