Passing an empty array as default value of an optional parameter

Question

How does one define a function that takes an optional array with an empty array as default?

public void DoSomething(int index, ushort[] array = new ushort[] {},
 bool thirdParam = true)

results in:

Default parameter value for 'array' must be a compile-time constant.

This is not a duplicate. It is specific in asking how to set a default value for an array, not a string. There is a real difference in approaches — , Feb 02 '18 at 09:49

Lasse V. Karlsen · Accepted Answer · 2020-03-31T19:53:58.237

160

You can't create compile-time constants of object references.

The only valid compile-time constant you can use is null, so change your code to this:

public void DoSomething(int index, ushort[] array = null,
  bool thirdParam = true)

And inside your method do this:

array = array ?? new ushort[0];

(from comments) From C# 8 onwards you can also use the shorter syntax:

array ??= new ushort[0];

edited Mar 31 '20 at 19:53

answered Aug 13 '10 at 20:22

Lasse V. Karlsen

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yes, that does come to mind, except passing null is not the same as an empty array. I was hoping for a way to do it without having to dress the param inside the method. The bit about ompile-time constants of object references makes sense. Thanks! – MandoMando Aug 13 '10 at 20:37
that throws ArgumentNullException, anybody knows why? – Ayyash May 12 '15 at 16:14
4

Yes, you're not fixing the case that you are actually receiving `null`, did you use the second statement from my answer? – Lasse V. Karlsen May 12 '15 at 16:16
I just wanted to mention that C# 8 now allows a prettier syntax. `array ??= new ushort[0];` – Éric Bergeron Nov 27 '19 at 22:47
Null-coalescing assignment operator `??=` is available in C# 8 – Kuroro Mar 31 '20 at 17:37
Probably you can consider `Array.Empty()` – Yousef Nov 02 '20 at 21:47
And now tweak that for `record`s ;) ... it is annoying ;) – Thomas Nov 10 '20 at 21:16

score 28 · Answer 2 · answered Feb 28 '13 at 20:10

If you can make the array the last argument you could also do this:

public void DoSomething(int index, bool wasThirdParam = true, params ushort[] array)

The compiler will automatically pass an empty array if it is not specified, and you get the added flexibility to either pass an array as a single argument or put the elements directly as variable length arguments to your method.

score 7 · Answer 3 · answered Apr 05 '13 at 23:03

7

I know it's an old question, and whilst this answer doesn't directly solve how to get around the limitations imposed by the compiler, method overloading is an alternative:

   public void DoSomething(int index, bool thirdParam = true){
        DoSomething(index, new ushort[] {}, thirdParam);
   }

   public void DoSomething(int index, ushort[] array, bool thirdParam = true){

      ...
   }

answered Apr 05 '13 at 23:03

Darius

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I am not sure whether a compiler could then infer and optimize a mutual recursion instead of an usual *(though yes, I know e.g. there is an option `-foptimize-sibling-calls` in g++, but it is C++, I am not sure whether C# compilers can it)*. – Hi-Angel Jun 02 '15 at 10:32

Passing an empty array as default value of an optional parameter

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