如何基于数据行动态添加控件？

Question

我想根据数据库中的值动态添加控件

我试过，但收到错误

代码：

aspx.cs

using (OleDbCommand cmd = new OleDbCommand("Select * from visa_details where emp_id = '"+ empid +"'", DbConnection))
using (OleDbDataAdapter da = new OleDbDataAdapter(cmd))
{

    OleDbDataReader DR1 = cmd.ExecuteReader();
    int y = 0;
    while (DR1.Read())
    {
       string visaNumb = DR1[2].ToString();
       string visaCountry = DR1[3].ToString();
       string visaType = DR1[4].ToString();
       string visaEntry = DR1[5].ToString();
       string expiryDate = DR1[6].ToString();

       y++;
       for (int i = 0; i < y; i++)
       {
          VisaUserControl userconrol = new VisaUserControl();
          //Here I get "Object reference null error"
          userconrol.TextVisaNumber = visaNumb;
          userconrol.VisaCountry = visaCountry;
          userconrol.VisaType = visaType;
          userconrol.VisaEntry = visaEntry;
          userconrol.ExpiryDate = expiryDate;

          this.Controls.Add(userconrol);
       }
      }
    }      

的.ascx

<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="VisaUserControl.ascx.cs" Inherits="Portal.VisaUserControl" %>
<%@ Register Assembly="BasicFrame.WebControls.BasicDatePicker" Namespace="BasicFrame.WebControls" TagPrefix="dp" %>
<asp:ScriptManager ID="scriptmanager1" runat="server"></asp:ScriptManager>
<div id="divreg" runat="server">
<table id="tbl" runat="server">
<tr>
<td>
    <asp:Label ID="lbl2" runat="server"></asp:Label>
</td>
</tr>
<tr>
<td> Visa Number:</td>
<td><asp:TextBox ID="txtUser" Width="160px" runat="server"/></td>
<td> Country Name:</td>
<td><asp:DropDownList ID="dropCountry" Width="165px" runat="server"></asp:DropDownList>   </td>
</tr>
<tr>
<td> Type of Visa:</td>
<td><asp:DropDownList ID="dropVisa" Width="165px" runat="server" OnSelectedIndexChanged="dropVisa_SelectedIndexChanged"></asp:DropDownList></td>
<td> Type of Entry:</td>
<td><asp:DropDownList ID="dropEntry" Width="165px" runat="server"></asp:DropDownList> </td>
</tr>
<tr>
<td> Expiry Date</td>
<td><asp:TextBox ID="txtDate" runat="server"></asp:TextBox>
    <ajaxToolkit:CalendarExtender ID="CalendarExtender1" runat="server" 
                      TargetControlID="txtDate" PopupButtonID="Imgbtnfromdate" Format="dd/MM/yyyy">
                  </ajaxToolkit:CalendarExtender>
</td>
<td>
<asp:Button ID="btnRemove" Text="Remove" runat="server" OnClick="btnRemove_Click" />
</td>
</tr>
</table>
</div>

.ascx.cs

    protected void Page_Load(object sender, EventArgs e)
    {
        txtUser.Text = Request.Form[txtUser.UniqueID];
        dropCountry.SelectedValue = Request.Form[dropCountry.UniqueID];
        dropVisa.SelectedValue = Request.Form[dropVisa.UniqueID];
        dropEntry.SelectedValue = Request.Form[dropEntry.UniqueID];
        txtDate.Text = Request.Form[txtDate.UniqueID];
    }

    public string TextVisaNumber
    {
        get { return txtUser.Text; }
        set { txtUser.Text = value; }
    }

    public string VisaCountry
    {
        get { return dropCountry.SelectedValue; }
        set { dropCountry.SelectedValue = value; }
    }

    public string VisaType
    {
        get { return dropVisa.SelectedValue; }
        set { dropVisa.SelectedValue = value; }
    }

    public string VisaEntry
    {
        get { return dropEntry.SelectedValue; }
        set { dropEntry.SelectedValue = value; }
    }

    public string ExpiryDate
    {
        get
        {
            return txtDate.Text;
        }
        set
        {
            txtDate.Text = value;
        }
    }

有什么想法吗？提前致谢