C #"?? "And "? "Operator

The condition operator is associated to the right and is calculated from left to right, so? B: C? D: E should be understood as? B :( C? D: E ). The nullable data type is implemented in C #2.0, Abandon pair object Data if （ object == null . At the same time, you will find that, In database applications, some fields in the Database allow null values, and the introduction of the nullable type in the C # world exactly corresponds , Ing from (C #) objects to relational database objects, or vice versa, the conversion is relatively smooth.   In order to convert the nullable data type into non-nullable data, there is such an operator "?? (Two question marks) ", the double question mark Operator means to take the assigned value ?? Left, if the left is null , Take the assigned value ?? On the right, For example int Y = x ?? -1 if X is null, the value of Y is-1. At this time, you can change the second code at the top: string strParam= Request.Params[ "param" ]?? "" ; Hey, is it more concise. There are also "? (Single question mark) "modifier, abbreviated form of system. nullable, such int ? Represents an empty integer, For example: int ? A = 1 or int ? b= null . The nullable type has a hasvalue bool Type Read-only property, This attribute is true It indicates that the instance is a non-empty instance and contains a known value, Hasvalue is false When the value attribute is accessed, system. invalidoperationexception occurs.Reprinted: http://www.cnblogs.com/refactor/archive/2012/05/30/2526147.html