i have a dropdownlist which i am populating using visual studio extender i want to populate a textbox from that dropdown list? how do i do that ?

What ever said in the above solution is correct but there is noneed of accessing the database one more as we are having the required value in the dropdown list itself.
 
so the dropdown list event handler can be modified like this.
 

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
       TextBox1.Text=DropDownList1.SelectedItem.Value.ToString();
    }

If you are need of more information related to the user selection then you can access the database one more time.