注册时向角色添加用户

我使用asp.net应用程序中内置的注册模块来注册新用户。我试图在用户注册时设置角色，但由于某种原因，我无法访问后端代码中的文本框控件以将新用户添加到所选角色，我也试图用此代码找到文本框，但它返回null，这意味着无法找到该控件:

 TextBox txtUsername = (TextBox)Page.FindControl("UserName");
            Roles.AddUserToRole(txtUsername.Text, "User");  

还有，这里是register模块的html标记:

   <asp:CreateUserWizardStep ID="RegisterUserWizardStep" runat="server">
                <ContentTemplate>
                    <h2>
                        Create a New Account
                    </h2>
                    <p>
                        Use the form below to create a new account.
                    </p>
                    <p>
                        Passwords are required to be a minimum of <%= Membership.MinRequiredPasswordLength %> characters in length.
                    </p>
                    <span class="failureNotification">
                        <asp:Literal ID="ErrorMessage" runat="server"></asp:Literal>
                    </span>
                    <asp:ValidationSummary ID="RegisterUserValidationSummary" runat="server" CssClass="failureNotification" 
                         ValidationGroup="RegisterUserValidationGroup"/>
                    <div class="accountInfo">
                        <fieldset class="register">
                            <legend>Account Information</legend>
                            <p>
                                <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>
                            </p>
                            <p>
                                <asp:Label ID="EmailLabel" runat="server" AssociatedControlID="Email">E-mail:</asp:Label>
                                <asp:TextBox ID="Email" runat="server" CssClass="textEntry"></asp:TextBox>
                                <asp:RequiredFieldValidator ID="EmailRequired" runat="server" ControlToValidate="Email" 
                                     CssClass="failureNotification" ErrorMessage="E-mail is required." ToolTip="E-mail is required." 
                                     ValidationGroup="RegisterUserValidationGroup">*</asp:RequiredFieldValidator>
                            </p>
                            <p>
                                <asp:Label ID="PasswordLabel" runat="server" AssociatedControlID="Password">Password:</asp:Label>
                                <asp:TextBox ID="Password" runat="server" CssClass="passwordEntry" TextMode="Password"></asp:TextBox>
                                <asp:RequiredFieldValidator ID="PasswordRequired" runat="server" ControlToValidate="Password" 
                                     CssClass="failureNotification" ErrorMessage="Password is required." ToolTip="Password is required." 
                                     ValidationGroup="RegisterUserValidationGroup">*</asp:RequiredFieldValidator>
                            </p>
                            <p>
                                <asp:Label ID="ConfirmPasswordLabel" runat="server" AssociatedControlID="ConfirmPassword">Confirm Password:</asp:Label>
                                <asp:TextBox ID="ConfirmPassword" runat="server" CssClass="passwordEntry" TextMode="Password"></asp:TextBox>
                                <asp:RequiredFieldValidator ControlToValidate="ConfirmPassword" CssClass="failureNotification" Display="Dynamic" 
                                     ErrorMessage="Confirm Password is required." ID="ConfirmPasswordRequired" runat="server" 
                                     ToolTip="Confirm Password is required." ValidationGroup="RegisterUserValidationGroup">*</asp:RequiredFieldValidator>
                                <asp:CompareValidator ID="PasswordCompare" runat="server" ControlToCompare="Password" ControlToValidate="ConfirmPassword" 
                                     CssClass="failureNotification" Display="Dynamic" ErrorMessage="The Password and Confirmation Password must match."
                                     ValidationGroup="RegisterUserValidationGroup">*</asp:CompareValidator>
                            </p>
                        </fieldset>
                        <p class="submitButton">
                            <asp:Button ID="CreateUserButton" runat="server" CommandName="MoveNext" Text="Create User" 
                                 ValidationGroup="RegisterUserValidationGroup"/>
                        </p>
                    </div>
                </ContentTemplate>
                <CustomNavigationTemplate>
                </CustomNavigationTemplate>
            </asp:CreateUserWizardStep>

你知道我做错了什么吗?谢谢,拉齐奥人

private TextBox getUserNameTextBox(ControlCollection ctls)
    {
        foreach (Control c in ctls)
        {
            if (c is System.Web.UI.WebControls.TextBox)
            {
                if (c.ID == "UserName")
                    return c;
            }
            if (c.HasControls())
            {
                getAllCtl(c.Controls);

            }
        }
        return null;
    }
YourUserNameTextBox = getuserNameTextBox(Page.Controls);