分两种类型，value type and reference type.

By default, value type owns a default value. For integer, the default value is 0. The value type can't be null. Non nullable

But for reference types, included class, interface, delegates, arrays, objects, 这些引用类型可以=null。Nullable

? to make non nullable type nullable.

(typeName)to make nullable type non nullable(explicit conversion).

但是有时候值类型也需要等于空值呀，だから？を採用する。

bool IsMajor

bool？ IsMajor=null

当一个是非的问题bool值表示用户的选择yes or no option，用户没有选择的时候

if(IsMajor==true){

}else

if(IsMajor==false) or if(!IsMajor.value){

 

}else{Console.WriteLine("User is not answer the que")}

?? Null coalescing operator

int? a=10;

int b=a??0; equals{

if(a==null){b=0;}else{b=a.Value; or b=(int)a;} //Direct Assignment will be fault. b=a

}

Double Question Mark