C# Linq merge two dictionaries

Question

How to make the following method more functional-linq-style?

    public static Dictionary<T, T> MergeDict<T, T>(Dictionary<T, T> a, Dictionary<T, T> b)
    {
        var e = new Dictionary<T, T>();

        a.Concat(b).ToList().ForEach(pair =>
        {
            e[pair.Key] = pair.Value;
        });

        return e;
    }

What behavior do you intend if there is a key collision between the two dictionaries? — Eric J., Apr 05 '12 at 19:37

score 15 · Accepted Answer · answered Apr 05 '12 at 19:43

15

To continue your duplicate discarding ways, just group up and take a winning item in the group (such as the Last one).

first.Concat(second)
  .GroupBy(kvp => kvp.Key, kvp => kvp.Value)
  .ToDictionary(g => g.Key, g => g.Last());

answered Apr 05 '12 at 19:43

Amy B

108,202
21
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This will actually have the opposite behavior - Values from "first" will be kept, where the OP will always get values from "second" (ie: `g.Last()` or reverse concat) – Reed Copsey Apr 05 '12 at 19:43
@ReedCopsey included by edit, thanks. – Amy B Apr 05 '12 at 19:46
1

@davidb I think this is the most elegant solution – michaelr524 Apr 07 '12 at 21:44

score 4 · Answer 2 · answered Apr 05 '12 at 19:37

You could use ToDictionary directly:

public static Dictionary<T, T> MergeDict<T, T>(Dictionary<T, T> a, Dictionary<T, T> b)
{
     return a.Concat(b).ToDictionary(kvp => kvp.Key, kvp => kvp.Value);
}

Be aware that this will raise an exception in the case of duplicate keys.

If you need to handle duplicate keys, you'll need to decide how you want this to be handled. For example, this will remove duplicates from "b":

public static Dictionary<T, T> MergeDict<T, T>(Dictionary<T, T> a, Dictionary<T, T> b)
{
     return a.Concat(b.Where(kvp => !a.ContainsKey(kvp.Key)).ToDictionary(kvp => kvp.Key, kvp => kvp.Value);
}

To get the same behavior as the original, you could go the other way (keeps KeyValuePair values from "b"):

public static Dictionary<T, T> MergeDict<T, T>(Dictionary<T, T> a, Dictionary<T, T> b)
{
     return b.Concat(a.Where(kvp => !b.ContainsKey(kvp.Key)).ToDictionary(kvp => kvp.Key, kvp => kvp.Value);
}

Behaves differently towards duplicates. This version throws. — H H, Apr 05 '12 at 19:40
@reedcopsey looks good, now I wonder what would be the most efficient solution :) — michaelr524, Apr 07 '12 at 21:42

score 0 · Answer 3 · answered Apr 05 '12 at 19:39

0

Perhaps something like this.

public static IDictionary<T, U> Merge<T, U>(this IDictionary<T, U> first, IDictionary<T, U> second)
{
    return first.Concat(second).ToDictionary(kvp => kvp.Key, kvp => kvp.Value);
}

answered Apr 05 '12 at 19:39

Amy Sutedja

432
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C# Linq merge two dictionaries

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