How to run a class method in an aspx file?

I have a recived an old web project, which I have translated from VB to C#.
All works fine except one thing, a dropdownlist which is databound will not run a class method.

I'll try to explain better, I have:

1. SqlDataSource sqlOwner with current select: SELECT SYSTEM_ID, OWNER FROM C_OWNER
2. Dropdownlist ddlOwner which has sqlOwner as DataSourceID and OWNER as DataTextField(see code 2)
3. A Common.cs class which lies under App_Code(see code 3)

The custom binding for the ddlOwner is: Common.GetSelectedDDLValue(Eval("OWNER"), string.Empty)

But when I try to run this code I get an error, see attachment error_msg.png

I manage to run the Common methods from the underlying aspx.cs files but not from the aspx files.
I'm kinda new to this, but I though the reason for the App_Code folder was so that the Classes/Methods in there could be used in the aspx files?
Can anyone help me with this, as I have a deadline soon on this project and need to get it working.

Regards
Marius

2. 
<asp:DropDownList ID="ddlOwner" runat="server" Width="350px"
                            DataSourceID="sqlOwner" AppendDataBoundItems="true" DataTextField="OWNER" DataValueField="SYSTEM_ID"  
                            SelectedValue='<%# Common.GetSelectedDDLValue(Eval("OWNER"), string.Empty) %>' >
                            <asp:ListItem Text="" Value="" />
</asp:DropDownList>
 
3.
public class Common
{
        public static string GetSelectedDDLValue(object evalValue, string notSelectedValue)
        {
           //CODE
        }
}

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error-msg.PNG