Creating a custom Key in a dictionary

Question

I'm new to C# and I'm trying to create a custom key for a dictionary.

Here is how I create the class

public class ImageKey
{
    public int RId;
    public int EId;
    public int LId;

    public ImageKey(int rId, int eId, int lId)
    {
        RId = rId;
        EId = eId;
        LId = lId;
    }


    public bool Equals(ImageKey x, ImageKey y)
    {
        return x.RId == y.RId && x.LId == y.LId &&
              x.EId == y.EId;
    }

    public int GetHashCode(ImageKey obj)
    {
        int hash = 13;
        hash = (hash * 7) + obj.RId.GetHashCode();
        hash = (hash * 7) + obj.EId.GetHashCode();
        hash = (hash * 7) + obj.LId.GetHashCode();
        return hash;
    }

}

I use it as a hashset like that

HashSet<ImageKey> resourcesId = new HashSet<ImageKey>();

and as a Dictionary

var enteries = new Dictionary<ImageKey, int>();

but this statement fails for ContainsKey when I iterate over and try to see if there is a matched key

ImageKey key = new ImageKey(rId, eId, lId);
bool knownId = enteries.ContainsKey(key);
if (!knownId)
{
   enteries.Add(key, new IgaEntry());
}

`x.LId == y.Ld` is that a typo? More so, your constructor doesn't make sense. — Yuval Itzchakov, Oct 16 '14 at 17:51
I believe you would need to use the `override` keyword for "Equals" and "GetHashCode" for this to work. — McGarnagle, Oct 16 '14 at 17:53
Your code does not compile. I think you need to modify either the property names used in your constructor or the actual property names in the class. There are no members named 'ResourceId', 'EditionId', and 'LocationId'. — Rufus L, Oct 16 '14 at 17:56
@RufusL there maybe typos, I will fix them. I just wrote it out of my head. — andre_lamothe, Oct 16 '14 at 17:58

score 4 · Accepted Answer · edited May 23 '17 at 10:25

4

You class needs to override object.Equals, Implement IEquatable<T> interface, or implement IEqualityComparer<T> and pass it as a keyComparer to Dictionary's constructor. Also you need to override GetHashCode method to avoid unexpected behavior, make sure you do it right

Otherwise, your class will be compared by reference.

Refer Marc's answer here for a sample.

edited May 23 '17 at 10:25

Community

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answered Oct 16 '14 at 17:54

Sriram Sakthivel

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Creating a custom Key in a dictionary

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