Use of question marks in C #

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. <Xmlnamespace prefix = "o" ns = "urn: schemas-microsoft-com: office"/>