c# - Is it better to return null or empty collection?

That's kind of a general question (but I'm using C#), what's the best way (best practice), do you return null or empty collection for a method that has a collection as a return type ?

Answer

Empty collection. Always.

This sucks:

if(myInstance.CollectionProperty != null)
{
  foreach(var item in myInstance.CollectionProperty)
    /* arrgh */
}

It is considered a best practice to NEVER return null when returning a collection or enumerable. ALWAYS return an empty enumerable/collection. It prevents the aforementioned nonsense, and prevents your car getting egged by co-workers and users of your classes.

When talking about properties, always set your property once and forget it

public List Foos {public get; private set;}

public Bar() { Foos = new List(); }

In .NET 4.6.1, you can condense this quite a lot:

public List Foos { get; } = new List();

When talking about methods that return enumerables, you can easily return an empty enumerable instead of null...

public IEnumerable GetMyFoos()
{
  return InnerGetFoos() ?? Enumerable.Empty();
}

Using Enumerable.Empty() can be seen as more efficient than returning, for example, a new empty collection or array.