ajax验证用户名是否存在

Reg.aspx
...
<SCRIPT language="javascript"> 
   function checkuser() {
    var txtuid=document.getElementById("txtUid");
    Reg2.GetCheckUserName(txtuid.value,docallback);
   }
   function docallback(res) {
    alert(res.value);
    res = null;
   }
</SCRIPT>
...
<asp:TextBox id="txtUid" runat="server" Width="234px"></asp:TextBox>
        <asp:RegularExpressionValidator id="RegularExpressionValidator3" runat="server" ValidationExpression="^[a-zA-Z]\w{4,20}$"
         Display="Dynamic" ControlToValidate="txtUid" ErrorMessage="The word you entered was not allowed"></asp:RegularExpressionValidator>
        <asp:Button id="btCheckUserName" runat="server" Text="Check Availablilily of User ID"></asp:Button>
Reg.aspx.cs
...
privcate void Page_Load(object sender , System.EventArgs e)
{
     Ajax.Utility.RegisterTypeForAjax(typeof(EkoShop.Reg2));
}
...
[Ajax.AjaxMethod()]
  public string GetCheckUserName(string str)
  {
   string strUserName="";
   if(str==string.Empty)
   {
    return strUserName+="Please enter your name!";
   }
   else
   {
    validate=new Validate();
    if(validate.UserNameOnly(str))
    {
     return strUserName+="The UserName is exists!";
    }
    else
    {
     return strUserName+="The UserName is no exists!You are Reg.";
    }
   }
  }