How to combine two dictionaries without looping?

Question

var d3 = d1.Concat(d2).ToDictionary(x => x.Key, x => x.Value);

You can use Concat:

Dictionary<string, object> d1 = new Dictionary<string, object>();
d1.Add("a", new object());
d1.Add("b", new object());
Dictionary<string, object> d2 = new Dictionary<string, object>();
d2.Add("c", new object());
d2.Add("d", new object());

Dictionary<string, object> d3 = d1.Concat(d2).ToDictionary(e => e.Key, e => e.Value);

foreach (var item in d3)
{
    Console.WriteLine(item.Key);
}

First up, it's not possible without looping. Whether that loop is done in a (extension) method is irrelevent, it still requires a loop.

I'm actually going to recommend doing it manually. All the other answers given require using two extention methods (Concat - ToDictionary and SelectMany - ToDictionary) and thus looping twice. If you are doing this to optimise your code, it will be faster to do a loop over dictionary B and add it's contents to dictionary A.

Edit: After further investigation, the Concat operation would only occur during the ToDictionary call, but I still think a custom extension method would be more efficient.

If you want to reduce your code size, then just make an extension method:

public static class DictionaryExtensions
{
    public static IDictionary<TKey,TVal> Merge<TKey,TVal>(this IDictionary<TKey,TVal> dictA, IDictionary<TKey,TVal> dictB)
    {
        IDictionary<TKey,TVal> output = new Dictionary<TKey,TVal>(dictA);

        foreach (KeyValuePair<TKey,TVal> pair in dictB)
        {
            // TODO: Check for collisions?
            output.Add(pair.Key, Pair.Value);
        }

        return output;
    }
}

Then you can use it by importing ('using') the DictionaryExtensions namespace and writing:

IDictionary<string,objet> output = dictA.Merge(dictB);

I have made the method act like the objects are immutable, but you could easily modify it to not return a new dictionary and just merge into dictA.

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