什么是 （ <type> ？）？

Question

I was looking through some code the other day and I saw something like (int?) and I dont think Ive ever see that before. What does it mean when you use a ? after a type?

Answer 1

It is short for Nullable<T> .

Nullable types in C#

This is a generic struct that can wrap a value-type to add the value null . To make the use of this type more convenient C# adds quite a bit of compiler-magic. Such as the short-name T? , lifted operators,...

The following thread on SO is interesting too: ? (nullable) operator in C#

Answer 2

It's a variation/alternative of the Nullable<Type> . Have seen it used a lot with DateTime to avoid the default DateTime value which gives an error in DB columns related to dates. Quite useful actually.

Answer 3

int? is syntactic sugar for Nullable<int> .

Answer 4

That is the shorthand syntax for Nullable<T> (or in your case Nullable<int> ).

This is used when you need value types to be null, such as int , Boolean and DateTime .

Answer 5

它意味着Nullable，所以我们的value-type变量可以为null

Answer 6

the ? after the type implies that the type can have null value besides its normal values.

I've seen the use mostly for database related types where you have Nullable columns

Answer 7

As other people have said int? is short for Nullable<int> .

This article is a couple of years old now but it's a good explanation of nullable types