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.

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
);

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.