button.visible

Question

hi
I have a web page.In this page i have a listBox and a butto i want to when i clicked on the ListBox my button is hide.
then i write bellow code 
ListBox_SelectedIndexChanged(object sender, EventArgs e)
{button1.visible=false;}
 
when i trace this code false visibilty assign to button1 rightly.
please say me what's mistake and how can i correct this.tnx a lot