Simple C# pure extension methods for dictionaries

DictionaryExtensions.cs

using System.Collections.Generic;

public static class DictionaryExtensions
{
    public static IDictionary<T, U> Merge<T, U>(this IDictionary<T, U> target, IDictionary<T, U> source)
    {
        var result = new Dictionary<T, U>(target);
        foreach (var pair in target) {
            if (source.ContainsKey(pair.Key)) {
                result[pair.Key] = source[pair.Key];
            }
        }
        foreach (var pair in source) {
            if (!target.ContainsKey(pair.Key)) {
                result[pair.Key] = pair.Value;
            }
        }
        return result;
    }

    public static IDictionary<T, U> Insert<T, U>(this IDictionary<T, U> source, T key, U value, bool before = false)
    {
        IDictionary<T, U> result;
        switch (before) {
            default:
                result = new Dictionary<T, U>();
                result.Add(key, value);
                return result.Merge(source);
            case false:
                result = new Dictionary<T, U>(source);
                result.Add(key, value);
                return result;
        }
    }
}