DBNull和可空类型

  private static int GetAge()
        {
            DataTable dt = SqlHelper.ExecuteDataTable("select * from T_Customers where id=4");
            T_Customers cust = new T_Customers();
            DataRow row = dt.Rows[0];
            //int i = null;//因为在C#中，int、bool、DateTime等值类型的数据不可以为null
            //string,Person等属于引用类型，可以为null

            //值类型：拷贝传递，不可以为null
            //引用类型：引用传递，可以为null。

            //如果数据库里面存的是NULL，那么用row["TelPhone"]
            //取出来的数据的值是DBNull.value，ADO.Net对于C#中int不可以为null
            //数据库中int可以为null的一个权衡
            //string s1 = null;
            //Person p1 = null;
            //DBNull.Value; null
            //null,DBNULL

            //DBNull.Value不等于null
            // MessageBox.Show((DBNull.Value == null).ToString());
            //if (row["TelPhone"]==DBNull.Value)//判断字段值是否为null的方法
            if (row.IsNull("TelPhone"))//更简单的判断数据库中字段值为null的方法
            {
                cust.TelPhone = null;
            }
            else
            {
                cust.TelPhone = (string)row["TelPhone"];
            }

            if (row.IsNull("Age"))
            {
                return -1;
            }
            else
            {
                int age = (int)row["Age"];
                return age;
            }

            //dataGridView1.DataSource
            //    =  new T_CustomersBLL().ListAll();
        }

namespace 可空类型
{
    class Program
    {
        static void Main(string[] args)
        {
            //object obj = null;
            //object i = obj ?? DBNull.Value;

            //int? i = null;
            //i = 90;
            //i++;
            //Console.WriteLine(i);

            ////在值类型后加?就成为了可空类型

            //int? age = GetAge();
            //Console.WriteLine(age.ToString());
            ////int age2 = GetAge();
            ////int age2 = (int)GetAge();
            //if (age.HasValue)//判断可空类型的对象是否为null
            //{
            //    int i2 = age.Value;
            //    //int i = (int)age;
            //}
            //else
            //{
            //    Console.WriteLine("nulnullnull");
            //}

            ////int w1 = 50;
            ////int? w2 = w1;
            ////int w3 = w2;

            Console.ReadKey();
        }

        public static int? GetAge()
        {
            if (DateTime.Now.Second % 2 == 0)
            {
                return 20;
            }
            else
            {
                return null;
            }
        }
    }
}