filter expression with or condition is not workng

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C#

ASP.NET

SQL

Hi,

My problem is I am using filter expression in gridview with two control parameters with or operator. But or operator is not working ?

I mean i have to fill both textbox control then data display otherwise default gridview is showing.

My code is

<asp:SqlDataSource ID="SqlDataSource2" runat="server"
                            ConnectionString="<%$ ConnectionStrings:hospital %>"
                            DeleteCommand="DELETE FROM [Purchaseentry] WHERE [Billno] = @Billno"
                            SelectCommand="SELECT * FROM [Purchaseentry]  " FilterExpression="Convert(bill , 'System.String') LIKE '{0}%' OR itemname Like '{1}%'"
                            UpdateCommand="StoredUpdatepurchase" UpdateCommandType="StoredProcedure"
                              >
                            <FilterParameters>
                               <asp:ControlParameter Name="bill" ControlID="txtSearch" PropertyName="Text" ConvertEmptyStringToNull="true"   />
                                  <asp:ControlParameter Name="itemname" ControlID="txtitemname" PropertyName="Text" ConvertEmptyStringToNull="true"   />

                           </FilterParameters>