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Merging Dictionary objects

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Say you have two (or more) Dictionary objects and want their contents merged, this can be done with LINQ very conveniently like so.

var mergedDictionary = dictionaries
    .SelectMany(e => e)
    .ToLookup(e => e.Key, e => e.Value)
    .ToDictionary(e => e.Key, e => e.First());

There is a major issue with this method though in that it’s quite horrible performance-wise, the reason being because of the amount of conversions and unnecessary work that is happening. Firstly SelectMany is flattening the IEnumerables into a single IEnumerable, then we’re converting the new IEnumerable into a Lookup and then we’re converting the Lookup into a Dictionary taking only the first value for any key in the Lookup to resolve duplicates.

This is one of the problems with LINQ, everything is so simple and easy to use, but if you don’t understand what is going on then a LINQ query can have a serious hit on the performance of an algorithm.

The way I would go about it would be to create an extension method on Dictionary<TKey, TValue> and loop through each Dictionary and value, merging it in if the key doesn’t exist in the source Dictionary.

public static class DictionaryExtensionMethods
{
    public static void MergeIn<TKey, TValue>(
        this Dictionary<TKey, TValue> main,
        params Dictionary<TKey, TValue>[] dictionaries)
    {
        foreach (var dictionary in dictionaries)
        {
            foreach (var item in dictionary)
            {
                if (!main.ContainsKey(item.Key))
                {
                    main[item.Key] = item.Value;
                }
            }
        }
    }
}

This method doesn’t create a new Dictionary but rather merges everything in to it, it would be very easy to modify this to a method that returns a new Dictionary.

public static Dictionary<TKey, TValue> MergeIn<TKey, TValue>(
    params Dictionary<TKey, TValue>[] dictionaries)
{
    var result = new Dictionary<TKey, TValue>();
    foreach (var dictionary in dictionaries)
    {
        foreach (var item in dictionary)
        {
            if (!result.ContainsKey(item.Key))
            {
                result[item.Key] = item.Value;
            }
        }
    }
    return result;
}

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