use update panel like this
<asp:UpdatePanel ID ="updt1" runat ="server" >
<ContentTemplate >
<asp:TextBox ID="txt" runat ="server" >
</asp:TextBox>
</ContentTemplate>
<Triggers >
<asp:AsyncPostBackTrigger ControlID ="txt" EventName ="TextChanged" />
</Triggers>
</asp:UpdatePanel>
in code behind:
Protected Sub txt_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles txt.TextChanged
Response.Write("hai")
End Sub