Meaning of the ? operator in C# for Properties

Question

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? (nullable) operator in C#

In System.Windows.Media.Animation I see the code as follows:

    public double? By { get; set; }

What does the ? operator do here? Does anyone know?

I've tried to google this but it's hard to search for the operator if you don't know what its called by name. I've checked the page on Operators (http://msdn.microsoft.com/en-us/library/6a71f45d(v=vs.80).aspx) but the ? operator is not listed there.

Thanks!

nullable type: http://msdn.microsoft.com/en-us/library/1t3y8s4s(v=vs.80).aspx — CRice, Jul 29 '11 at 06:58
normally a question like this would be closed as duplicate, but's its true - very difficult to google for "c# ? operator". — RPM1984, Jul 29 '11 at 07:00
...but googling "C# type with question mark" works. Even after typing "c# type with" google suggest the ending. — Renatas M., Jul 29 '11 at 07:11
Problem here is the (understandable) mistaken belief that ? is an operator, which makes searching for it much harder. — Dan Diplo, Jul 29 '11 at 09:59

Konrad Rudolph · Accepted Answer · 2011-07-29T07:11:17.213

18

The ? is a type decorator. T? is the same as Nullable<T>, i.e. a nullable value type.

The documentation of the By property explains why it’s used here:

The property controls how A DoubleAnimation progresses; but instead of setting the By property, you can also set the From and To properties (or either) to control animation progress. Every combination of properties (except To and By) is allowed, so there needs to be a way to signal that a property is not set – hence it’s nullable.

Use the By property when you want to animate a value "by" a certain amount, rather than specifying a starting or ending value. You may also use the By property with the From property.

edited Jul 29 '11 at 07:11

answered Jul 29 '11 at 06:58

Konrad Rudolph

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Side note: a way to use Nullable types: you should first check if `By.HasValue` is set to `true`, and next access its value with `By.Value` – Nicholas Sizer Jul 29 '11 at 07:03
Thanks! That's pretty cool. Why would they use it for this class? – swinefeaster Jul 29 '11 at 07:07
For any number of reasons, so no idea in this case :P – Nicholas Sizer Jul 29 '11 at 07:10
Awesome thanks. From is also nullable too. – swinefeaster Jul 29 '11 at 07:20

score 6 · Answer 2 · answered Jul 29 '11 at 06:58

6

The ? means it's nullable (the value can be set to null.

Nullable Types (C# Programming Guide)

answered Jul 29 '11 at 06:58

Tim

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score 2 · Answer 3 · answered Jul 29 '11 at 06:58

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This is not an operator. Rather, this is a special shorthand syntax for declaring nullable values.

answered Jul 29 '11 at 06:58

Anton Gogolev

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score 1 · Answer 4 · answered Jul 29 '11 at 06:58

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It is nullable propertie, that means that you can set By = null, without ? you'll get error that double cant be null

answered Jul 29 '11 at 06:58

Renatas M.

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score 1 · Answer 5 · answered Jul 29 '11 at 06:59

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? stands for Nullable types, this qualifies By in your case to hold a null value which is not possible for a value type

answered Jul 29 '11 at 06:59

V4Vendetta

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score 1 · Answer 6 · answered Jul 29 '11 at 06:59

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It means the type is nullable.

See this page for details.

answered Jul 29 '11 at 06:59

AgentConundrum

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score 1 · Answer 7 · answered Jul 29 '11 at 07:01

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this is syntactic sugar handled by the C# compiler.

It basically treat "double?" as Nullable which allows the value to be null. It basically wraps the double value inside another object.

answered Jul 29 '11 at 07:01

Meaning of the ? operator in C# for Properties

7 Answers7