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I just started using code contracts in my project. However, I have a problem with my repository implementation, which queries my database using the Entity Framework.

I have the following method:

public IEnumerable<Organization> GetAllOrganizations()
{
    return _uow.CreateSet<Party>().OfType<Organization>().AsEnumerable();
}

The method returns a collection containing all organizations in the database, or an empty collection there's not organizations in the database.

However, this is not okay according to CodeContracts, which give me the error: "requires unproven: source != null"

What is it trying to tell me? I can satisfy code contracts by using Contract.Assume, to assume that it would always find something, but then I need to do that in all methods that reads data from the database.

Am I missing something here, or is it intended behavior when you're working with databases and LINQ?

Vern
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  • What is "source"? Do any of the methods you're calling have contracts? – Anon. Feb 09 '11 at 23:36
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    Is [this](http://stackoverflow.com/questions/3050279/how-to-avoid-source-null-when-using-code-contracts-and-linq-to-sql) question any help? – adrianbanks Feb 10 '11 at 00:01
  • @Anon: I have no idea what "source" is. I'm not aware if there is contracts on the methods in the Entity Framework, but I don't think so. @adrianbanks: No sorry, I cannot see how that can help me. – Vern Feb 10 '11 at 19:13

1 Answers1

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My guess is that one of the methods CreateSet, OfType and AsEnumerable is declared as an extension method with a this-parameter called "source", and CodeContrcts can't prove it is not null. Also, don't you need to add a Requires clause to specify that _uow is non-null on entry?

CreateSet appears to be the extension method since it doesn't appear in MSDN. If the method should never return null, you can enforce this by adding this contract to CreateSet:

Contract.Ensures(Contract.Result<T>() != null);

When the CodeContracts analyzer sees this rule, it will take it as proof that the input to OfType will not be null, and the warning should go away.

Jim Counts
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dc42
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  • It's most likely `AsEnumerable`, which has a `source != null` precondition. I'm not sure if there are many (any?) contracts on methods in the Entity Framework, so neither `CreateSet` nor `OfType` have postconditions on them. You might need to create your own wrapper functions. – porges Feb 10 '11 at 04:15
  • You can only set a precondition if the caller of the code is responsible for fulfilling that precondition. In this case, it seems like _uow is an internal responsibility, not that of the caller. – koenmetsu Feb 10 '11 at 07:36
  • you're right that I need a Requires clause to specify that _uow is non-null. But after adding that I'm still getting the error. @Porges: I think you're right about AsEnumerable being the cause here. I've done some testing with AsEnumerable and I got the problem here aswell. It seems like a pretty tedious task to write wrapper functions for lots of functions - when using 3rd party APIs that doesn't use code contracts. I'll have to reconsider using code contracts at all. – Vern Feb 10 '11 at 19:07
  • @Vern: yeah, I think a big feature would be for users to be able to declare contracts on 3rd-party libraries. – porges Feb 14 '11 at 20:21
  • You should not use Requires() for internal fields like KoMet says. – Stephen Drew Jun 27 '11 at 14:41
  • The other way is to create a SafeOfType extension method, that ensures that result is not null and then consists of `return source == null ? Enumerable.Empty : source.OfType()` – Bob Vale Jan 23 '12 at 10:11