Try this
public List<item> getrID()
{
List<item> IDAll = new List<item>();
int ID;
string queryStr = "Select Item.ID FROM Item JOIN Food on Item.ID= Food.ID ";
SqlConnection conn = new SqlConnection(_connStr);
SqlCommand cmd = new SqlCommand(queryStr, conn);
conn.Open();
SqlDataReader dr = cmd.ExecuteReader();
while (dr.Read())
{
string id = dr["ID"].ToString();
ID = int.Parse(id);
IDAll.Add(new Item (ID));
}
conn.Close();
dr.Close();
dr.Dispose();
return IDAll;
}</item></item></item>