"Grouping" dictionary by value

You can use GroupBy.

Dictionary<int,List<int>> groups = 
             prices.GroupBy(x => x.Value)
                   .ToDictionary(x => x.Key, x => x.Select(i => i.Key).ToList());

Here is my reply. When the dictionaries get large, you will likely find the GroupBy() extension methods less efficient than you would like, as they provide many guarantees that you don't need, such as retaining order.

public static class DictionaryExtensions 
{
    public static IDictionary<TValue,List<TKey>> Reverse<TKey,TValue>(this IDictionary<TKey,TValue> src) 
    {
        var result = new Dictionary<TValue,List<TKey>>();

        foreach (var pair in src) 
        {
            List<TKey> keyList;

            if (!result.TryGetValue(pair.Value, out keyList)) 
            {
                keyList = new List<TKey>();
                result[pair.Value] = keyList;
            }

            keyList.Add(pair.Key);
        }

        return result;
    }
}

And an example to use in LinqPad:

void Main()
{
    var prices = new Dictionary<int, int>();
    prices.Add(1, 100);
    prices.Add(2, 200);
    prices.Add(3, 100);
    prices.Add(4, 300);

    // Dump method is provided by LinqPad.
    prices.Reverse().Dump();
}

var prices = new Dictionary<int, int>();
prices.Add(1, 100);
prices.Add(2, 200);
prices.Add(3, 100);
prices.Add(4, 300);

Dictionary<int,List<int>> test  = 
                   prices.GroupBy(r=> r.Value)
                  .ToDictionary(t=> t.Key, t=> t.Select(r=> r.Key).ToList());

You can use GroupBy followed by the Func<TSource, TKey>, Func<TSource, TElement> overload of Enumerable.ToDictionary:

var d = prices.GroupBy(x => x.Value).ToDictionary(x => x.Key, x => x.ToList());