C# ？ and ？？

Nullable Types in C#

One of the "late breaking" features in C# 2.0 is what is known as "Nullable Types". The details can be found in the C# 2.0 language spec.

Nullable types address the scenario where you want to be able to have a primitive type with a null (or unknown) value. This is common in database scenarios, but is also useful in other situations.

In the past, there were several ways of doing this:

• A boxed value type. This is not strongly-typed at compile-time, and involves doing a heap allocation for every type.
• A class wrapper for the value type. This is strongly-typed, but still involves a heap allocation, and the you have to write the wrapper.
• A struct wrapper that supports the concept of nullability. This is a good solution, but you have to write it yourself.

To make this easier, in VS 2005, we're introducing a new type named "Nullable", that looks something like this (it's actually more complex than this, but I want to keep the example simple):

struct Nullable<T>{    public bool HasValue;    public T Value;}

You can use this struct directly, but we've also added some shortcut syntax to make the resulting code much cleaner. The first is the introduction of a new syntax for declaring a nullable type. Rather than typing:

Nullable<int> x = new Nullable<int>(125);

I can write:

int? x = 125;

which is much simpler. Similarly, rather than needed to write a null test as:

if (x.HasValue) {...}

you can use a familiar comparison to null:

if (x != null) {...}

Finally, we have support to make writing expressions easier. If I wanted to add two nullable ints together and preserve null values, if I didn't have language support, I would need to write:

Nullable<int> x = new Nullable<int>(125);Nullable<int> y = new Nullable<int>(33);Nullable<int> z =  (x.HasValue && y.HasValue) ?            new Nullable<int>(x.Value + y.Value) : Nullable<int>.NullValue;

At least I think that's what I'd have to write - it's complex enough that I'm not sure this code works. This is ugly enough that it makes using Nullable without compiler support a whole lot of work. With the compiler support, you write:

int? x = 125;int? y = 33;
int? z = x + y;
The ?? operator returns the left-hand operand if it is not null, or else it returns the right operand.
A nullable type can contain a value, or it can be undefined. 
The ?? operator defines the default value to be returned when a nullable type is assigned to a non-nullable type. 
If you try to assign a nullable type to a non-nullable type without using the ?? operator, 
you will generate a compile-time error. 
If you use a cast, and the nullable type is currently undefined, an InvalidOperationException exception will be thrown. 
// nullable_type_operator.cs
using System;class MainClass{    static int? GetNullableInt()    {        return null;    }    static string GetStringValue()    {        return null;    }    static void Main()    {        // ?? operator example.        int? x = null;        // y = x, unless x is null, in which case y = -1.        int y = x ?? -1;        // Assign i to return value of method, unless        // return value is null, in which case assign        // default value of int to i.        int i = GetNullableInt() ?? default(int);        string s = GetStringValue();        // ?? also works with reference types.         // Display contents of s, unless s is null,         // in which case display "Unspecified".        Console.WriteLine(s ?? "Unspecified");    }}

Mark：Becuase of "?" we can know the value is zero or not inited。