SelectedIndexChanged Event of GridView? URGENT!

A having a grid which fetches data from the database dynamically.The Code is as follows,

<asp:GridView ID="gvdesig" runat="server" AutoGenerateColumns="False" OnRowDataBound="gvdesig_RowDataBound"
                                                 CellPadding="3"  onrowcommand="gvdesig_RowCommand"  CssClass="GridViewStyle" Width="90%" Height="180px" 
                                                  OnSorting="gvdesig_Sorting" OnPageIndexChanging="gvdesig_PageIndexChanging"  OnSelectedIndexChanged="gvdesig_OnSelectedIndexChanged">
                                                    <Columns>
                                                        <asp:BoundField DataField="DesignationID" HeaderText="ID" ItemStyle-Width="40px" ItemStyle-HorizontalAlign="Center"  />
                                                        <asp:BoundField DataField="Designation" HeaderText="Name" ItemStyle-Width="200px" ItemStyle-HorizontalAlign="Center" />

                                                        <asp:TemplateField HeaderText="Select" ItemStyle-Width="40px" ItemStyle-HorizontalAlign="Center">
                                                        <ItemTemplate>
                                                            <asp:Button ID="btnselect" runat="server" CommandName="select" Text="select" CausesValidation="false" />
                                                        </ItemTemplate>
                                                        </asp:TemplateField>
                                                    </Columns>
                                                  
                                        </asp:GridView>

 protected void gvdesig_OnSelectedIndexChanged(object sender, EventArgs e)
    {

        txtdesig.Text = gvdesig.SelectedRow.Cells[1].Text;
        lblDesig.ToolTip = gvdesig.SelectedRow.Cells[0].Text;
        ddldept.SelectedItem.Text = gvdesig.SelectedRow.Cells[2].Text;

    }

At first while executing i got the following error when the grid row is selected,

Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ Page EnableEventValidation="true" %> in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation.

I added EnableEventValidation="false" in my page directive,now it does'nt throw error but still the SelectedIndexChanged Event is not firing ? Its urgent Please Guide me i dont know where i am wrong. Thanks in advance