无法访问从aspx到cs的变量

Question

我有问题。我在c＃中创建了asp.net网页。我使用visuals studio为您提供的默认母版页创建了它。在register.aspx中有一个用户名的文本框，如tihs：

                            <asp:Label ID="UserNameLabel" runat="server" AssociatedControlID="UserName">User Name:</asp:Label>
                            <asp:TextBox ID="UserName" runat="server" CssClass="textEntry"></asp:TextBox>
                            <asp:RequiredFieldValidator ID="UserNameRequired" runat="server" ControlToValidate="UserName" 
                                 CssClass="failureNotification" ErrorMessage="User Name is required." ToolTip="User Name is required." 
                                 ValidationGroup="RegisterUserValidationGroup">*</asp:RequiredFieldValidator>

在cs文件夹中，在按钮下面我放了一个代码，将注册用户放入数据库，但是找不到文本框的变量。这有什么问题？

protected void RegisterUser_CreatedUser(object sender, EventArgs e)
{
    //FormsAuthentication.SetAuthCookie(RegisterUser.UserName, false 

    //string continueUrl = RegisterUser.ContinueDestinationPageUrl;
    //if (String.IsNullOrEmpty(continueUrl))
    //{
    //    continueUrl = "~/";
    //}
    //Response.Redirect(continueUrl);

        try
        {
            SqlConnection objcon = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["ConnectionString"].ToString());
            DataTable objdatatable = new DataTable();
            string validationquery = "select * from Uporabniki where username=`" + UserName.Text + "`";
            SqlDataAdapter objdavalidate = new SqlDataAdapter(validationquery, objcon);
            objdavalidate.SelectCommand.CommandType = CommandType.Text;
            objdavalidate.Fill(objdatatable);
            if (objdatatable.Rows.Count > 0)
            {
               ...