What does double? mean in C#?

Question

Possible Duplicate:
C# newbie: what’s the difference between “bool” and “bool?” ?

Hi, While reading the code of the NUnit project's assert class, I came across this particular construct -

public static void AreEqual(double expected, double? actual, double delta)
{
     AssertDoublesAreEqual(expected, (double)actual, delta ,null, null);
}

In this function the second input parameter is entered as double?. The interesting thing is that this code compiles without issue in VS2010 (C# 4.0). Anyone know why this is NOT throwing an error ? Why is double? considered a valid keyword and is there any special significance to the ?.

See [C# newbie: what's the difference between "bool" and "bool?" ?](http://stackoverflow.com/questions/1181491/c-newbie-whats-the-difference-between-bool-and-bool) — Matthew Flaschen, Jun 07 '10 at 10:04
Stop downvoting the guy, it's a valid question and it's hard for someone who doesn't know the nullable operator to find the answer. Just close the question as exact duplicate. — Diadistis, Jun 07 '10 at 10:07
What's up with those downvotes? The question mark suffix isn't very search engine friendly. — cfern, Jun 07 '10 at 10:07

score 25 · Accepted Answer · answered Jun 07 '10 at 10:06

25

double? is just shorthand for Nullable<double>; basically, a double that can be null. But the code is not very safe. If actual is null, (double)actual will throw an exception.

answered Jun 07 '10 at 10:06

Gorpik

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+1 for warning for warning about the `InvalidOperationException`. – Steven Jun 07 '10 at 10:13

score 7 · Answer 2 · answered Jun 07 '10 at 10:01

7

It's a nullable type. So it's a double that can also be null.

See here for more info.

answered Jun 07 '10 at 10:01

Hans Olsson

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score 3 · Answer 3 · answered Jun 07 '10 at 10:04

3

I believe this means that the parameter could also be called null.

answered Jun 07 '10 at 10:04

Sir Graystar

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score 3 · Answer 4 · edited Jun 07 '10 at 10:10

The syntax T? is shorthand for System.Nullable. Following line of code declares nullable type.

double? actual =null;

You can not assign null values to .NET value types (structs) including c# primitive types like int and double so in .NET 2.0 there is concept of nullable type is added to assign null value.

for example :

int? num = null;
        if (num.HasValue == true)
        {
            System.Console.WriteLine("num = " + num.Value);
        }
        else
        {
            System.Console.WriteLine("num = Null");
        }

What does double? mean in C#?

4 Answers4