Usage of ?, ?., ??, ??= operators in C#

// The reference type can be used to represent a non-existent value, but the value type cannot.  For example:string str = null;
int i = null;//Compilation error
// In order to make the value type also use nullable types,//    You can use "?" to represent it, and the expression is "T?".  For example:int i?;            //Denotes a nullable typeDataTime time?; //Indicates the time that can be vacancied

// Default values ​​for defining reference types and nullable types.// If the left operator of this operator is not Null, this operator returns the left operand,//         Otherwise, return the right operand.// Return a when a is not empty, return b when it is nullvar c = a ?? b;

// Perform the following operations when not null.  For example:
// The following two pieces of code are equivalent, that is, the subsequent logic will continue to be executed when it is not null?.;
 = Person == null ? null : ;

// C# 8.0 and later can use the null merge assignment operator ??=,// This operator is only when the evaluation result of the left operand is null,// Assign the value of the operand to the right to the operand to the left.// If the calculation result of the left operand is non-null,//         Then the ??= operator will not calculate its right operand.
List<int> numbers = null;
int? i = null;
 
numbers??= new List<int>();
(i ??= 17);
(i ??= 20);
 
(("", numbers));//output：17 17
(i);//output 17