1. var implicit type

var is a special variable type, which can be used to represent any type of variable

be careful:

(1) var cannot be used as a member of a class. It can only be used to declare temporary variables, that is, it is usually written in a function statement block

(2) var must be initialized, but the type is determined after initialization

var i = 5;
var s = "srea";
var array = new int[] { 1, 2, 3, 4 };
var list = new List<int>();

2. Set object initial value

When declaring an object, you can initialize public member variables and attributes by directly writing braces

class Person {
    private int money;
    public bool sex;

    public string Name {
        get;
        set;
    }

    public int Age {
        get;
        set;
    }
}

Person p = new Person { sex = true, Age = 17, Name = "person" }; //Parameterless construction parentheses can be omitted
Person p2 = new Person() { sex = true };

If it is a parameterized structure, you can directly pass in the relevant parameters in parentheses

3. Set set initial value

When declaring a collection object, you can also directly initialize the internal properties through braces

int[] array = new int[] { 1, 2, 3, 4, 5 };
List<int> list = new List<int>() { 1, 2, 3, 4, 5 }; // Parentheses can also be omitted
List<Person> listPerson = new List<Person>() {
    new Person(),
    new Person(){Age = 10},
    new Person(){sex = true, Name = "test"}
};

Dictionary<int, string> dic = new Dictionary<int, string>() {
    {1, "123" },
    {2, "222" }
};

4. Anonymous type

var variables can be declared as custom anonymous types

var v = new { age = 10, money = 11, name = "Ming" };
Console.WriteLine("{0}, {1}, {2}", v.age, v.money, v.name);

Cannot add a Lambda expression to an anonymous type

5. Nullable type

(1) The value type cannot be assigned null, that is, int a = null; No

(2) When declaring, add?, after the value type?, Can be assigned null

int? a = null;  

(3) Judge whether it is empty

if (a.HasValue) {
    Console.WriteLine(a);
    // Equivalent to
    Console.WriteLine(a.Value);
}

Note: a cannot be directly assigned to a variable of type int, that is, int v = a; Is an error, and the corresponding Value must be obtained through Value: int v = a.Value;

(4) Safely get nullable type value

If blank, the default value of the default return value type

Console.WriteLine(a.GetValueOrDefault());

You can also specify a default value

Console.WriteLine(a.GetValueOrDefault(100));

But the value of a is not assigned to 100

(5) Usage with reference type:

object o = null;
if (o != null) {
    o.ToString();
}

Action action = null;
if (action != null)
    action.Invoke();

Can be abbreviated as

o?.ToString();

action?.Invoke();

For arrays:

int[] array = null;
Console.WriteLine(array?[0]);

It is equivalent to a syntax sugar, which can help automatically determine whether it is empty. If it is empty, the following methods will not be executed

6. Empty merge operator

Empty merge operator??

Format: left value?? Right value

If the left value is null, the right value is returned; otherwise, the left value is returned

Any null able type can be used

Similar to trinocular operator

int? intV = null;
int intI = intV == null ? 100 : intV.Value;
intI = intV ?? 100;

7. Interpolate string

Key symbols:$

Use $to construct a string, so that variables can be spliced in the string

string name = "ming";
int age = 19;
Console.WriteLine($"hello world,{name},{age}");

8. A brief description of single sentence logic

It is generally said that braces can be omitted from a single sentence, mainly referring to the writing of attributes and functions in a class

public string Name {
    get => "test";
    set => name = value;
}

public int Add(int a, int b) => a + b;
public void Speak(string str) => Console.WriteLine(str);