怎么查询页面上所有textbox，并赋值

如何查询页面上所有textbox，并赋值。
我的代码如下（asp.net2005）
For Each c As Control In Page.Controls
  If TypeOf c Is TextBox Then
  Select Case c.ClientID
  Case "TextBox1"
  DirectCast(c, TextBox).Text = "a1"
  Case "TextBox2"
  DirectCast(c, TextBox).Text = "a2"
  Case "TextBox3"
  DirectCast(c, TextBox).Text = "a3"
  End Select
  End If
  Next

不知道哪里出错，就是不能赋值。请各路高手指点

[解决办法]

VB.NET code

For Each c As Control In Me.Controls             If c Is TextBox Then                 Select Case c.ClientID                     Case "TextBox1"                         DirectCast(c, TextBox).Text = "a1"                     Case "TextBox2"                         DirectCast(c, TextBox).Text = "a2"                     Case "TextBox3"                         DirectCast(c, TextBox).Text = "a3"                 End Select             End If         Next
[解决办法]
string id = string.Empty;
           string controlID = string.Empty;
           for (int i = 0; i < this.Controls.Count; i++)
           {
               foreach (System.Web.UI.Control control in this.Controls[i].Controls)
               {
                   if (control is TextBox)
                   {
                       controlID = (control as TextBox).ID;
                       string[] textBoxHeads = textBoxHead.Split(',');
                       for (int j = 0; j < textBoxHeads.Length; j++)
                       {
                           if (controlID.IndexOf(textBoxHeads[j]) == 0)
                           {
                               id = controlID.Substring(textBoxHeads[j].Length);
                               if (dt.ContainCol(id))
                               {
                                   //try
                                   //{
                                   (control as TextBox).Text = dt.GetValue(id).Trim();
                                   //}
                                   //catch
                                   //{
                                   //}
                               }
                           }
                       }

                   }
                   else if (control is HtmlInputHidden)
                   {
                       controlID = (control as HtmlInputHidden).ID;
                       string[] valueListHeads = hiValueHead.Split(',');
                       for (int j = 0; j < valueListHeads.Length; j++)
                       {
                           if (controlID.IndexOf(valueListHeads[j]) == 0)
                           {
                               id = controlID.Substring(valueListHeads[j].Length);
                               if (dt.ContainCol(id))
                               {
                                   (control as HtmlInputHidden).Value = dt.GetValue(id).Trim();
                               }
                           }
                       }
                   }
                   else if (control is HtmlInputText)
                   {
                       controlID = (control as HtmlInputText).ID;
                       string[] valueListHeads = hiValueHead.Split(',');
                       for (int j = 0; j < valueListHeads.Length; j++)
                       {
                           if (controlID.IndexOf(valueListHeads[j]) == 0) 
 
                           {
                               id = controlID.Substring(valueListHeads[j].Length);
                               if (dt.ContainCol(id))
                               {
                                   (control as HtmlInputText).Value = dt.GetValue(id).Trim();
                               }
                           }
                       }
                   }
                   else if (control is DropDownList)
                   {
                       controlID = (control as DropDownList).ID;
                       string[] dropDownListHeads = dropDownListHead.Split(',');
                       for (int j = 0; j < dropDownListHeads.Length; j++)
                       {
                           if (controlID.IndexOf(dropDownListHeads[j]) == 0)
                           {
                               id = controlID.Substring(dropDownListHeads[j].Length);
                               if (dt.ContainCol(id))
                               {
                                   (control as DropDownList).SelectedValue = dt.GetValue(id).Trim();
                               }
                           }
                       }
                   }
                   //(control as TextBox).Text = "";
               }
           }