C# ขอถามเกี่ยวกับ SqlDataReader เช็คค่าว่าเป็น null หรือไม่

                        ArrayList getSize(string gene1, ArrayList data)
                        SqlConnection con;
                        SqlCommand Cmd = new SqlCommand();
                        SqlDataReader dtReader;
                        string genome1 = gene1;
                        string g1;
                        con = new SqlConnection();
                        con.ConnectionString = "...";
                        Cmd.CommandText = "SELECT DISTINCT(genomeCluster) As gecluster FROM dbo.genomeFromFile where geneID like '%" + genome1 + "%'";
                        Cmd.Connection = con;
                        Cmd.Connection.Open();
                        dtReader = Cmd.ExecuteReader();
                        dtReader.Read();
                        do
                        {
                                g1 = dtReader["gecluster"].ToString();
                                data.Add(g1);
                        }
                        while (dtReader.Read());
                        dtReader.Close();
                        con.Close();
                        return data;
                }