问题
Possible Duplicates:
?? Null Coalescing Operator --> What does coalescing mean?
What do two question marks together mean in C#?
I couldn't find this question being asked here so I figured I would ask it. What does a double question mark do in C#?
Example:
x = y ?? z;
回答1:
This is a null coalescing operator. The method above states x is assigned y's value, unless y is null, in which case it is assigned z's value.
回答2:
From Wikipedia:
It's the null-coalesce operator and shorthand for this:
x = (y != null ? y : z);
回答3:
Use y if not null, otherwise use z.
回答4:
If a the value y is null then the value z is assigned.
For example:
x = Person.Name ?? "No Name";
If name is null x will have the value "No Name"
回答5:
If y is null x will be set to z.
回答6:
.Net framework 2.0 onwards allow null values to Nullable value types.
here in this case, it says x equals y if it has some value (ie not null) or else equals z
回答7:
As others have stated, it is the null coalescing operator.
MSDN information on this:
https://docs.microsoft.com/dotnet/csharp/language-reference/operators/null-coalescing-operator
来源：https://stackoverflow.com/questions/1608174/what-does-a-double-question-mark-do-in-c