optional/null-able OUT parameter in C#

Question

I have a method that has several overrides. In one of the more expanded overrides, I want to return an OUT parameter but not in my simpler overrides. For example:

public bool IsPossible(string param1, int param2)
public bool IsPossible(string param1, int param2, out bool param3)

The way I am currently achieving this, is like so:

public bool IsPossible(string param1, int param2) {
    bool temp;
    return IsPossible(param1, param2, out temp);
}

Is there a better way to achieve this? Can I (or should I) use a null-able out parameter?

score 19 · Accepted Answer · answered Jun 30 '11 at 18:38

19

That looks fine to me. A out cannot be optional for technical reasons (it needs to point to a valid instance).

answered Jun 30 '11 at 18:38

Lucero

59,176
9
122
152

Having a hard time believing the reason for not supporting this is technical more than marketing priorities. Seems the compiler could just create a temp location just as your example--and also for Refs by init'ing with Default(T). Notably, "out/ref (inout)" arguments are [supported in SQL Server](http://stackoverflow.com/questions/3267523). – crokusek Jan 20 '14 at 20:37
1

It's worth to mention, that in C# 7.0 you can use Discards with Underscore sign. Read more here: https://stackoverflow.com/a/42924200/1319086 – Jonatan Dragon Feb 01 '19 at 12:52
VB.net does it. C# should automatically allow out/ref to be optional, and fill in a dummy variable when needed in the background. – Brain2000 Apr 08 '22 at 15:12

aaroncatlin · Answer 2 · 2021-08-03T11:17:23.660

17

Since C# 7.0 you can pass a Discard (_) into the method:

return IsPossible(param1, param2, out _);

See: https://learn.microsoft.com/en-us/dotnet/csharp/discards

edited Aug 03 '21 at 11:17

answered Mar 14 '19 at 17:23

aaroncatlin

3,203
1
16
27

1

Good answer, but you forgot to write **out** `return IsPossible(param1, param2, out _);` – colaus Jul 28 '21 at 05:50
Thanks. Amended my answer – aaroncatlin Aug 03 '21 at 11:17

score 7 · Answer 3 · answered Jun 30 '11 at 19:43

OP:

Can I (or should I) use a null-able out parameter?

A nullable out parameter won't do you much good. This:

public bool IsPossible( string param1, int param2, out bool? param3 )
{
  ...
}

doesn't make param3 optional. It changes the semantics of what param3 is. A bool? (aka Nullable<bool>) widens the domain of param3 from 2 values ( true and false ) to 3, adding a third value (null), usually interpreted as missing/unknown. The caller still must supply all three arguments.

Overloading IsPossible() by providing an overload -- the "don't care" scenario -- that discards the reference parameter is the proper solution.

score 4 · Answer 4 · answered Jun 30 '11 at 18:37

4

A 'ref' or 'out' parameter cannot have a default value.

The way you've done it is just fine. Sorry, no silver bullet.

answered Jun 30 '11 at 18:37

agent-j

27,335
5
52
79

score 4 · Answer 5 · answered Jun 30 '11 at 18:39

Personally I think it is fine "as is". Another approach, though, is to return a composite return value (perhaps an immutable struct with 2 properties); that avoids the fact that some people avoid out. I am not one of those people :p

Re the question; indeed, out/ref can't be optional.

If you wanted to get unnecessarily fancy you could give the conposite return-type a conversion to bool to allow implicit if tests.

score 0 · Answer 6 · answered Jun 30 '11 at 18:38

0

Yes, Nullable variables are great. You would then do something like this.

public bool? IsPossible(string param1, int param2);

var ans = IsPossible("Parm1", 1);
if (ans.HasValue)
{
   //working.
 }

answered Jun 30 '11 at 18:38

Jethro

5,896
3
23
24

I don't find this API style hugely obvious to the caller, personally. Subjective, though. – Marc Gravell Jun 30 '11 at 18:56
What do you mean obvoius to the caller? – Jethro Jun 30 '11 at 18:58
4

well, for starters - there were previously 4 possible outcomes; there are now 3; what are they equivalent of, etc? what does it mean for IsPossible to be null? vs non-null but false? presumable true means "yes it is possible", but the other two are less clear-cut to me. – Marc Gravell Jun 30 '11 at 19:59

optional/null-able OUT parameter in C#

6 Answers6