Why elvis operator used in "if" statement is NOT a boolean algebra anymore?

Question

This code below:

class B
{
    public bool FinishedDownloading { get; set; } = false;
};

class A
{
    public B DownloadStream { get; set; }
};

A a = new A();

Console.WriteLine(a.DownloadStream?.FinishedDownloading != false);
Console.WriteLine(a.DownloadStream?.FinishedDownloading == true);

Outputs:

True
False

How on earth a.DownloadStream?.FinishedDownloading != false is true when DownloadStream is null and FinishedDownloading is false?!

Let's forget about what happend and focus on the result.

We know this statement a.DownloadStream?.FinishedDownloading != false is true, because it's not false, so in our universe when something is true then it should be true, beacause 1 == 1, right?

So why when we do a.DownloadStream?.FinishedDownloading == true the result is now false?! Madness.

I think I kind of know what's going on here. It's because when DownloadStream is null then FinishedDownloading boolean value is not accessible, thus left side is null, so C# "thinks":

a.DownloadStream?.FinishedDownloading != false

Ok let's check if null value is not equal to boolean "false" value. Hmm, it certainly does not, so let's output true.
a.DownloadStream?.FinishedDownloading == true

Ok let's compare a null value with boolean "true" value. Hmm, that doesn't make any sense. It's not true, so let's output false.

Does anyone know if this is really the case? I find it very confusing and bug prone.

Because if `a.DownLoadStream` is null, you get `null != false`, which is true. — MakePeaceGreatAgain, Nov 30 '18 at 12:59
If a.DownloadStream == null then your expression boils down to 'if null == true' which is False — hsmiths, Nov 30 '18 at 12:59
_"How on earth a.DownloadStream?.FinishedDownloading != false is true"_ - If you write something like this `if(null != false)`, you will get - `Warning: CS0472 The result of the expression is always 'true' since a value of type 'bool' is never equal to 'null' of type 'bool?'` — SᴇM, Nov 30 '18 at 13:02
@BartłomiejStyś You need to have `if(null != false)` in your code in order to see that warning, it is for testing purpose, to understand what's going on. `null` represents value of `DownloadStream` when it is `null`. — SᴇM, Nov 30 '18 at 13:06
@BartłomiejStyś: use: `if (new bool?() != false){}` because that is equal to your case. You will get the warning: _"The result of the expression is always 'true' since a value of type 'bool' is never equal to 'null' of type 'bool?'"_ — Tim Schmelter, Nov 30 '18 at 13:07
I guess you need something like `!a.DownloadStream?.FinishedDownloading ?? false` when your `DownloadStream` is `null`. — SᴇM, Nov 30 '18 at 13:16
Your last approach makes perfectly sense and is the best way to avoid this issue and to keep the code readable: `bool finished=a.DownloadStream?.FinishedDownloading == true;`. What happens here is that the `==` operator will convert the `bool?` to a `bool` to make this comparison possible. This is called [lifted operators](https://stackoverflow.com/questions/3370110/what-are-lifted-operators). — Tim Schmelter, Nov 30 '18 at 13:22

score 8 · Accepted Answer · edited Nov 30 '18 at 13:05

You're forgetting that Nullable<bool> (also known as bool?) and bool are two different types with a different list of possible values.

bool can either be true or false
bool? can be true, false or null

Since you've never initialized the DownloadStream property, it is therefore null. a.DownloadStream?.FinishedDownloading therefore returns a bool? with value null because ?. propagates the null.

And null != false is true since they are two completely different values.

If you initialize the DownloadStream property, e.g. by doing:

class A
{
    public B DownloadStream { get; set; } = new B();
};

Then you will see that a.DownloadStream?.FinishedDownloading != false will return false, which is what you're expecting.

Why elvis operator used in "if" statement is NOT a boolean algebra anymore?

1 Answers1