如果对象不为null，如何将属性值分配给var ONLY

Question

In my code, is there a shorthand that I can use to assign a variable the value of a object's property ONLY if the object isn't null?

string username = SomeUserObject.Username;     // fails if null

I know I can do a check like if(SomeUserObject != null) but I think I saw a shorthand for this kind of test.

I tried:

string username = SomeUserObject ?? "" : SomeUserObject.Username;

But that doesn't work.

Answer 1

您在第二个语法上略有偏差。

string name = SomeUserObject != null ? SomeUserObject.Username : string.Empty;

Answer 2

我认为，你最接近的是：

string username = SomeUserObject == null ? null : SomeUserObject.Username;

Answer 3

In c# 6.0 you can now do

string username = SomeUserObject?.Username;

username will be set to null if SomeUSerObject is null. If you want it to get the value "", you can do

string username = SomeUserObject?.Username ?? "";

Answer 4

这可能就像你将得到的那样接近：

string username = (SomeUserObject != null) ? SomeUserObject.Username : null;

Answer 5

You can use ? : as others have suggested but you might want to consider the Null object pattern where you create a special static User User.NotloggedIn and use that instead of null everywhere.

Then it becomes easy to always do .Username .

Other benefits: you get / can generate different exceptions for the case (null) where you didn't assign a variable and (not logged in) where that user isn't allowed to do something.

Your NotloggedIn user can be a derived class from User, say NotLoggedIn that overrides methods and throws exceptions on things you can't do when not logged in, like make payments, send emails, ...

As a derived class from User you get some fairly nice syntactic sugar as you can do things like if (someuser is NotLoggedIn) ...

Answer 6

You're thinking of the ternary operator.

string username = SomeUserObject == null ? "" : SomeUserObject.Username;

See http://msdn.microsoft.com/en-us/library/ty67wk28.aspx for more details.

Answer 7

它被称为空合并，并执行如下：

string username = SomeUserObject.Username ?? ""