Question

I just found this lambda expression:

myCustomerList.GroupBy(cust => cust.CustomerId).Select(grp => grp.First());

Correct me if I am wrong, but with this lambda you can distinct the myCustomerList on the CustomerId and that’s exaclty what I need. But I am trying to figure out how it works.

The first step is the groupby: this result in a dictionary, IGouping<long, Customer> with the CustomerId as the key of the dictionary.

Second a select takes place and this is the part I don’t get. The select selects a customer, but how can it select a Customer from a dictionary? You need a key for this, because of the group by. Where’s that key? And how is First() helping here?

Can you tell me in detail how the last part works?

Answer 1

It’s not selecting it from the dictionary – it’s saying for each grouping in the result of GroupBy, select the first entry. Note that IGrouping<TKey, TElement> implements IEnumerable<TElement>.

Basically a group has two things:

• A key
• A list of elements

This is selecting the first element from each group.