Question

Some background info;

• LanguageResource is the base class
• LanguageTranslatorResource and LanguageEditorResource inherit from LanguageResource
• LanguageEditorResource defines an IsDirty property
• LanguageResourceCollection is a collection of LanguageResource
• LanguageResourceCollection internally holds LanguageResources in Dictionary<string, LanguageResource> _dict
• LanguageResourceCollection.GetEnumerator() returns _dict.Values.GetEnumerator()

I have a LanguageResourceCollection _resources that contains only LanguageEditorResource objects and want to use LINQ to enumerate those that are dirty so I have tried the following. My specific questions are in bold.

1. _resources.Where(r => (r as LanguageEditorResource).IsDirty)

   neither Where not other LINQ methods are displayed by Intellisense but I code it anyway and am told ‘LanguageResourceCollection does not contain a definition for ‘Where’ and no extension method…’.

   Why does the way that LanguageResourceCollection implements IEnumerable preclude it from supporting LINQ?

2. If I change the query to

   (_resources as IEnumerable<LanguageEditorResource>).Where(r => r.IsDirty)

   Intellisense displays the LINQ methods and the solution compiles. But at runtime I get an ArgumentNullException ‘Value cannot be null. Parameter name: source’.

   Is this a problem in my LINQ code?
   Is it a problem with the general design of the classes?
   How can I dig into what LINQ generates to try and see what the problem is?

My aim with this question is not to get a solution for the specific problem, as I will have to solve it now using other (non LINQ) means, but rather to try and improve my understanding of LINQ and learn how I can improve the design of my classes to work better with LINQ.

Answer 1

It sounds like your collection implements IEnumerable, not IEnumerable<T>, hence you need:

_resources.Cast<LanguageEditorResource>().Where(r => r.IsDirty) 

Note that Enumerable.Where is defined on IEnumerable<T>, not IEnumerable – if you have the non-generic type, you need to use Cast<T> (or OfType<T>) to get the right type. The difference being that Cast<T> will throw an exception if it finds something that isn’t a T, where-as OfType<T> simply ignores anything that isn’t a T. Since you’ve stated that your collection only contains LanguageEditorResource, it is reasonable to check that assumption using Cast<T>, rather than silently drop data.

Check also that you have ‘using System.Linq’ (and are referencing System.Core (.NET 3.5; else LINQBridge with .NET 2.0) to get the Where extension method(s).

Actually, it would be worth having your collection implement IEnumerable<LanguageResource> – which you could do quite simply using either the Cast<T> method, or an iterator block (yield return).

[edit] To build on Richard Poole’s note – you could write your own generic container here, presumably with T : LanguageResource (and using that T in the Dictionary<string,T>, and implementing IEnumerable<T> or ICollection<T>). Just a thought.