struct XfhNullable<T> where T : struct
{
  private T innerValue;
  //这个属性很重要
  public bool HasValue { set; get; }
  public T Value
  {
    get
    {
      return HasValue ? innerValue: throw new InvalidOperationException();
    }
  }
  public XfhNullable(T value)
  {
    this.innerValue= value;
    HasValue = true;
  }
  public T GetValueOrDefault(T value)
  {
    return HasValue ? this.innerValue: value;
  }
  public T GetValueOrDefault()
  {
    return this.innerValue;
  }
}

using static System.Console;
class Program
{
  static void Main()
  {
    //使用结构体默认的无参构造函数进行实例化
    XfhNullable<int> num = new XfhNullable<int>();
    WriteLine(num.HasValue);
    WriteLine(null_num.GetValueOrDefault());
  }
}

public static implicit operator XfhNullable<T>(T? nullabelValue)
{
  if (nullabelValue== null)
  {
    return new XfhNullable<T>();
  }
  return new XfhNullable<T>(nullabelValue.Value);
}

static void Main()
{
  XfhNullable<int> null_num = null;
  WriteLine(null_num.HasValue);
}

public static bool operator ==(XfhNullable<T> cn, object obj)
{
  if (cn.HasValue)
  {
    return false;
  }
  return true;
}
public static bool operator !=(XfhNullable<T> cn, object obj)
{
  return !(cn == obj);
}

static void Main()
{
  XfhNullable<int> null_num = null;
  WriteLine(null_num == null);
}

public static implicit operator XfhNullable<T>(T value)
{
  return new XfhNullable<T>(value);
}
public static explicit operator T(XfhNullable<T> value)
{
  return value.innerValue;
}

static void Main()
{
  XfhNullable<int> null_num = null;
  null_num = 12;//int类型隐式转换为XfhNullable<int>类型
  WriteLine(null_num == null);
  WriteLine(null_num.Value);
  int i = (int)null_num;//XfhNullable<int>类型强制转换为int类型
  WriteLine(i);
}

static void Main()
{
  XfhNullable<int> null_num = 12;
  WriteLine(null_num.GetType());
}

//因为Object类中的GetType方法不允许子类重写（避免子类隐藏自己的实际类型）
//所以这里使用关键字new来隐藏Object类中的GetType方法
public new Type GetType()
{
  return innerValue.GetType();
}