Setting Session Variable from Dropdown List

I have a dropdown list with this source code:
        <asp:DropDownList ID="ddlProject" runat="server" DataSourceID="SqlProject" DataTextField="ProjectName"
            DataValueField="ProjectName">   

I am trying to get the value of dropdown box to be a session value so all subsequent pages will reference the ProjectName selected.  If the value in the dropdown list changes, the session value will change as well. 
I tried the following code behind statement:
Protected Sub ddlProject_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ddlProject.SelectedIndexChanged
        Session["Projectname"]=ddlProject
    End Sub
I get an error:
Property access must assign t ...