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, Jun 29 '16 at 15:30
See [What exactly does ‘lifted’ mean?](https://blogs.msdn.microsoft.com/ericlippert/2007/06/27/what-exactly-does-lifted-mean/) — Lucas Trzesniewski, Jun 29 '16 at 15:30
If you make the property "public bool? B" on class A, does it work? — Brian Mains, Jun 29 '16 at 15:31
And note VB.NET took a different way here. I.e. both of these work as expected, though that does mean there's other interesting inconsistencies. — Mark Hurd, Jul 06 '16 at 08:17

score 79 · Answer 1 · edited May 23 '17 at 12:18

79

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 '17 at 12:18

Community

1
1

answered Jun 29 '16 at 15:31

Heinzi

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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. – Dee J. Doena Jun 29 '16 at 15:35
1

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. – Adam Caviness Aug 25 '16 at 18:35

René Vogt · Accepted Answer · 2016-06-29T15:38:44.883

41

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 '16 at 15:38

answered Jun 29 '16 at 15:31

René Vogt

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Would `if (a?.B == true)` give the same effect? – TheLethalCoder Jun 29 '16 at 15:37
4

@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`. – Jun 29 '16 at 17:16
2

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)`. – Joshua Taylor Jun 29 '16 at 19:17
@JoshuaTaylor it's not comparing a `bool` to `true`, but a `bool?` – René Vogt Jun 29 '16 at 19:29
2

@RenéVogt Yes, I understand that; my point is that it's jarring to see a place where `== true` *is* the right thing. – Joshua Taylor Jun 29 '16 at 19:36
Also will (b?.A == false) give a similar effect? – Krish Munot Jul 23 '16 at 17:42
@KrishMunot what do you mean? there is no variable called `b` in the question. – René Vogt Jul 23 '16 at 19:32
@RenéVogt They obviously mistyped :-/ (a?.B == false) works as well. The issue here is null. – Saturn K Aug 07 '17 at 22:31

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

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