Trying to understand ?. (null-conditional) operator in C#

Question

I have this very simple example:

class Program
{
    class A
    {
        public bool B;
    }

    static void Main()
    {
        System.Collections.ArrayList list = null;

        if (list?.Count > 0)
        {
            System.Console.WriteLine("Contains elements");
        }

        A a = null;

        if (a?.B)
        {
            System.Console.WriteLine("Is initialized");
        }
    }
}

The line if (list?.Count > 0) compiles perfectly which means that if list is null, the expression Count > 0 becomes false by default.

However, the line if (a?.B) throws a compiler error saying I can't implicitly convert bool? to bool.

Why is one different from the other?

@Veverke it is not the .NET framework version that conrolls this, it is the compiler you use. .? was introduces in C# 6 which was included with Visual Studio 2015 and newer — Scott Chamberlain
– Scott Chamberlain, Commented Jun 29, 2016 at 15:30
If you make the property "public bool? B" on class A, does it work? — Brian Mains
– Brian Mains, Commented Jun 29, 2016 at 15:31

Community · Accepted Answer · 2017-05-23 12:18:15Z

81

list?.Count > 0: Here you compare an int? to an int, yielding a bool, since lifted comparison operators return a bool, not a bool?.
a?.B: Here, you have a bool?. if, however, requires a bool.

edited May 23, 2017 at 12:18

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answered Jun 29, 2016 at 15:31

Heinzi

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2 Comments

Dee J. Doena Over a year ago

Thanks. I didn't realize that list?.Count basically becomes int? tmp = list?.Count and then of course tmp > 0 evaluates to false. This was the missing piece.

Adam Caviness Over a year ago

An important point is "if, however, requires a bool". This is why one can express a?.B == true and it should compile. As obvious as that is, it can elude someone looking too closely at syntax.

René Vogt · Accepted Answer · 2016-06-29 15:38:44Z

43

In your first case (list?.Count) the operator returns an int? - a nullable int.
The > operator is defined for nullable integers so that if the int? has no value (is null), the comparison will return false.

In your second example (a?.B) a bool? is returned (because if a is null, neither true nor false but null is returned). And bool? cannot be used in an if statement as the if statement requires a (non-nullable) bool.

You can change that statement to:

if (a?.B ?? false)

to make it work again. So the null-coalescing operator (??) returns false when the null-conditional operator (?.) returned null.

Or (as TheLethalCoder suggested):

if (a?.B == true)

edited Jun 29, 2016 at 15:38

answered Jun 29, 2016 at 15:31

René Vogt

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8 Comments

TheLethalCoder Over a year ago

Would if (a?.B == true) give the same effect?

user743382 Over a year ago

@TheLethalCoder While C# limits bool values to false and true, the .NET Framework does not, and non-C# methods may give you what can be in pseudo-C# as (bool) 2. Such a value is true enough to enter an if block if used as the condition, but does not compare equal to true.

Joshua Taylor Over a year ago

Whether if (a?.B == true) works or not, when I see it, I have the gut instinct to chime in "don't compare booleans to true (or false), use them directly," as would normally be appropriate when someone writes if (someCondition == true) instead of if (someCondition). I'm not sure what to make of a construction that ends up encouraging (... == true).

René Vogt Over a year ago

@JoshuaTaylor it's not comparing a bool to true, but a bool?

Joshua Taylor Over a year ago

@RenéVogt Yes, I understand that; my point is that it's jarring to see a place where == true is the right thing.

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