How Can I Efficiently Merge Multiple Dictionaries in C#?

Merge Multiple Dictionaries Efficiently in C#

When working with multiple dictionaries in C#, merging them into a single dictionary can be a common task. Here's a robust and efficient method to accomplish this:

public static Dictionary<TKey, TValue> Merge<TKey, TValue>(IEnumerable<Dictionary<TKey, TValue>> dictionaries)
{
    return dictionaries.SelectMany(dict => dict).ToDictionary(pair => pair.Key, pair => pair.Value);
}

This extension method takes an enumerable collection of dictionaries as an argument and flattens them into a single dictionary by selecting all key-value pairs from each dictionary using SelectMany. It then converts the result into a new dictionary using ToDictionary with the keys and values from the flattened pairs.

If the original dictionaries contain duplicate keys, the behavior of this method depends on the version of .NET being used. In .NET versions prior to 8, it will throw an exception. In .NET 8 and later, the newly defined ToDictionary overload will ignore duplicate keys.

For more advanced scenarios, you can also use ToLookup to create a lookup dictionary that allows multiple values per key. However, converting that to a dictionary with unique keys requires additional steps, as demonstrated in the provided reference code.

Whether you choose to overwrite duplicate keys or handle them differently, the key insight is to leverage the power of LINQ's SelectMany operator to flatten the dictionaries and create a new one efficiently.

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