i am facing this exception that 

Unable to cast object of type 'System.String' to type 'System.Byte[]'.

<asp:TemplateField HeaderText="Image">
    <ItemTemplate>
        <%--<asp:Image ID="img" runat="server" ImageUrl='<%# Eval("Data").ToString() %>' Height="100px" Width="100px" />--%>
        <asp:Image  ID="Data" runat="server" Width="100px" Height="80px"  ImageUrl='<%# "data:image/jpg;base64," + Convert.ToBase64String((byte[])Eval("Data")) %>' />
    </ItemTemplate>
</asp:TemplateField>