How do you get the duplicate key that ToDictionary() has failed on?

Question

I'm creating a Dictionary object, using IEnumerable's ToDictionary() extension method:

var dictionary = new Dictionary<string, MyType>     (myCollection.ToDictionary<MyType, string>(k => k.Key)); 

When it executes, it throws the following ArgumentException:

An item with the same key has already been added.

How do I get it to tell me what the duplicate key is?

Answer 1

Get the duplicate keys:

var duplicateKeys =   myCollection  .GroupBy(k => k.Key)  .Where(g => g.Count() > 1)  .Select(g => g.Key);