请问下面的代码，为什么跑到 .Fill 的部分会报“此结构不得为值类型”！
C# code        
        private DataTable SelectData(string strCommand, string strTableName)
        {
            mySqlDataAdapter.SelectCommand.Connection = mySqlConnection;
            mySqlDataAdapter. = strCommand;
            dataSet = new DataSet();
            mySqlDataAdapter.Fill(dataSet, strTableName);
            dataTable = dataSet.Tables[strTableName];
            return dataTable;
        }