Is there a way to implement and make use of a "NOT null coalescing" operator?

Question

Is there a not null coalescing operator in C# which in case could be used such as:

public void Foo(string arg1)
{
    Bar b = arg1 !?? Bar.Parse(arg1);   
}

The following case made me think of it:

public void SomeMethod(string strStartDate)
{
    DateTime? dtStartDate = strStartDate !?? DateTime.ParseExact(strStartDate, "dd.MM.yyyy", System.Globalization.CultureInfo.InvariantCulture);
}

I might not have strStartDate information, which in case will be null but if i do; i'm always certain that it will be in expected format. So instead of initializing dtStartDate = null and trying to parse and set the value within try catch block. It seems to be more useful.

I suppose the answer is no (and there is no such operator !?? or anything else) I wonder if there's a way to implement this logic, would it be worth and what would be the cases that it comes useful.

Kun

why don't you use just conditional ?: Operator ? – Coit

what do you mean by "not null coalescing"? If you wrote what it did in regular "this compiles now" code (using either if or conditional), what would it look like? – Obstinate

@Selman22 Isn't null coalescing a syntactic sugar for tenary? You could x = x != null ? x : y instead of x = x ?? y aswell. I suppose this is not the case, sorry kardeşim – Benkley

@Zefnus you can NOT add a new operator to an existing language.If you want it design and create your own language, kardeşim. – Coit

@Zefnus no, the ?? operator is syntactic sugar for "take the first non-null expression"; it is also possible to represent that in a condtional, but that doesn't mean that ?? is syntactic sugar for a conditional – Obstinate

@MarcGravell well i suppose the syntactic sugar i was looking for was "take the first null expression" in case. And if none it would take a value – Benkley

The "take the first null expression" is under consideration (at least, as far as member access) for C# 6 - see my answer – Obstinate

Some other questions with a similar topic: Possible pitfalls of using this (extension method based) shorthand, Evil use of Maybe monad and extension methods in C#?, How to check for nulls in a deep lambda expression?. – Rochette

Answer 1

Mads Torgersen has publicly said that a null-propagating operator is under consideration for the next version of C# (but also emphasised that this doesn't mean it will be there). This would allow code like:

var value = someValue?.Method()?.AnotherMethod();

where the ?. returns null if the operand (on the left) is null, else will evaluate the right hand side. I suspect that would get you a lot of the way here, especially if combined with (say) extension methods; for example:

DateTime? dtStartDate = strStartDate?.MyParse();

where:

static DateTime MyParse(this string value) {
    return DateTime.ParseExact(value, "dd.MM.yyyy",
         System.Globalization.CultureInfo.InvariantCulture
);

However! You could do the same thing right now just using extension methods:

DateTime? dtStartDate = strStartDate.MyParse();

static DateTime? MyParse(this string value) {
    if(value == null) return null;
    return DateTime.ParseExact(value, "dd.MM.yyyy",
         System.Globalization.CultureInfo.InvariantCulture
);

Obstinate

That is the answer, thank you. I considered your right now suggestion as alternative for using conditional operator – Benkley

Do you know if any consideration was given to, instead of only having an operator that maps null to null, having foo ?.bar : baz be equivalent [but for foo being evaluated only once] to foo ? foo.bar : baz? That would I think both make the syntax more useful and also bring it closer to ? :. – Clairvoyant

Answer 2

Just use the ternary conditional operator ?::

DateTime? dtStartDate = strStartDate == null ? null : DateTime.ParseExact(…)

An operator you proposed isn’t actually easily doable because it has a non-consistent type:

DateTime? a = (string)b !?? (DateTime)c;

For this expression to work, the compiler would need to know at compile time that b is null, so that the (null) string value can be assigned to a.

Rochette