Web Forms :: Trying To Populate The Value (s) Of A Listbox That Has Multiple Selections?

I am trying to populate the value(s) of a listbox that has multiple selections:

Assignment Preference:<br />

<asp:ListBox ID="assignmentPreferenceList" SelectionMode="Multiple" runat="server">

<asp:ListItem Value="">None</asp:ListItem>

<asp:ListItem Value="CONTRACTING">CONTRACTING</asp:ListItem>

<asp:ListItem Value="DIRECT HIRE">DIRECT HIRE</asp:ListItem>

<asp:ListItem Value="CONTRACTING-DIRECT HIRE">CONTRACTING-DIRECT HIRE</asp:ListItem>

</asp:ListBox>

I am using a datareader in code behind to "loop" through the values:

string sql4 = "Select * from assignmentPref where tID=" + (dr["id"].ToString());

SqlCommand comm4 = new SqlCommand(sql4, con3);

SqlDataReader dr4 = comm4.ExecuteReader();

while (dr4.Read())

{

assignmentPreferenceList.SelectedValue = (dr["assignmentPref"].ToString());

}
Works great with 1 assignment preference but with 2 or 3 it craps out.