The usage of question mark "?" in C#

tags: C#

There are two scenarios for question marks in C#, single question mark and double question mark. The single question mark is a ternary operator used in conditional expressions; the double question mark has two uses: nullable type Modifiers and null coalescing operators.

(1) Conditional expression ternary operator: such as x>y?a:b, that is, when x>y, the expression takes the value a, otherwise it takes the value b;

c = x > y ? a : b;
//Equivalent to 
if (x > y) {
    c = a;
}
else {
    c = b;
}

(2) Nullable type modifier: value types such as int and bool cannot be assigned to null (null reference), in order to make the value type also assign a null reference to indicate a value that does not exist ,introducedSystem.Nullable<T> (T is a value type), the abbreviation is T?, such as int?System.Nullable<int>；

int? x = null;//Nullable type default value is null, and value type such as int default value is 0, bool default value is false, etc.
bool? result = true;
/*
         When x is null and is referenced, it will raise the "nullable object must have a value" exception
         When a nullable type is referenced or involved in an operation, the Value property will be called. This exception will be raised when the nullable type is not assigned
*/
int y = x + 1;
x = 1;
 y = x.Value;//When assigning a nullable type to a value type, use the Value attribute

(3) Null combination operator: used to define the default value of nullable types and reference types, it is a right associative operator;

int y = x ?? 0;//When x is null, y takes the value 0, otherwise it takes the value x.Value. This operator can simplify the null judgment of the nullable type
 int y = null == x? 0: x.Value;//It is equivalent to the above statement

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