string connStr = @"Data Source=D:\database.mdb; Provider=Microsoft.Jet.OleDb.4.0;"; //数据库连接字符串 
OleDbConnection conn = new OleDbConnection(connStr); 

//数据库和客户端在同一台机器上 
using System.Data.OracleClient; 
string connStr = "data source=orcl;user=user;password=pwd;"; 
OracleConnection conn = new OracleConnection(connStr); 
//客户端和数据库不在同一台机器，可以使用下面的连接方式 
using System.Data.OleDb; 
//HOST是Oracle数据库服务器地址，PORT是Oracle数据库端口，SERVICE_NAME是数据库名称 
string connStr = "Provider=OraOLEDB.Oracle.1;Data Source=(DESCRIPTION =(ADDRESS_LIST =(ADDRESS = (PROTOCOL = TCP)(HOST = 127.0.0.1)(PORT = 1521)))(CONNECT_DATA =(SERVICE_NAME = orcl)));User ID=message;Password=message;"; 
OleDbConnection conn = new OleDbConnection(connStr); 

using System.Data.SqlClient; 
string connStr = "data source=127.0.0.1;initial catalog=database;user id=sa;pwd=sa"; 
SqlConnection conn = new SqlConnection(connStr); 

using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Text; 
using System.Data.SqlClient; 
 
namespace Login 
{ 
 class Program 
 { 
  static void Main(string[] args) 
  { 
   //新建一个数据库连接 
   using(SqlConnection conn = new SqlConnection(GetConnectString())) 
   { 
    conn.Open();//打开数据库 
    //Console.WriteLine("数据库打开成功!"); 
    //创建数据库命令 
    SqlCommand cmd = conn.CreateCommand(); 
    //创建查询语句 
    cmd.CommandText = "SELECT * FROM userinfo"; 
    //从数据库中读取数据流存入reader中 
    SqlDataReader reader = cmd.ExecuteReader();     
     
    //从reader中读取下一行数据,如果没有数据,reader.Read()返回flase 
    while (reader.Read()) 
    { 
     //reader.GetOrdinal("id")是得到ID所在列的index, 
     //reader.GetInt32(int n)这是将第n列的数据以Int32的格式返回 
     //reader.GetString(int n)这是将第n列的数据以string 格式返回 
     int id = reader.GetInt32(reader.GetOrdinal("id")); 
     string name = reader.GetString(reader.GetOrdinal("name")); 
     string pwd = reader.GetString(reader.GetOrdinal("password")); 
     int age = reader.GetInt32(reader.GetOrdinal("age")); 
     string sex = reader.GetString(reader.GetOrdinal("sex")); 
     string phone = reader.GetString(reader.GetOrdinal("phone")); 
     string address = reader.GetString(reader.GetOrdinal("Address")); 
 
     //格式输出数据 
     Console.Write("ID:{0},Name:{1},PWD:{2},Age:{3},Sex:{4},Phone{5},Address:{6}\n", id, name, pwd, age, sex, phone, address); 
    } 
   } 
   Console.ReadKey(); 
  } 
  //得到一个数据库连接字符串 
  static string GetConnectString() 
  { 
   return "Data Source=(local);Initial Catalog=db1;Integrated Security=SSPI;"; 
  } 
 } 
} 

DataTable dt;        //假设字段为name, dt已经保存了数据 
dt.rows[0]["name"] == System.DBNull.Value; //判断第一行数据的name字段是否为空 

DataTable dt;    //假设字段为name, dt已经保存了数据 
dt.rows[0].IsNull("name"); //判断第一行数据的name字段是否为空 

DataTable dt;       //假设字段为name, dt已经保存了数据 
dt.rows[0]["name"].ToString() == ""; //判断第一行数据的name字段是否为空