Proposal: C# null conditional assignment

[HermanEldering]

With the introduction of the null conditional operator it might make sense to also add null conditional assignment, for instance like this:

targetVariable ?= sourceVariable;

Which would be functional equivalent to:

// normal method
if (sourceVariable != null) targetVariable = sourceVariable; 
// slightly shorter method, but less clear (imo)
targetVariable = sourceVariable ?? targetVariable;

[gafter]

I'm trying to understand why this would be a good thing.

[HermanEldering]

It would be similar to the ?. operator in that it reduces the verbosity of the code, so it's advantages are similar. Admittedly, it is probably a less frequent operation. A disadvantage, compared with the null conditional operator, is that is less clear which variable is checked for null.

[whoisj]

I can see some value in an operator like this, though I'm not sure that it brings clarity of any kind.

Basically, the proposal is for a null-coalescing-assignment operator. It does reduce some verbosity in some cases, but there's a ton of holes in the current design like: how do you know which choice it made? It seems to a case for simply coalescing the following statement into a single statement:

object myRef = Init();

object testRef;
if ((testRef = Update()) != null)
{
    myRef = testRef;
}

// use myRef ...

into

object myRef = Init();
myRef ?= Update();
// use myRef ...

While I suppose I could see uses for this, I'm not it makes for overly clear code. Additionally, in the flow of things I don't have a way to know if the value of myRef is its initial value or the reallocated value and have surrendered flow control. If I explicitly do not care about its value, and only care that it has a value the null-coalescing operator is how to do default values.

object myRef = Update() ?? Init();
// use myRef ...

This is even more concise and is fairly explicit in that the value after the ?? is the default to value.

[paulomorgado]

Would it make a lot more sense to allow ?? to be composed with = (like +, -, etc.)?

targetVariable ??= sourceVariable;

[whoisj]

@paulomorgado interesting idea, but I don't think that fulfills the contract of the null-coalescing-assignment which is to say "assign a value but if it is null, use this default value instead".

The ?= / ??= operators are saying "assign a value if the it is not null, otherwise do not assign the value", which is still a like odd but maybe useful.

[paulomorgado]

That kind of is different than everything already in C#. Isn't it?