Deep understanding of c # nullable types

Why can't value type variable is empty

nullIn the memory is full0存在的.Value type of the variable values stored in memory will automatically add a fully behind the value0The value of is used to limit the value range of value type variables.Reference types do not have this mechanism,So reference types can be null,while the value type cannot be null.But because the database of all types can be empty,So hopefully the value type can be null.

C#1The way to give the value type is empty

名称	具体操作
魔值	Specific a number as a null value,例如DateTimeA null value in the database is passed to the database as0001-01-01 00:00:00
reference type wrapper	boxing and unboxing
Additional non-duo logo	Use another value type for null value judgment

Nullable<T>

属性1：HasValue,判断是否有值

属性2：Value,真实的值

方法GetValueOrDefault,If there is a real value, return that value,如果没有就返回默认值,You can add a value type parameter as a default value to the formal parameter of the method.

C#A hollow value is equal to a null value or no value,A null value is less than any other value.

C#2Syntactic sugar for nullable types

When the declaration value types,使用?修饰符,例如：int? i; This method is actually a simplified operation to be boxed value type.

Nullable types accept implicit conversions of any type,But nullable types must be explicitly converted to other types,and cannot be null when converted to pinch type.

C#中的??The operator can be seen as a simplified version of the ternary operation that evaluates to null.例如：

int? a = 4;
int b = 3;
int c = a ?? b;

a为空时,c等于b;a不为空时,c等于a.其中amust be a nullable type otherwise it is meaningless,如果c为值类型,那么amust have a value orb必须为值类型.

??operators can also be used together,例如：

int? a = 3;
int? b = 2;
int c = 3;
int d = a ?? b ?? c;

The execution mechanism of the entire statement is the same as above,But is the start of the first from the right,start judgingb和c(ps:I felt very anti-human at first.,Because other operators start from the left first).

总结：To assign a null value type variables,add just after the modifier when declaring?,使用??can have a sorting effect.