C# List as Dictionary key

Question

I have a dictionary which is keyed by a List:

private Dictionary<List<custom_obj>, string> Lookup;

I'm trying to use ContainsKey, but it doesn't seem to be working, and I have no idea why. Here is the debug information from my Visual Studio Immediate Window:

?Lookup.Keys.ElementAt(7)[0]
{custom_obj}
    Direction: Down
    SID: 2540
?Lookup.Keys.ElementAt(7)[1]
{custom_obj}
    Direction: Down
    SID: 2550
searchObject[0]
{custom_obj}
    Direction: Down
    SID: 2540
searchObject[1]
{custom_obj}
    Direction: Down
    SID: 2550
?Lookup.ContainsKey(searchObject)
false

In my common sense, that last ContainsKey should be true. Hopefully I've included enough information here... any ideas?

Thanks!

score 18 · Accepted Answer · answered Apr 04 '12 at 23:15

The List<custom_obj> instance acting as a key is referentially unequal to the instance referred to by searchObject.

If you want the dictionary to use the values in the list instead of referential equality to find matching keys, you must supply an IEqualityComparer in the constructor of the dictionary (since you can't override Equals and GetHashCode in List<T>).

Kendall Frey · Answer 2 · 2014-12-16T20:31:36.540

14

You have two separate Lists that contain the same elements. The correct way to find out if two lists are equal is with the SequenceEqual method.

You cannot by default do what you are trying to do. You can however, write a custom IEqualityComparer and pass it into the Dictionary constructor.

Here is a sample generic IEqualityComparer:

class ListComparer<T> : IEqualityComparer<List<T>>
{
    public bool Equals(List<T> x, List<T> y)
    {
        return x.SequenceEqual(y);
    }

    public int GetHashCode(List<T> obj)
    {
        int hashcode = 0;
        foreach (T t in obj)
        {
            hashcode ^= t.GetHashCode();
        }
        return hashcode;
    }
}

You may want to improve on the GetHashCode implementation, as this was a quick-and-dirty solution.

edited Dec 16 '14 at 20:31

answered Apr 04 '12 at 23:15

Kendall Frey

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GetHashCode is missing a return. – nvja Dec 16 '14 at 18:45
1

Quick note: As the answer hints at, you should not use this Comparer. The equals method is sensitive to order but the hash method is not. – Alex N. Jun 16 '16 at 19:35
You can use `StructuralComparisons` class (since .NET 4.0 and Visual Studio 2010), see [this answer](https://stackoverflow.com/a/54492137). – Jeppe Stig Nielsen Feb 02 '19 at 10:39
Starting from .NET Core 2.1 or .NET Framework 4.6.1, you can use [System.HashCode](https://learn.microsoft.com/en-us/dotnet/api/system.hashcode?view=netcore-2.1), which might be better than implementing your own hash. See [this answer](https://stackoverflow.com/a/51716512/2491528). – Federico Jul 30 '21 at 09:04

score 3 · Answer 3 · answered Apr 04 '12 at 23:15

3

This will only work if the actual list instance used in the lookup is the same as the instance that was added as a key. It will not compare the list contents. This is the same behavior you will get if you try to compare two List objects directly.

answered Apr 04 '12 at 23:15

David Nelson

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score 0 · Answer 4 · answered Apr 04 '12 at 23:16

0

Are you certain that the instance you are using in your lookup method is the same instance that is among your dictionary's keys? That is the only thing I can think of.

answered Apr 04 '12 at 23:16

Sean H

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C# List as Dictionary key

4 Answers4

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