可空类型为什么可以为空？也许会被面试到哦。。…

  1 [Serializable, StructLayout(LayoutKind.Sequential), __DynamicallyInvokable]
  2 public struct Nullable<T> where T: struct
  3 {
  4     private bool hasValue;
  5     internal T value;
  6     [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable]
  7     public Nullable(T value)
  8     {
  9         this.value = value;
 10         this.hasValue = true;
 11     }
 12 
 13     [__DynamicallyInvokable]
 14     public bool HasValue
 15     {
 16         [__DynamicallyInvokable, TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")]
 17         get
 18         {
 19             return this.hasValue;
 20         }
 21     }
 22     [__DynamicallyInvokable]
 23     public T Value
 24     {
 25         [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable]
 26         get
 27         {
 28             if (!this.HasValue)
 29             {
 30                 ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_NoValue);
 31             }
 32             return this.value;
 33         }
 34     }
 35     [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), __DynamicallyInvokable]
 36     public T GetValueOrDefault()
 37     {
 38         return this.value;
 39     }
 40 
 41     [__DynamicallyInvokable]
 42     public T GetValueOrDefault(T defaultValue)
 43     {
 44         if (!this.HasValue)
 45         {
 46             return defaultValue;
 47         }
 48         return this.value;
 49     }
 50 
 51     [__DynamicallyInvokable]
 52     public override bool Equals(object other)
 53     {
 54         if (!this.HasValue)
 55         {
 56             return (other == null);
 57         }
 58         if (other == null)
 59         {
 60             return false;
 61         }
 62         return this.value.Equals(other);
 63     }
 64 
 65     [__DynamicallyInvokable]
 66     public override int GetHashCode()
 67     {
 68         if (!this.HasValue)
 69         {
 70             return 0;
 71         }
 72         return this.value.GetHashCode();
 73     }
 74 
 75     [__DynamicallyInvokable]
 76     public override string ToString()
 77     {
 78         if (!this.HasValue)
 79         {
 80             return "";
 81         }
 82         return this.value.ToString();
 83     }
 84 
 85     [__DynamicallyInvokable]
 86     public static implicit operator T?(T value)
 87     {
 88         return new T?(value);
 89     }
 90 
 91     [__DynamicallyInvokable]
 92     public static explicit operator T(T? value)
 93     {
 94         return value.Value;
 95     }
 96 }
 97 
 98  
 99 Collapse Methods
100  

1     class Program
2     {
3         static void Main(string[] args)
4         {
5             Nullable<Int32> i = null;
6         }
7     }

1     class Program
2     {
3         static void Main(string[] args)
4         {
5             Nullable<Int32> i = 0;
6         }
7     }

1     class Program
2     {
3         static void Main(string[] args)
4         {
5             Nullable<Int32> i = new Nullable<Int32>();
6         }
7     }

1     class Program
2     {
3         static void Main(string[] args)
4         {
5             Program p = null;
6         }
7     }