static property which returns class instance

Question

I would like to use something similar as link! Part of the source code looks like that:

public struct Vector3
{
    public static Vector3 zero { get; }
}

This function returns an already initialized Vector (which 0 on every axis). Is this possible, because it is a struct? That's what I have:

public class WorldBlock
{
    public enum e_WorldBlockType
    {
        Undef,
        Earth,
        Stone,
        Air // no block set
    }

    public static WorldBlock Empty { get; }
    public e_WorldBlockType BlockType { get; set; }
    public bool IsWalkable { get; set; }

    public WorldBlock(e_WorldBlockType blockType = e_WorldBlockType.Undef, bool isWalkable = false)
    {
        this.BlockType = blockType;
        this.IsWalkable = isWalkable;
    }
}

But I get this: error CS0840: 'WorldBlock.Empty.get' must have a body because it is not marked abstract or extern. The property can be automatically implemented when you define both accessors

How should I write the body?

EDIT: If I add for example a private set, it compiles, but if I use it, I get a null pointer exception.

possible duplicate of [Singleton: How should it be used](http://stackoverflow.com/questions/86582/singleton-how-should-it-be-used) — 123 456 789 0, Jan 08 '14 at 19:26
I have edited your title. Please see, "[Should questions include “tags” in their titles?](http://meta.stackexchange.com/questions/19190/)", where the consensus is "no, they should not". — John Saunders, Jan 08 '14 at 19:27
How many people will re-invent their own, buggy `Lazy`, when answering this question? — Dennis, Jan 08 '14 at 19:32

Håkan Edling · Answer 1 · 2014-01-08T22:56:53.140

1

If you only want an empty WorldBlock I suggest you write:

public static readonly WorldBlock Empty = new WorldBlock();

Instead of your Property. However it's kind of dodgy since the setters of the "Empty" object is public, so there's really no guarantee that it's actually empty.

edited Jan 08 '14 at 22:56

answered Jan 08 '14 at 19:31

Håkan Edling

2,723
1
12
15

+1. Since `WorldBlock` is mutable, there's no much sense in `WorldBlock.Empty`. Also, you have a typo - field name is missing. – Dennis Jan 08 '14 at 19:38

score 0 · Accepted Answer · answered Jan 08 '14 at 19:26

0

Something like this:

private static WorldBlock __empty = ...;

public static WorldBlock Empty {
  get {
    return WorldBlock __empty;
  }
}

answered Jan 08 '14 at 19:26

ichramm

6,437
19
30

I think, that might be the solution, give me a sec – Jan 08 '14 at 19:29
Can smbd tell me, if this holds just one instance of an empty WorldBlock, so if I had 20 instances of WorldBlock, there is just one `__empty` instance? – Jan 08 '14 at 19:39
@immerhart: `WorldBlock.Empty == new WorldBlock()` will evaluate to `false`. Is this normal behavior for you? – Dennis Jan 08 '14 at 19:47
Don't know why... What would you suggest? – Jan 08 '14 at 19:52

score 0 · Answer 3 · answered Jan 08 '14 at 19:26

You have to do:

public static WorldBlock Empty
{
  get{return someThing; }
}

Replace someThing with whatever you were wanting Empty to store.... If thats null or whatever.

From your title... it looks like you want to return an instance of the class. If you only ever want one instance of this class... you're looking for the Singleton design pattern.

score 0 · Answer 4 · answered Jan 08 '14 at 19:26

Should your "Empty" instances be shared and be the same? If so, the following body is probably what you are looking for.

private static WorldBlock emptyInstance;
public static WorldBlock Empty
{
   get
   {
      if (emptyInstance == null)
      {
         //initialize it
      }
      return emptyInstance;
   }
}

static property which returns class instance

4 Answers4