FilterExpression Syntax In C#

I have a some search components to filter a GridView follows:

<asp:DropDownList ID="ddFilterType" runat="server" ToolTip="Select field to search">
   <asp:ListItem Selected="True">(ALL)</asp:ListItem>
   <asp:ListItem>SKU</asp:ListItem>
   <asp:ListItem>Vendor Name</asp:ListItem>
   <asp:ListItem>Item Name</asp:ListItem>
</asp:DropDownList>
<asp:TextBox ID="txtSearchFilter" runat="server"></asp:TextBox>
<asp:LinkButton ID="btnSearchFilter" runat="server">Filter</asp:LinkButton>

and C# code as follows:

protected void Page_Load(object sender, EventArgs e)
{
   if (IsPostBack)
   {
      if (ddFilterType.SelectedValue != "ALL")
      {
         dsItems.FilterExpression = (ddFilterType.SelectedValue = txtSearchFilter.Text);
         gvItems.DataBind();
      }
      else { gvItems.DataBind(); }
   }
}

But when I run the search, I get:
Exception Details: System.Data.EvaluateException: Cannot find column [the value in txtSearchFilter.Text].

How should I do my C# syntax to get this to work? Also, I would like to add further logic to check if the field is not the SKU field and have my expression do a LIKE expression instead of checking for equality. Any help with this is much appreciated.

Marc