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 — Ralf Bönning
– Ralf Bönning, Commented Oct 11, 2016 at 6:07
@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
– Michael McMullin, Commented Oct 11, 2016 at 6:22
Possible duplicate of Setting the default value of a C# Optional Parameter — hatchet - done with SOverflow
– hatchet - done with SOverflow, Commented Oct 11, 2016 at 20:20

Gilad Green · Accepted Answer · 2017-09-21 07:43:34Z

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, 2017 at 7:43

answered Oct 11, 2016 at 6:05

Gilad Green

37.3k7 gold badges67 silver badges99 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

Salah Akbari Over a year ago

Also the priceLevels property should has setter.

Michael McMullin Over a year ago

The null-coalescing operator on its own makes it 'better'!

Adjit Over a year ago

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?

Gilad Green Over a year ago

@Adjit - Cubicle.Jockey comment below about the setters.

Ivan Stoev Over a year ago

@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.

|

jegtugado · Accepted Answer · 2016-10-11 06:27:00Z

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, 2016 at 6:27

answered Oct 11, 2016 at 6:06

jegtugado

5,1511 gold badge14 silver badges35 bronze badges

3 Comments

Salah Akbari Over a year ago

Also the priceLevels property should has setter.

jegtugado Over a year ago

Added a private setter. Thanks.

Cubicle.Jockey Over a year ago

If C# 6.0 the private set is unnecessary if the value is set in the constructor only.

Mathew · Accepted Answer · 2016-10-11 06:06:34Z

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, 2016 at 6:06

Mathew

4,8574 gold badges31 silver badges38 bronze badges

3 Comments

Mathew Over a year ago

@DongWei You surely can do it. Try it in linqpad.

xqMogvKW Over a year ago

：Sorry I should paste what I said to OP instead of you :)

Mrinal Kamboj Over a year ago

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 · Accepted Answer · 2016-10-11 06:52:45Z

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, 2016 at 6:52

answered Oct 11, 2016 at 6:24

Mrinal Kamboj

11.6k5 gold badges45 silver badges82 bronze badges

Comments

Sweeper · Accepted Answer · 2016-10-11 06:44:09Z

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, 2016 at 6:44

answered Oct 11, 2016 at 6:10

Sweeper

292k23 gold badges260 silver badges437 bronze badges

Comments

Ralf Bönning · Accepted Answer · 2016-10-11 06:11:02Z

3

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.

answered Oct 11, 2016 at 6:11

Ralf Bönning

15.7k6 gold badges56 silver badges71 bronze badges

Collectives™ on Stack Overflow

C# Optional Array Parameter for Class

6 Answers 6

6 Comments

3 Comments

3 Comments

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

6 Comments

3 Comments

3 Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related