Merging dictionaries in C#

public static Dictionary<TKey,TValue>
                 Merge<TKey,TValue>(Dictionary<TKey,TValue>[] dictionaries);

public static Dictionary<TKey,TValue>
                 Merge<TKey,TValue>(IEnumerable<Dictionary<TKey,TValue>> dictionaries);

var result = dictionaries.SelectMany(dict => dict)
                         .ToDictionary(pair => pair.Key, pair => pair.Value);

var result = dictionaries.SelectMany(dict => dict)
                         .ToLookup(pair => pair.Key, pair => pair.Value)
                         .ToDictionary(group => group.Key, group => group.First());

public static class DictionaryExtensions
{
    // Works in C#3/VS2080:
    // Returns a new dictionary of this ... others merged leftward.
    // Keeps the type of 'this', which must be default-instantiable.
    // Example:
    //   result = map.MergeLeft(other1, other2, ...)
    public static T MergeLeft<T,K,V>(this T me, params IDictionary<K,V>[] others)
        where T : IDictionary<K,V>, new()
    {
        T newMap = new T();
        foreach (IDictionary<K,V> src in
            (new List<IDictionary<K,V>> { me }).Concat(others)) {
            // ^-- echk. Not quite there type-system.
            foreach (KeyValuePair<K,V> p in src) {
                newMap[p.Key] = p.Value;
            }
        }
        return newMap;
    }

}

using System.Collections.Generic;
...
public static Dictionary<TKey, TValue>
    Merge<TKey,TValue>(IEnumerable<Dictionary<TKey, TValue>> dictionaries)
{
    var result = new Dictionary<TKey, TValue>();
    foreach (var dict in dictionaries)
        foreach (var x in dict)
            result[x.Key] = x.Value;
    return result;
}

static Dictionary<TKey, TValue>
    Merge<TKey, TValue>(this IEnumerable<Dictionary<TKey, TValue>> enumerable)
{
    return enumerable.SelectMany(x => x).ToDictionary(x => x.Key, y => y.Value);
}

Dictionary<String, String> allTables = new Dictionary<String, String>();
allTables = tables1.Union(tables2).ToDictionary(pair => pair.Key, pair => pair.Value);

public static IDictionary<TKey, TValue> Merge<TKey, TValue>(IEnumerable<IDictionary<TKey, TValue>> dictionaries)
{
    // ...
}

public static IDictionary<TKey, TValue> Merge<TKey, TValue>(params IDictionary<TKey, TValue>[] dictionaries)
{
    return Merge((IEnumerable<TKey, TValue>) dictionaries);
}

using System.Collections.Generic;
namespace HelperMethods
{
    public static class MergeDictionaries
    {
        public static void Merge<TKey, TValue>(this IDictionary<TKey, TValue> first, IDictionary<TKey, TValue> second)
        {
            if (second == null) return;
            if (first == null) first = new Dictionary<TKey, TValue>();
            foreach (var item in second)
                if (!first.ContainsKey(item.Key))
                    first.Add(item.Key, item.Value);
        }
    }
}

dictionaryFrom.ToList().ForEach(x => dictionaryTo.Add(x.Key, x.Value));