// C# 4 Wrox , Page 122
// using generic structures , using Nullable<T>
// casting  Nullable<T> to T using operatoe overloads
public struct Nullable<T> where T:<struct>
{
public Nullable (T value)
{
this.hasValue = true;
this.value = value;
}
private bool hasValue;
public bool HasValue
{
get{ return hasValue;}
}
private T value
Public T value
{
get { if (!hasValue)
{throw new InvalidOperationException("no value")}
}
return value;
}
}
public static explicit operator T ( Nullable<T>  value)
{
return  value.Value;
public static implicit operator Nullable<T> (T  value)
{
return new  Nullable<T>(value);
}
public override string ToString(){
if(!HasValue)  
return  String.Empty;
return this.value.ToString();}
}
}
class MPEP  {public static void Main(){
 Nullable<int>  x;
x = 4;
X+ = 3;
if(x.HasValue){
int  y = x.Value;
}
x = null;
}}

private T value
public T Value
{
  get { 
    if (!hasValue)
    {
      throw new InvalidOperationException("no value");
    }
    return value;
  }
}

public static explicit operator T (Nullable<T>  value)