Select Deselect Checkbox in gridview

I had a dropdown containing Users. When User is selected from dropdown corresponding Roles should be checked in Gridview. I had done it as shown below on Dropdown selected change event but it's not giving correct result.
<table align="center"> <tr> <td> <asp:Label ID="lblMsg" runat="server" ForeColor="Red"></asp:Label></td> <td></td> </tr> <%--<tr> <td><asp:TextBox ID="txtrolename" runat="server"></asp:TextBox></td> <td> <asp:Button ID="btnCreateRole" runat="server" Text="Create Role" onclick="btnCreateRole_Click" /> </td> </tr>--%> <tr> <td> <b>Select User:</b> </td> <td><asp:DropDownList ID="ddlUsers" runat="server" AutoPostBack=&quo ...