将GridView头模板中的文本框作为sqldatasource select中的参数传递

我有一个使用sqldatasource显示数据库表内容的GridView。这个部分(包括插入、更新和删除行)工作得很好。我想做的是添加一个“搜索”功能,以限制返回到用户输入名称的行。我有一个要搜索的名称文本框和HeaderTemplate中的“搜索”按钮,例如:

 <HeaderTemplate>
    <asp:Button ID="btnShowAll" runat="server" CausesValidation="False" CommandName="ShowAll" Text="Show All" Visible="false" />
    <asp:Button ID="btnSearch" runat="server" CausesValidation="True" CommandName="Search" Text="Search" ValidationGroup="vldSearch" />
    <asp:TextBox ID="txtSearchName" runat="server"></asp:TextBox>
    <asp:RequiredFieldValidator ID="vldSearchName" runat="server" ErrorMessage="You have to provide an attorney name to search for." Text="*" ControlToValidate="txtSearchName" ValidationGroup="vldSearch" ForeColor="White"></asp:RequiredFieldValidator>
 </HeaderTemplate>   

现在,在rowcommand事件处理程序中,我有以下代码:

 Case "Search"
   If Page.IsValid Then
       Dim btnSearch As Button = Me.dgAttorneys.HeaderRow.FindControl("btnSearch")
       Dim btnShowAll As Button = Me.dgAttorneys.HeaderRow.FindControl("btnShowAll")
       Dim txtSearchName As TextBox = Me.dgAttorneys.HeaderRow.FindControl("txtSearchName")
       Me.sqlAttorneys.SelectCommand = "SELECT [userID], [login], [username], [password] FROM [attorney] WHERE [username] LIKE '%' + @username + '%' ORDER BY [login]"
       Dim parmUserName As New ControlParameter("username", "txtSearchName", "Text")
       Me.sqlAttorneys.SelectParameters.Add(parmUserName)
       Me.dgAttorneys.DataBind()
       Me.dgAttorneys.PageIndex = 0
       btnSearch.Visible = False
       btnShowAll.Visible = True
       txtSearchName.Visible = False
   End If

数据绑定失败,错误为:

在controlParameter“username”中找不到控件“txtSearchName”。

关于实现这一目标的替代方法有什么建议吗?

谢谢。