LINQ: Exception, Null or Object?

I have run into a problem.  LINQ didn’t behave as I had expected when trying to query an object from a list.  So I set out to figure out what I didn’t know.  Turns out I am not alone in this since I have been asked about it several times. This post  will look at what you can expect to happen when you query a list of objects that does not contain the item you are looking for.  This can produce three different outcomes depending on the way you approach it.  You will either get an exception, a null object or an empty object.

Before we begin we need a class to give us some useful data to query against.

I created a real simple Item object with just an id and name, as well as, a simple class to build up a IQueryable<Item> for us to run some LINQ queries against.

   1:      public class Item

   2:      {

   3:          public int Id { get; set; }

   4:   

   5:          public string Name { get; set; }

   6:      }

   7:   

   8:      public class ItemData

   9:      {

  10:          public IQueryable<Item> GetItems()

  11:          {

  12:              var users = new List<Item>

  13:                  {

  14:                      new Item {Id=1, Name = "Item 1"},

  15:                      new Item {Id=2, Name = "Item 2"},

  16:                      new Item {Id=3, Name = "Item 3"},

  17:                      new Item {Id=4, Name = "Item 4"},

  18:                      new Item {Id=5, Name = "Item 5"},

  19:                  };

  20:   

  21:              return users.AsQueryable();

  22:          }

  23:      }

Single

var single = items.Single(x => x.Id == 10);

Single returns the single, specific item from a sequence of values that matches your query.  Single throws an exception if there is not exactly one element in the sequence.  In my testing when looking for an item not in the sequence it threw [System.InvalidOperationException] = {"Sequence contains no matching element"}.

Single with DefaultIfEmpty

var singleDefaultIfEmtpy = items.DefaultIfEmpty(new Item()).Single(x => x.Id == 10);

So what changes of we add the .DefaultIfEmpty(new Item()) to the query?  None at all, it still throws [System.InvalidOperationException] = {"Sequence contains no matching element"}.

SingleOrDefault

var singleOrDefault = items.SingleOrDefault(x => x.Id == 10);

This one is a little better, it doesn’t throw an exception it returns null.  Why didn’t it return an empty Item object?  You’d think that would be the default and that it could figure it out because it knows what type of object we are querying for.  Nope, if you don’t specify the default value you get null.

SingleOrDefault with DefaultIfEmpty

Ok, so lets specify a default value with .DefaultIfEmpty.

var singleOrDefaultIfEmpty = items.DefaultIfEmpty(new Item())

                .SingleOrDefault(x => x.Id == 10);

I expected to get a new Item object, but guess what I got null.  I couldn’t figure out why, the documentation states ‘The SingleOrDefault method does not provide a way to specify a default value. If you want to specify a default value other than default(TSource), use the DefaultIfEmpty(Of TSource)(IEnumerable(Of TSource), TSource) method as described in the Example section.’

I did that yet I still got a null, and not an empty Item object.  Hopefully, you guys can tell me why.

First and FirstOrDefault, Last and LastOrDefault

First, FirstOrDefault and Last and LastOrDefault gave the same results as their respective counterparts.  It appears that the OrDefault doesn’t mean anything.

Where

Let’s look at using Where(), cause hey let’s be honest this is the way I write most of my queries anyway.  Where by itself returns IQueryable<T> which what we expect, but let’s look at what happens when use DefaultIfEmpty by itself and with First and FirstOrDefault.

Where with DefaultIfEmpty

var whereDefaultIfEmpty = items.Where(x => x.Id == 10)

                .DefaultIfEmpty(new Item());

Hey, we are finally starting to get some results, this returned a new Item object.

Where with First

Ok, so what if we put First?

var whereFirst = items.Where(x => x.Id == 10).First();

Oh, here is our old friend InvalidOperationException, no joy!

Where with First and DefaultIfEmpty

var whereFirstDefaultIfEmpty = items.Where(x => x.Id == 10)

                .DefaultIfEmpty(new Item())

                .First();

This return a new Item object. Ok, so now we are starting to see when DefaulIfEmpty helps. Remember just using it with First without Where just threw an exception.

Where with FirstOrDefault

var whereFirstOrDefault = items.Where(x => x.Id == 10).FirstOrDefault();

This behaved no different than just using FirstOrDefaul(x=> x.Id == 10), we got a null.

Where with FirstOrDefault and DefaultIfEmpty

Ok, so DefaultIfEmpty made a difference when using First, let’s hope it does the same for FirstOrDefault.

var whereFirstOrDefaultIfEmpty = items.Where(x => x.Id == 10)

                .DefaultIfEmpty(new Item())

                .FirstOrDefault();

Eureka! A new Item object.

Summary

From what I was able to figure out reading the online documentation and my own testing it I recommend using DefaultIfEmpty() with Where() alone or with FirstOrDefault.  But remember using it with Single, SingleOrDefault, First, FirstOrDefault, Last, or LastOrDeafult will not get you the default object, you will either get an InvalidOperationException or  null.

Now that I understand how and when DefaultIfEmpty makes a difference I can create LINQ queries that return the results I expect and I will be able to predict if I will get an Exception, Null or Object.

If you have experienced something different, disagree, or have a better way to approach this please post a reply.  The discussion just makes all of us better.