C medium? This form of

1. Nullable type modifier （？）
　　 Citation type can use empty citation to indicate a non-existent value , The value type usually cannot be empty .
　　 for example ：stringstr=null; That's right. ,inti=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 show , The way of expression is "T？"
　　 for example ：int? Indicates nullable shaping ,DateTime? Indicates the time that can be empty .
　　T? It's actually System.Nullable( Generic structure ） The abbreviation of ,
　　 That means when you use T？ The compiler will compile T？ Translate it into System.Nullable The way .
　　 for example ：int?, After compilation System.Nullable The way .
2. Three yuan （ Operator ） expression （?:)
　　 for example ：x?y:z Show that if the expression x by true, Then come back y;
　　 If x by false, Then come back z, Is to omit if{}else{} In a simple way .
3. Null merge operator (??)
　　 Default values used to define nullable types and citation types .
　　 If the left operand of this operator is not null, Then this operator will return the left operand , Otherwise, the right operand .
　　 for example ：a??b When a by null When you come back b,a Not for null When you come back a Oneself .
　　 The null merge operator is the right join operator , That is to say, it is combined from right to left during operation .
　　 Such as ,“a??b??c” By the way of “a??(b??c)” Calculation .
4.NULL Look at the operator （?.）