C# Language Tutorial => Using Extension methods to build new...

Example

You can create extension methods to improve usability for nested collections like a Dictionary with a List<T> value.

Consider the following extension methods:

public static class DictListExtensions
{
    public static void Add<TKey, TValue, TCollection>(this Dictionary<TKey, TCollection> dict, TKey key, TValue value)
            where TCollection : ICollection<TValue>, new()
    {
        TCollection list;
        if (!dict.TryGetValue(key, out list))
        {
            list = new TCollection();
            dict.Add(key, list);
        }

        list.Add(value);
    }

    public static bool Remove<TKey, TValue, TCollection>(this Dictionary<TKey, TCollection> dict, TKey key, TValue value)
        where TCollection : ICollection<TValue>
    {
        TCollection list;
        if (!dict.TryGetValue(key, out list))
        {
            return false;
        }

        var ret = list.Remove(value);
        if (list.Count == 0)
        {
            dict.Remove(key);
        }
        return ret;
    }
}

you can use the extension methods as follows:

var dictList = new Dictionary<string, List<int>>();

dictList.Add("example", 5);
dictList.Add("example", 10);
dictList.Add("example", 15);

Console.WriteLine(String.Join(", ", dictList["example"])); // 5, 10, 15

dictList.Remove("example", 5);
dictList.Remove("example", 10);

Console.WriteLine(String.Join(", ", dictList["example"])); // 15

dictList.Remove("example", 15);

Console.WriteLine(dictList.ContainsKey("example")); // False

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C# Language Extension Methods Using Extension methods to build new collection types (e.g. DictList)

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