C# Optional Array Parameter for Class

Question

I know this can be done using null so I have a workaround for that, but I was wondering if there was a better way that I can have an optional int[] parameter for a class?

class PriceLevels
{
    public int[] priceLevels { get; }
    private readonly int[] defaultPriceLevels = { 2, 3, 3, 4, 5, 6 };

    public PriceLevels(int[] newPriceLevels = defaultPriceLevels)
    {
        priceLevels = newPriceLevels;
    }
}

This gives me an error saying it is an invalid expression defaultPriceLevels must be constant. How can I fix this?

One workaround that I can thing of is this, but I don't really like the solution

class PriceLevels
{
    public int[] priceLevels { get; }
    private readonly int[] defaultPriceLevels = { 2, 3, 3, 4, 5, 6 };

    public PriceLevels(int[] newPriceLevels = null)
    {
        if(newPriceLevels == null) 
            priceLevels = defaultPriceLevels;
        else 
            priceLevels = newPriceLevels;
    }
}

Possible duplicate of [Passing an empty array as default value of an optional parameter](http://stackoverflow.com/questions/3480382/passing-an-empty-array-as-default-value-of-an-optional-parameter) — Ralf Bönning, Oct 11 '16 at 06:07
Default Value by assignment for a function's input value can be the same as "const" assignment rules……So yours cannot be implemented directly until now. — xqMogvKW, Oct 11 '16 at 06:12
@S.Akbari You're probably right. However, if the idea is to only allow `priceLevels` to be set on initialization (hence the default value as a fallback), then it's valid. — Michael McMullin, Oct 11 '16 at 06:22
Possible duplicate of [Setting the default value of a C# Optional Parameter](http://stackoverflow.com/questions/2729639/setting-the-default-value-of-a-c-sharp-optional-parameter) — hatchet - done with SOverflow, Oct 11 '16 at 20:20

Gilad Green · Accepted Answer · 2017-09-21T07:43:34.160

17

A better design all together would be to have 2 constructors (constructor overload) one that gets a int[] and another that doesn't:

class PriceLevels
{
    public int[] priceLevels { get; set; }
    private readonly int[] defaultPriceLevels = { 2, 3, 3, 4, 5, 6 };

    public PriceLevels()
    {
        priceLevels = defaultPriceLevels;
    }

    public PriceLevels(int[] newPriceLevels)
    {
       priceLevels = newPriceLevels;
    }
}

If not, don't know if i'd call this "better" but you can use the params keyword:

class PriceLevels
{
    public int[] priceLevels { get; set; }
    private readonly int[] defaultPriceLevels = { 2, 3, 3, 4, 5, 6 };

    public PriceLevels(params int[] newPriceLevels)
    {
        priceLevels = newPriceLevels.Length == 0 ? defaultPriceLevels : newPriceLevels;
    }
}

Also, depending on design, I'm not convinced that it is PriceLevels responsibility to decide what the default values are and maybe it should get it as a dependency in any case - See SOLID and Dependency Injection. Then you'd have only 1 constructor:

class PriceLevels
{
    public int[] priceLevels { get; set; }

    public PriceLevels(int[] newPriceLevels)
    {
       priceLevels = newPriceLevels;
    }
}

edited Sep 21 '17 at 07:43

answered Oct 11 '16 at 06:05

Gilad Green

36,708
7
61
95

Also the `priceLevels` property should has setter. – Salah Akbari Oct 11 '16 at 06:13
The null-coalescing operator on its own makes it 'better'! – Michael McMullin Oct 11 '16 at 06:14
Thanks, I think I will probably go with your final suggestion and re-think my structure. Also, curious about this setter thing... why is it necessary that I have a setter (even if it is private) if all I am doing is setting that value upon initialization of the class? – Adjit Oct 11 '16 at 06:28
@Adjit - Cubicle.Jockey comment below about the setters. – Gilad Green Oct 11 '16 at 06:53
@Gilad, I don't think the first part with `params` works - if you call it w/o parameters, you still receive **empty array**, not `null`. – Ivan Stoev Oct 11 '16 at 11:01
@IvanStoev - correct. Tested and you are right. Missed it. Fixed the problem. Thanks :) – Gilad Green Oct 11 '16 at 11:07

jegtugado · Answer 2 · 2016-10-11T06:27:00.273

9

You can overload your constructor.

