C＃ ?,?.,?? .....

Reprinted here

1. Nullable type modifier （?）

Reference types can use empty references to represent a value that does not exist , The value type is usually not represented as empty .
for example ：string str=null; That's right. ,int i=null; The compiler will report an error .
To make the value type also nullable , You can use nullable types , Use the nullable type modifier "？“ To express , In the form of "T？” for example ：int? Represents a nullable shape ,DateTime? Indicates a time that can be empty .
T? It's actually System.Nullable( Generic structure ） Abbreviated form , That means when you use T？ The compiler will compile T？ Translate it into System.Nullable In the form of .
for example ：int?, When compiled, it's System.Nullable In the form of .

2. Three yuan （ Operator ） expression （?: )

for example ：x?y:z Means if the expression x by true, Then return to y; If x by false, Then return to z, It's the omission if{}else{} In a simple form .

3. Empty merge operator (??)

Default values for defining nullable and reference types .
If the left operand of this operator is not null, Then this operator will return the left operand , Otherwise, return the right operand .
for example ：a??b When a by null When you return to b,a Not for null When you return to a In itself . The empty merge operator is the right merge operator , That is to say, it is combined from right to left during operation . Such as ,“a??b??c” In the form of “a??(b??c)” Calculation .

4. NULL Check operators （?.）

For example, we want to get a Point Of the first point of the sequence X coordinate , The first feeling would say ： int firstX = points.First().X; however , The old bird will tell you , Not here NULL Check , The correct version is like this ：

int? firstX = null;
if (points != null)
{
    
　　var first = points.FirstOrDefault();
　　if (first != null)
　　firstX = first.X;
} 

That's right... That's right , Code extraction becomes much harder to read . stay C# 6.0 in , Introduced a ?. Operator , The previous code can be changed to the following form ：
int? firstX = points?.FirstOrDefault()?.X; We can also see its basic usage from this example ： If the object is NULL, Then, the subsequent operation of getting members is not performed , Go straight back to NULL
It should be noted that , because "?.“ The operator can return NULL, When the returned member type is struct When it comes to type ,”?.“ and ”." The return value type of the operator is different .

Point p = new Point(3, 2);
Console.WriteLine(p.X.GetType() == typeof(int)); //true
Console.WriteLine(p?.X.GetType() == typeof(int?)); //true 

5. "?[]" Operator ：

int? first = customers?[0].Orders.Count(); 

If the array customers by null, Then the expression returns null; Otherwise, return the array access result .

6.??=

Only if the left operand evaluates to null when , To use operators ??= Assign the value of its right operand to the left . This is quite easy to use