<connectionStrings>
 <add name="connstr" connectionString="Data Source=.;Initial Catalog=Milk ;User Id=sa;Password=521521521;"></add>
 </connectionStrings>

public static readonly string connstr =
ConfigurationManager.ConnectionStrings["connstr"].ConnectionString;

//SqlConnection为建立和数据库连接的对象
using(SqlConnection conn = new SqlConnection(connstr))
{
conn.Open();//打开连接
//通过连接创建一个向数据库发命令（Command）的对象SqlCommand
using(SqlCommand cmd = conn.CreateCommand())
{
cmd.CommandText =”Insert into T_Student*(Name,Age) values(‘XXX'，18)”;// CommandText要执行的SQL语句
cmd.ExecuteNonQuery();//执行
}
}

using(SqlConnection conn = new SqlConnection(connstr))
{
conn.Open();//打开连接
//通过连接创建一个向数据库发命令（Command）的对象SqlCommand
using(SqlCommand cmd = conn.CreateCommand())
{
cmd.CommandText =”select Name from T_Student where Name=”XXX””;// CommandText要执行的SQL语句
cmd.ExecuteScalar();//执行
}
}

using(SqlConnection conn = new SqlConnection(connstr))
{
conn.Open();//打开连接
//通过连接创建一个向数据库发命令（Command）的对象SqlCommand
using(SqlCommand cmd = conn.CreateCommand())
{
cmd.CommandText =”select * from T_Student where Age<18”;
using(SqlDataReader reader=cmd.ExecuteReader()
{
  while(reader.Read())
{
 string name=reader.GetString(1);//获得第一列列的值
 int age=reader.GetIn32(2); //获得第2列列的值
 Console.WriteLine(name);
 Console.WriteLine(age.ToString());
}
}
}
}

DataSet dataset=new DataSet（）；
SqlDataAdapter adapter=new SqlDataAdapter(cmd);
adapter.Fill(dataset);

DataTable table=dataset.Tables[0];//一般情况下只有一个Tables，当同时执行多个select语句时有多个Tables。

DataRowCollection rows=table.Rows;
for(int i=0;i<rows.Count;i++)
{
  DataRow row=rows[i];
int age=(int)row[“Age”];//遍历年龄
}

public static class SqlHelper
  {
    public static readonly string connstr =
      ConfigurationManager.ConnectionStrings["connstr"].ConnectionString;
    public static SqlConnection OpenConnection()//建立连接
    {
      SqlConnection conn = new SqlConnection(connstr);
      conn.Open();
      return conn;
    }
    public static int ExecuteNonQuery(string cmdText,
      params SqlParameter[] parameters)//注意看有使用长度可变参数进行了简化
    {
      using (SqlConnection conn = new SqlConnection(connstr))
      {
        conn.Open();
        return ExecuteNonQuery(conn, cmdText, parameters);
      }
    }
    public static object ExecuteScalar(string cmdText,
      params SqlParameter[] parameters)
    {
      using (SqlConnection conn = new SqlConnection(connstr))
      {
        conn.Open();
        return ExecuteScalar(conn, cmdText, parameters);
      }
    }
    public static DataTable ExecuteDataTable(string cmdText,
      params SqlParameter[] parameters)
    {
      using (SqlConnection conn = new SqlConnection(connstr))
      {
        conn.Open();
        return ExecuteDataTable(conn, cmdText, parameters);
      }
    }
    public static int ExecuteNonQuery(SqlConnection conn,string cmdText,
      params SqlParameter[] parameters)
    {
      using (SqlCommand cmd = conn.CreateCommand())
      {
        cmd.CommandText = cmdText;
        cmd.Parameters.AddRange(parameters);
        return cmd.ExecuteNonQuery();//返回执行了多少行
      }
    }
    public static object ExecuteScalar(SqlConnection conn, string cmdText,
      params SqlParameter[] parameters)
    {
      using (SqlCommand cmd = conn.CreateCommand())
      {
        cmd.CommandText = cmdText;
        cmd.Parameters.AddRange(parameters);
        return cmd.ExecuteScalar();
      }
    }
    public static DataTable ExecuteDataTable(SqlConnection conn, string cmdText,
      params SqlParameter[] parameters)
    {
      using (SqlCommand cmd = conn.CreateCommand())
      {
        cmd.CommandText = cmdText;
        cmd.Parameters.AddRange(parameters);
        using (SqlDataAdapter adapter = new SqlDataAdapter(cmd))
        {
          DataTable dt = new DataTable();
          adapter.Fill(dt);
          return dt;
        }
      }
    }
    public static object ToDBValue(this object value)
    {
      return value == null ? DBNull.Value : value;
    }
    public static object FromDBValue(this object dbValue)
    {
      return dbValue == DBNull.Value ? null : dbValue;
    }
  }

object objName;
string name=tbName.Text;
if(name.Length<0)
{
  objName=DBNull.Value;
}else
{
  objName=name;
}

if（row[“Name”]==DBNull.Value）
{
 name=null;
}
else
{
 name=(string)row[“Name”];
}

public static object ToDBValue(this object value)

    {
      return value == null ? DBNull.Value : value;
    }

    public static object FromDBValue(this object dbValue)
    {
      return dbValue == DBNull.Value ? null : dbValue;
    }