I Used mistakenly an incorrect instance of Form1!!
in form1,there is a button that when click it,it shows form2.i wrote this code in click event of it:
Form1 frm1=new Form1();
Form2 frm2=new Form2(frm1);
and this was incorrect,because i made additional instance of Form1.
And I Change the code like this:
Form2 frm2=new Form2(this);