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Dictionary<TKey, TValue> is great for doing fast lookups using the strongly-typed key; sometimes, however, I need to do lookups based on a unique combination of two (or more) variables. My "design pattern" for accomplishing this is to concatenate the values together using a separator, like this:

String compoundKey = productID.ToString + "~" + colorID.ToString;
myDict.Add(compoundKey, myObject);

This method seems a bit hacky. This answer uses a struct but I'm not convinced that it is any better.

What is the preferable way of making dictionaries work with compound keys?

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poke
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2 Answers2

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If your key is made up of two parts, the most obvious solution is a variable which has two parts. The "classical" way to produce this is a struct:

struct MyKey {
    public string PartA;
    public string PartB;
}

.NET will hash both fields for Dictionary<MyKey, object>. It's better to use a struct, not a class, because by default the identity of a class is defined by where the reference points (similar to the concept of pointers in C or C++) and not the values of the fields. You can, of course, override the Object.GetHashCode method in your class - but it's less work to just use a struct.

In practice, .NET already has a convenience for such situations: A struct with 2 fields can be represented as a Tuple<string, string>. You can then define:

var d = new Dictionary<Tuple<string, string>, object>

And you will index with d[new Tuple<string, string>("some value", "some other value")>].

This looks a bit bulky, so you can add using MyKey = System.Tuple<string, string> to the beginning of your .cs file. Then you will be able to index with d[new MyKey("some value", "some other value")]. The using a = b notation acts as a compile time search and replace.

Superbest
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    I believe the `>` inside `d[new MyKey("some value", "some other value")>]` (in the final paragraph) should be removed - appears to be a cut-and-paste typo. – Hutch Aug 11 '15 at 17:39
  • Note that if you make a struct for your pair, you should override GetHashCode or supply your own IEqualityComparer because the default GetHashCode for structs does *not* necessarily hash both fields! http://stackoverflow.com/questions/5926776/how-does-native-implementation-of-valuetype-gethashcode-work/5927853#5927853 – Jerry Federspiel Aug 11 '15 at 18:14
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What is the preferable way of making dictionaries working with compound keys?

My preferred method is to make a custom immutable class for the Key type. This requires a bit of work, but is very clean and clear, and works for any combination of types.

You can also customize the GetHashCode and equality computations if required, as well.

If you're using string or types that properly implement hashing and equality, you can use a Tuple<T1,T2>. If the values within your compound type are more complex, or you need custom equality/hashing (ie: only compare off an ID within an entity object instead of the all properties), a custom class allows you full control over how the equality and hashing are determined for your keys.

Reed Copsey
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  • What about using `Tuple`? – John Saunders Feb 21 '14 at 01:26
  • Just don't forget to make that class immutable. – alex.b Feb 21 '14 at 01:27
  • @JohnSaunders Tuples work fine, too - they're just ugly when you go to work with the type, and may or may not implement hashing appropriately, depending on the data stored in the key. – Reed Copsey Feb 21 '14 at 01:30
  • @Reed: this is the sort of thing I was trying to elicit. Could you add that to your answer, along with a description of the kind of class which, "depending on the data stored in the key" "may not implement hashing correctly". – John Saunders Feb 21 '14 at 01:32
  • @ReedCopsey: and one sentence about immutability please. – alex.b Feb 21 '14 at 01:35
  • @ReedCopsey: works great. I had already upvoted you, so take the thought as the deed: +2 – John Saunders Feb 21 '14 at 01:38
  • @ReedCopsey: Can you share a code example? I'm particularly interested in seeing an example of a key comprised of a String and an Int. – poke Feb 21 '14 at 01:52
  • @poke With a string and int, you could either use `Tuple` or make a class like this one: http://stackoverflow.com/a/3976305/65358 – Reed Copsey Feb 21 '14 at 18:23