数据集关系 - ArgumentNullExpression

Question

我正在使用Visual Studio上的C＃编写数据库，并在尝试添加关系时遇到问题。这是代码：

    static void ImportDatabase() {
        OleDbConnection mConn = new OleDbConnection(conn);
        string cmd = "SELECT * FROM ";
        string[] table = { "Artist", "Properties", "Record", "RecordLabel", "Records" };

        dataSet = new DataSet();

        for (int i = 0; i < 5; i++) {
            try {
                OleDbCommand mCmd = new OleDbCommand((cmd + table[i]), mConn);
                OleDbDataAdapter mDA = new OleDbDataAdapter(mCmd);

                mConn.Open();
                mDA.Fill(dataSet, table[i]);
            } catch (Exception ex) {
                Console.WriteLine("Error: Failed to retrieve the required data from the DataBase.\n{0}", ex.Message);
                return;
            } finally {
                mConn.Close();
            }

        }

        dataSet.Tables["Artist"].PrimaryKey = new DataColumn[] { dataSet.Tables["Artist"].Columns["ArtistID"] };
        dataSet.Tables["RecordLabel"].PrimaryKey = new DataColumn[] { dataSet.Tables["RecordLabel"].Columns["LabelID"] };
        dataSet.Tables["Record"].PrimaryKey = new DataColumn[] { dataSet.Tables["Record"].Columns["RecordID"] };

        dataSet.Relations.Add("Artist_Records", dataSet.Tables["Record"].Columns["ArtistID"], dataSet.Tables["Artist"].Columns["ArtistID"]);

        dataSet.Relations.Add("Label_Records", dataSet.Tables["Record"].Columns["LabelID"], dataSet.Tables["RecordLabel"].Columns["LabelID"]);


    }

我得到的错误是＆＃34;未处理的类型＆＃39; System.ArgumentNullException＆＃39;发生在System.Data.dll＆＃34 ;;我该怎么办？