C # question mark (?) Usage Summary

When I look at the MVC self-generated example today, C # has two strange question marks:

Public accountcontroller (iformsauthentication formsauth, imembershipservice Service)
{
Formsauth = formsauth??New formsauthenticationservice ();
Membershipservice = Service??New accountmembershipservice ();
}

Search online, summarized as follows:

1. Can be empty type modifier (?) :

The reference type can use null reference to indicate a nonexistent value, while the value type cannot be null.

For example, string STR = NULL; is correct, int I = NULL; the compiler reports an error.

To make the value type empty, you can use the void type, that is, the void type modifier "? "T? "

Example: Int? It indicates an integer that can be null, datetime? It indicates a time that can be empty.

T? It is actually the abbreviation form of system. nullable <t> (generic structure), that means when you use t? When the compiler compiles t? Compiled into the form of system. nullable <t>.

Example: Int ?, After compilation, it is in the form of system. nullable <int>.

2. Ternary (operator) expression (? :):

Example: X? Y: Z indicates that if expression X is true, return Y. If expression X is false, return Z, which is a simple form of if {} else {} omitted.

3. null merge operator (??) :

Defines the default values of the null and reference types. If the left operand of this operator is not null, this operator returns the left operand; otherwise, the right operand is returned.

Example: ?? B returns B if A is null, and a itself if A is not null.

The null merge operator is the right merge operator, that is, the Union from right to left during the operation. For example, "?? B ?? C ?? (B ?? C) "computing.