//连接字符串
string connectionString = @"Data Source=User;Initial Catalog=guanli;Integrated Security=True";
//建立连接
SqlConnection myCon = new SqlConnection(connectionString);
myCon.Open();
//读取所输入用户名的密码
SqlCommand myComm = myCon.CreateCommand();
string sql = "select username,password,usertype from T_User where(username = N'"+txtUsername.Text.Trim()+"')";
myComm.CommandText = sql;
SqlDataReader myReader = myComm.ExecuteReader();

能详细解说一下这段代码吗???

我喜欢这样的格式写：

SqlCommand cmd=new Sqlcommand("select ....",conn);

SqlDataReader read=cmd.ExecuteReader();

while(read.Read())
{
....
}