ASP.NET

 <asp:UpdatePanel ID="UserNameUpdatePanel" runat="server" UpdateMode="Conditional
">
                                    <ContentTemplate>                                       
                                        <asp:TextBox ID="UserNameTextBox" 
AutoPostBack="true" OnTextChanged="UserName_Changed" runat="server" MaxLength="50"></asp:TextBox>               
                                        <asp:Label ID ="litusername" runat="server" 
Text="*" ForeColor="Red"></asp:Label>
                                        <asp:RequiredFieldValidator ID="UserNameRequiredFieldValidator" runat="server" ErrorMessage="Required Field" 
                                            Display="Dynamic" 
ControlToValidate="UserNameTextBox" CssClass="RequiredField">
</asp:RequiredFieldValidator>
                                        <asp:Image ID="UserNameImage" 
runat="server" Visible="false" />                    
                                        <asp:Label ID="lblMessage" 
runat="server" CssClass="ErrorMessagesLabel" ></asp:Label> 
                                        <asp:RegularExpressionValidator ID="UserNameExpressionValidator" runat="server" 
ControlToValidate="UserNameTextBox"
  CssClass="RequiredField" 
ValidationExpression="[\w|]*" ErrorMessage="UserLogin Already exists"></asp:RegularExpressionValidator>               
</ContentTemplate>               
</asp:UpdatePanel>

protected void UserName_Changed(object sender, EventArgs e)
    {
        
    Business.User u = new User();
    lblMessage.Text = "";
    UserNameImage.ImageUrl = "";
    if (UserNameTextBox.Text != "")
    {
       string strUserName = UserNameTextBox.Text.Trim();
       List<string> UserNames = u.GetUserLogin();
       if (UserNameImage.Visible == false)
          UserNameImage.Visible = true;

        if (UserNames.Contains(strUserName))
        {
            UserNameImage.ImageUrl = "~/images/unacknowledged_x.gif";
            lblMessage.Text = "User Name Already Exists";
         }
         else
         {
           UserNameImage.ImageUrl = "~/images/acknowledged_checkmark.gif";
            lblMessage.Text = "";
         }
       }
       else
       {
          UserNameRequiredFieldValidator.Visible = true;
       }
        
    }

protected void Page_Load(object sender, EventArgs e)
{
if (MyChachItem == null)
{
// Kill the event so it is not triggerd 
MyAntemPlaceHolder.Visible = true;
ShowMessage(MyAntemPlaceHolder, "Can not continue, please click on the list button and start over.")                   
}
}

btnSave.Click -= btnSave_Click(..)