fix Selected item in list box is null

from https://stackoverflow.com/questions/5859885/selected-item-in-list-box-is-null

It's because you are running ListBox1.DataBind() on every cycle. Change to:

if (!IsPostBack)
{
    var query = from emp in db.Employees
                select emp.Employee_Name;

    ListBox1.DataSource = query;
    ListBox1.DataBind();
}