How to sort dictionary by DateTime value

Question

I have this Dictionary

Dictionary<Guid, DateTime> myDic = new Dictionary<Guid, DateTime>();

What is the best approach to sort it DESC by DateTime? Is it LINQ?

How it could be done in code?

Thank you!

Answer 1

You cannot sort a Dictionary without relying on details of current implementation, but you can process its entries in any specific order using LINQ's sorting functionality:

foreach (KeyValuePair<Guid,DateTime> pair in myDict.OrderByDescending(p => p.Value)) {
    // do some processing
}

Purely as a curiosity, you can pass ordered key-value pairs to Dictionary's constructor, and the resulting dictionary will enumerate in the order the items were provided in the enumeration to the constructor. However, relying on this functionality is very dangerous; you should not do it in your code.

Answer 2

You have the wrong data structure to achieve what you want. The purpose of a dictionary is to provide fast access by key. A SortedDictionary exists that sorts its own keys, but there is no dictionary that sorts its values because that doesn't make sense.

Assuming all you need is the DateTimes contained in myDic sorted descending, you could do:

var dateTimesDescending = myDic.Values.OrderByDescending(d => d);

That will allow you to enumerate the values in descending order, but will not sort the dictionary.