ASP.NET

<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
        <ContentTemplate>
        <asp:TextBox ID="UserNameTextBox" CssClass="PCMSStandardTextbox" OnTextChanged="UserName_Changed"
                runat="server" MaxLength="50"></asp:TextBox>
</ContentTemplate>
                <Triggers>
                <asp:AsyncPostBackTrigger ControlID="UserNameTextBox" EventName="UserName_Changed" />
                </Triggers>
                </asp:UpdatePanel>
                 <asp:UpdateProgress ID="UpdateProgress1" runat="server">
                <ProgressTemplate>
                 <span class="ProgressBar">
                Checking for Username availability...</span></ProgressTemplate></asp:UpdateProgress></td>  

protected void UserName_Changed(object sender, EventArgs e)
    {
        string strUserName = UserNameTextBox.Text.Trim();
        List<string> UserNames = u.GetUserLogin();
}