onchange will only be triggered on client side when you directly type something and leave the textbox. So nothing will happen when you set the textbox value in code behind.

If you want to handle the text change event on client side:

<script>  

     function test(txt){

         alert(txt.value);

     };

</script>



<asp:TextBox ID=txt runat=server onchange=test(this);></asp:TextBox>

If you want to handle text change event on server side, you could do this:

HTML:

<asp:TextBox ID=txt runat=server OnTextChanged=txt_OnTextChanged AutoPostBack=true></asp:TextBox>

CS:

protected void txt_OnTextChanged(object sender, EventArgs e)

{

     // Do something

}