How do I get the latest date from a collection of objects using LINQ?

Question

I have a list of objects and each object has a ExpirationDate property which is of type DateTime. I want to retrieve the latest date in the list. Is there an elegant way of doing that through LINQ?

Something like:

DateTime latestDate =  myCollection.Where(r=>r.ExpirationDate.HasValue).MaxDate(r=>r.ExpirationDate.Value); 

Answer 1

DateTime? latestDate = myCollection.Max(r => r.ExpirationDate); 

Intellisense should have given you the Max() method. =)

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Now, if you must assign it to a DateTime, I would consider doing thus:

DateTime latestDate = myCollection.Where(r => r.ExpirationDate.HasValue)                                   .Max(r => r.ExpirationDate)                                   .Value; 

This will not cover the case of an empty collection or a collection with only null ExpirationDates.

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Depending on your earlier logic, checking

myCollection.Any(r => r.ExpirationDate.HasValue) 

might be a good idea before trying to assign a value.

Answer 2

You are almost there. Use Max:

DateTime? biggest = myCollection.Max(r=>r.ExpirationDate); 

If all expiration dates are null or the collection is empty you will get Null as a result, otherwise the greatest date.

As you have commented on J. Sheens' answer that you want an DateTime as a result you will need to do something about any empty collection or no items with a value, you could do this with

DateTime biggest=myCollection.Max(r=>r.ExpirationDate) ?? DateTime.MinValue 

This will give you DateTime.MinValue instead of nulls in my previous example (it also has the advantage over using the any clause that it iterates the collection once). I picked MinValue as an example. You could use your own abitary date.

Using the DateTime? is better, because it allows you to use null for what it's meant to mean - undefined as MinValue could be a valid item in your list.