Getting Value from dictionary, when the key in an object

Question

I have an object PixelData:

public class PixelData
{
   public int X {get;set;}
   public int Y {get;set;}
}

pixel data is a key to a dictionary.

Dictionary<PixelData, int> dict

how do i use pixel data the right way?

Well as you haven't overridden `Equals` or `GetHashCode`, you'd need to have a reference to the original `PixelData` object used as the key. Additionally, even if you *did* override `Equals` and `GetHashCode`, you'd have a mutable key which is generally a bad idea. — Jon Skeet, Sep 10 '17 at 14:44

stelioslogothetis · Accepted Answer · 2017-09-10T15:19:02.633

3

A very simple solution would be to use a struct instead of a class for PixelData:

public struct PixelData
{
    public int X;
    public int Y;
}

var dict = new Dictionary<PixelData, int>();

You can read about the differences between structs and classes in C# here. Short version: structs are value types, unlike classes which are reference types. Therefore, if you want to retrieve a value from the dictionary, you don't need a reference to the original PixelData instance which was used as the key. You can go ahead and create a new instance, with the exact same X and Y you used for the key, and it will work just fine.

// Add new value to dictionary with new PixelData instance as the key
dict.Add(new PixelData { X = 1, Y = 1 }, 42); 

// Retrieving the value using a new, but identical instance of PixelData works just fine
int value = dict[new PixelData { X = 1, Y = 1 }]);

edited Sep 10 '17 at 15:19

answered Sep 10 '17 at 15:13

stelioslogothetis

9,371
3
28
53

Mutable structs are evil. https://stackoverflow.com/a/441357/3312 https://techiesimon.com/2017/02/18/mutable-structs-are-evil-yes-really/ http://www.tigranetworks.co.uk/blogs/electricdreams/the-evil-of-mutable-structs/ https://blogs.msdn.microsoft.com/ericlippert/2008/05/14/mutating-readonly-structs/ – Jesse C. Slicer Sep 10 '17 at 16:35
You'd also want to override gethashcode for efficiency I believe. – Yair Halberstadt Sep 10 '17 at 18:19
@YairHalberstadt Are you sure? It worked fine as is when I tested it out – stelioslogothetis Sep 10 '17 at 18:25
It will work fine, just possibly not as fast as it could be. – Yair Halberstadt Sep 10 '17 at 18:27
It turns out that since the struct contains only value types you don't need to override gethashcode. http://stackoverflow.com/a/5927853/ – Yair Halberstadt Sep 10 '17 at 18:40
Jesse, you are usually right, but in this case, keys are rarely changed. Thank you all guys. – tal Sep 10 '17 at 19:04
So why not just make them immutable? – Yair Halberstadt Sep 11 '17 at 12:00

score -1 · Answer 2 · answered Sep 10 '17 at 18:16

-1

If X and Y are public properties, then below query can be used to retrieve the matching dictionary value. This query assumes that 'searchingForItem' is the PixelData Item, you are looking to find in dict.

var matchingItem = dict.FirstOrDefault(p => p.Key.X.Equals(searchingForItem.X) && p.Key.Y.Equals(searchingForItem.Y));

answered Sep 10 '17 at 18:16

user7986086

52
4

That seems like a pretty topsy-turvy way to use a dictionary ;-) – Yair Halberstadt Sep 10 '17 at 18:21

Getting Value from dictionary, when the key in an object

2 Answers2