class PriceLevels
{
    public int[] priceLevels { get; private set; }
    private readonly int[] defaultPriceLevels = { 2, 3, 3, 4, 5, 6 };

    public PriceLevels()
    {
        priceLevels = defaultPriceLevels;
    }

    public PriceLevels(int[] newPriceLevels)
    {
       priceLevels = newPriceLevels;
    }
}

edited Oct 11 '16 at 06:27

answered Oct 11 '16 at 06:06

jegtugado

5,081
1
12
35

1

Also the priceLevels property should has setter. – Salah Akbari Oct 11 '16 at 06:13
Added a private setter. Thanks. – jegtugado Oct 11 '16 at 06:27
2

If C# 6.0 the private set is unnecessary if the value is set in the constructor only. – Cubicle.Jockey Oct 11 '16 at 06:48

score 7 · Answer 3 · answered Oct 11 '16 at 06:06

7

You could create another constructor with no parameters that would just pass your defaultPriceLevels variable to the constructor you already have? You'd have to change the variable to static too.

e.g.

class PriceLevels
{
    public int[] priceLevels { get; }
    private static int[] defaultPriceLevels = { 2, 3, 3, 4, 5, 6 };

    public PriceLevels(int[] newPriceLevels = null)
    {
        if (newPriceLevels == null) priceLevels = defaultPriceLevels;
        else priceLevels = newPriceLevels;
    }

    public PriceLevels() : this(defaultPriceLevels)
    { }
}

answered Oct 11 '16 at 06:06

Mathew

4,297
4
29
38

@DongWei You surely can do it. Try it in linqpad. – Mathew Oct 11 '16 at 06:11
：Sorry I should paste what I said to OP instead of you :) – xqMogvKW Oct 11 '16 at 06:14
1

Liked this solution but you shouldn't make the `newPriceLevels` optional in one of the Constructors, that in my view has no benefit +1 – Mrinal Kamboj Oct 11 '16 at 06:18

Mrinal Kamboj · Answer 4 · 2016-10-11T06:52:45.413

7

Another option in the Crowd :), which is closest to your original code

class PriceLevels
{
  public int[] priceLevels { get; set; }

  private readonly int[] defaultPriceLevels = { 2, 3, 3, 4, 5, 6 };

  public PriceLevels(int[] newPriceLevels = null)
  {
     priceLevels = newPriceLevels ?? defaultPriceLevels;
  }
}

edited Oct 11 '16 at 06:52

answered Oct 11 '16 at 06:24

Mrinal Kamboj

11,300
5
40
74

Sweeper · Answer 5 · 2016-10-11T06:44:09.463

4

Just create an overload of the constructor! It can be really easily done with one line of code!

public PriceLevels() : this(defaultPriceLevels) { }

Then, remove the default parameter value from the original constructor:

public PriceLevels(int[] newPriceLevels)

Actually, you also need to declare defaultPriceLevels as static. Hopefully you don't mind that.

edited Oct 11 '16 at 06:44

answered Oct 11 '16 at 06:10

Sweeper

213,210
22
193
313

score 3 · Answer 6 · answered Oct 11 '16 at 06:11

Optional array parameters are not possible in C#. From MSDN

Each optional parameter has a default value as part of its definition. If no argument is sent for that parameter, the default value is used. A default value must be one of the following types of expressions:

a constant expression;

an expression of the form new ValType(), where ValType is a value type, such as an enum or a struct;

an expression of the form default(ValType), where ValType is a value type.

Therefore a "workaround" has to be used. Either check for null (as you suggested) or you can use a default constructor.

C# Optional Array Parameter for Class

6 Answers6

Linked