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?
Get the duplicate keys:
var duplicateKeys =
myCollection
.GroupBy(k => k.Key)
.Where(g => g.Count() > 1)
.Select(g => g.Key);
If your specific situation makes it okay to only insert one of a set of objects with duplicate Key properties into your dictionary, you can avoid this error entirely by using the LINQ Distinct method prior to calling ToDictionary.
var dict = myCollection.Distinct().ToDictionary(x => x.Key);
Of course, the above will only work if the classes in your collection override Equals and GetHashCode in a way that only takes the Key property into account. If that's not the case, you'll need to make a custom IEqualityComparer<YourClass> that only compares the Key property.
var comparer = new MyClassKeyComparer();
var dict = myCollection.Distinct(comparer).ToDictionary(x => x.Key);
If you need to make sure that all instances in your collection end up in the dictionary, then using Distinct won't work for you.