windows MDI forms

Question

hello,

In MDI form i click child form A to show.

Then from that child form A i using button to show Child from b.

I need close Child form A before show child form B.

I am using following line coding but child A Not close ,
its show behind the child B.

this.Close();
B ChildB= new B();
ChildB.state = "Update";
ChildB.ShowDialog();