What does it mean to add a question mark after the data type ?, Data Type question mark

What does it mean to add a question mark after the data type ?, Data Type question mark

During. net development, I saw a piece of code of this type:

Public double? AskLeave {get; set ;}

I can't figure it out. Why do I need to add a question mark? Now I know why:

By default, attribute values of the double type cannot be null. If a question mark is added, the value can be null. In fact, the nullabl interface is implemented, and so on, if this question mark is added to other data types, this is also the reason.

 

When I saw the comments, I checked the materials on the Internet. If you are interested, you can check these three URLs to help you understand them.

Http://tieba.baidu.com/p/1343474351

Http://www.cnblogs.com/Holmes-Jin/archive/2012/01/13/2321841.html

Http://www.cnblogs.com/holly/archive/2010/02/02/1662256.html