Control Events not Opening in .CS page in ASP.net

Question

I am making a registration from and opening DropDownList event but it is not opening in code behind file (.cs) it is opening on designing page like 

 

<script runat="server">

protected void Ddl_course_SelectedIndexChanged(object sender, EventArgs e)

{

}

</script>

Can anyone tell me how to resolve this problem.