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static void Main(string[] args)
{
string connectString = "data source=127.0.0.1;initial catalog=test;integrated security=True;";
using (SqlConnection conn = new SqlConnection(connectString))
{
conn.Open();
SqlCommand command = conn.CreateCommand();
IDataReader reader;
string CmdText;
//ToList 是扩展函数,位于 Extensions 类中 实现 IDataReader 与 DataTable 扩展
//加载常规类型
CmdText = "select U.UserID from [UserRole] AS UR INNER JOIN [User] AS U ON U.UserID = UR.UserID INNER JOIN [Role] AS R ON R.RoleID = UR.RoleID";
command.CommandText = CmdText; //加载用户id
reader = command.ExecuteReader();
List<int> UserIDs = reader.ToList<int>(); //常规类型只会加载每一个字段,不区分大小名称
CmdText = "select U.UserName from [UserRole] AS UR INNER JOIN [User] AS U ON U.UserID = UR.UserID INNER JOIN [Role] AS R ON R.RoleID = UR.RoleID";
command.CommandText = CmdText; //加载用户名称
reader = command.ExecuteReader(); //加载完成必须重新执行,若想一次读取多次加载,请使用DataTable
List<string> UserNames = reader.ToList<string>();
//加载对象
CmdText = "select U.UserID,U.UserName,R.RoleID,R.RoleName from [UserRole] AS UR INNER JOIN [User] AS U ON U.UserID = UR.UserID INNER JOIN [Role] AS R ON R.RoleID = UR.RoleID";
command.CommandText = CmdText;
reader = command.ExecuteReader();
List<User> Users = reader.ToList<User>(); //用户集合
reader = command.ExecuteReader();
List<Role> Roles = reader.ToList<Role>(); //角色集合
//以上都是简单的对象,大多数人都有自己的包
//匿名对象(初始值确类型,类型要与数据库字段的类型可转换 名称一定要与字段名称相同,不区分大小写,看特性说明)
//简单
reader = command.ExecuteReader();
var u1 = reader.ToList(a => new { UserID = 0, UserName = "", RoleName = "" });
//简单,UserID 可为空(即数据库字段可为空值)
reader = command.ExecuteReader();
var u2 = reader.ToList(a => new { UserID = new Nullable<int>(), UserName = "", RoleName = "" });
//与对象混合
reader = command.ExecuteReader();
var u3 = reader.ToList(a => new { UserID = 0, UserName = "", user = new User(), role = new Role() });
//嵌套
reader = command.ExecuteReader();
var u4 = reader.ToList(a => new
{
UserID = 0,
UserName = "",
C = new
{
RoleID = 0,
RoleName = "",
user = new User(),
role = new Role(),
D = new { UserID = 0 }
}
});
}
}