ADO.NET读取MySQL数据库的三种方式：DataReader、DataSet、DataView

ADO.NET读取MySQL数据库有多种方式：DataReader、DataSet、DataView。
Command对象的ExecuteScalar方法查询数据库获取某个单个值，但是如果获取多行、多列可以用ExcecuteReader，ExcecuteReader返回一个DataReader的数据流对象。
DataSet是存在于内存中的数据库，不依赖于数据库的独立数据集合，内部是用XML来描述数据的，需要与DataAdapter来填充数据。
DataView表示用于排序、筛选、搜索、编辑和排序的DataTable的可绑定数据的自定义视图。
本文使用以上三种方式做了一个实例：

using MySql.Data.MySqlClient;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApp2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        MySqlConnection conn;
        string sql = @"SELECT * FROM users WHERE 1=1 ";
        DataSet dataSet;

        private void Form1_Load(object sender, EventArgs e)
        {
            string connStr = "server=localhost;Database=pikachu;uid=root;pwd=root;charset=utf8";
            conn = new MySqlConnection(connStr);
        }



        private void btnDataReader_Click(object sender, EventArgs e)
        {
            try
            {
                MySqlDataReader dataReader;
                

                if(conn.State != ConnectionState.Open)
                {
                    conn.Open();
                }

                if (conn != null && conn.State == ConnectionState.Open)
                {
                    MySqlCommand command = new MySqlCommand(sql, conn);
                    dataReader = command.ExecuteReader();
                    // MessageBox.Show("数据准备完毕！");
                    StringBuilder stringBuilder = new StringBuilder();

                    if (dataReader.HasRows)
                    {
                        while (dataReader.Read())
                        {
                            stringBuilder.Append(string.Format("ID:{0} 用户名：{1}\r\n", dataReader[0].ToString(),
                                dataReader[1].ToString()));
                        }
                    }
                    dataReader = null;
                    conn.Close();
                    MessageBox.Show(stringBuilder.ToString());
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

        private void btnDataSet_Click(object sender, EventArgs e)
        {
            try {
            if (conn.State != ConnectionState.Open)
            {
                conn.Open();
            }

            if (conn != null && conn.State == ConnectionState.Open)
            {
                MySqlDataAdapter dataAdapter = new MySqlDataAdapter();
                dataSet = new DataSet();
                dataAdapter.SelectCommand = new MySqlCommand(sql, conn);
                dataAdapter.Fill(dataSet);
                // 拼接字符串
                StringBuilder stringBuilder = new StringBuilder();
                // DataTable对象
                DataTable dataTable = dataSet.Tables[0];
                for (int i = 0; i < dataTable.Rows.Count; i++)
                {
                    stringBuilder.Append(string.Format("ID:{0}, 用户名：{1}\r\n",
                        dataTable.Rows[i][0], dataTable.Rows[i][1]));
                }
                conn.Close();
                MessageBox.Show(stringBuilder.ToString());
            }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

        private void btnDataView_Click(object sender, EventArgs e)
        {
            if (dataSet == null)
            {
                MessageBox.Show("dataSet不能为空");
                return;
            }
            DataView dataView = dataSet.Tables[0].DefaultView;
            dataView.RowFilter = "id>1";
            dataView.Sort = "id DESC";
            StringBuilder stringBuilder = new StringBuilder();
            foreach(DataRowView dataRowView in dataView)
            {
                for (int i = 0; i < dataView.Table.Columns.Count; i++)
                {
                    stringBuilder.Append(dataRowView[i].ToString() + "\r\n");
                }
            }
            MessageBox.Show(stringBuilder.ToString());
        }
    }
}