Available types in C ＃_ Unavailable type_ C double question mark_ C question mark point_ C null is not equal to

C# Available types in _ Type not available _C# ??_C#?._C# null!

One 、 Null type Nullable

When we talk about null When it's safe , A variable can have two states ：

• Nullable : It can be for null.
• Non-Nullable ： Not for null.

from C# 8.0 Start , By default, all reference types are Non-nullable.

“ Emptiness ” This can be modified by the following two new type operators ：

• ! ： from Nullable Change it to Non-Nullable
• ? ： from Non-Nullable Change it to Nullable

These two operators correspond to each other . You use these two operators to qualify variables , The compiler then ensures that... According to your qualifications null Security .

1、? Usage of operators

It is mainly used for corresponding databases null type .
Nullable：int? x;
x It's the basic data type , Therefore, by default, it cannot be null Of .
We use ? Operator to change it to be null Of .
x = null; The assignment is normal , No warning .

2、?. NULL Conditional operator

 Person per = null;
 string name = per?.Name;

No error will be reported here .

?[ NULL Operators can not only use ?. Access object properties and methods , You can also use ?[ Monitors whether the array or the object containing the indexer is empty

int[] are = new int[5] { 2, 3, 5, 7, 4 };

            int? value = are?.Length;

            int? result = are?[0];

3、?? Air transport operator

int[] are = new int[5] { 2, 3, 5, 7, 4 };

            int value = are?.Length??0;

            int result = are?[0]??0;

Example ：

Student stu2 = new();

string name = stu2.Name??" Zhang San ";

Console.WriteLine(name);// Print  " Zhang San ", similar js  in ||  Operator 

Two 、 An uncontrollable variable of nullable type Non-Nullable / null!

null! assignment
null! It's telling the compiler null No null value , This method only prompts in the coding process , Errors may not be reported during operation .

namespace ConsoleApp1
{
    public class Student
    {
        public int ID { get; set; }
        public string Name { get; set; }
    }


    public class School
    {

        // The current field cannot be set to empty 
        public Student StuInfo { get; set; } = null!;
    }
}

Student stu=new Student();
stu.ID = 10;
stu.Name = " Zhang San ";

Console.WriteLine(stu.ToString());


School schOne = new School();
schOne.StuInfo = null;

Console.WriteLine(" End of code execution ");

Compiler here , Tips   schOne.StuInfo = null;  Cannot be assigned to null.

more ：

C# data type 、 Variable 、 Scope

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