Question: Suppose I have a method that returns an array or list or some other collection. If something goes wrong inside or when there is simply no data to return, what is better - to return null or to return array (list, etc.) that has length (count) equal to 0?

In fact is it important, or it is just a matter of developer's preferences?

Answer: 
It's better to return an empty collection. This way when someone calls the function like so:

```
foreach(var i in items)
{

}

```

it doesn't throw a null reference exception on them.

Technically, you could argue empty vs. `null` means, but in reality a lot of people (myself included at times) forget to be defensive and don't check to see if the object is null before using it. It's playing into what other people assume is going to happen, which means less bugs and less angry users.
Here is an article by Josh Bloch explaining the same. It's written in Java's context but it should apply to C# equally well.
[Return zero-length arrays, not nulls](http://www.informit.com/articles/article.aspx?p=31551&seqNum=5)