List to implement IQueryable

Question

I'm trying to create a mock for my IRepository interface:

public interface IRepository<T> : ICollection<T>, IQueryable<T>
{
}

With this implementation:

public class RepositoryFake<T> : List<T>, IRepository<T>
{
    public Expression Expression
    {
        get
        {
            return this.AsQueryable().Expression;
        }
    }

    public Type ElementType
    {
        get
        {
            return this.AsQueryable().ElementType;
        }
    }

    public IQueryProvider Provider
    {
        get
        {
            return this.AsQueryable().Provider;
        }
    }
}

But when I use it, I'm getting StackOverflow exception. How to implement this interface correctly to be able to use just a List as a repository?

Usage is very simple

[Test]
public void Test()
{
    RepositoryFake<User> users = new RepositoryFake<User>();
    users.Add(new User());

    List<User> list = (from user in users 
                 where user.Id == "5"
                 select user).ToList();

    Assert.That(list, Is.Empty);
}

Here is screenshot of exception:

Exception

"But when I use it" -- show us the code? Show us the stack trace? — cdhowie, Jul 20 '11 at 16:39
What are you trying to do? My best guess is you are hoping for some magic. The behavior is completely expected. — leppie, Jul 20 '11 at 16:52

CodesInChaos · Accepted Answer · 2011-07-20T16:56:59.957

31

The reason for your problem is that if you perform AsQueryable it checks if the object already implements IQueryable and if yes returns it.

Use new EnumerableQuery<T>(this) instead of AsQueryable which doesn't perform this check.

Workaround for .net 3.5:

return ((IEnumerable<T>)this).Select(x=>x).AsQueryable()

First casts to IEnumerable<T> so the chosen Select method will be Enumerable.Select not Queryable.Select. The identity select will then return a new object that does not implement IQueryable<T>, so the check if it's implemented in AsQueryable fails.

edited Jul 20 '11 at 16:56

answered Jul 20 '11 at 16:40

CodesInChaos

106,488
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The constructor taking an `IEnumerable` is public. Check http://msdn.microsoft.com/en-us/library/cc190806.aspx At least on .net 4 – CodesInChaos Jul 20 '11 at 16:47
You included both the cast to `IEnumerable` and the `Select`? – CodesInChaos Jul 20 '11 at 16:53
1

Second option did the job. Thanks! – Sly Jul 20 '11 at 16:55
Seems very weird to me. In my particular case this caused our server to use 100% CPU, debugging revealed that all db rows were instantiated as objects in memory – dksh Jan 24 '14 at 05:32
@dksh This is designed to reuse `IQueryable` code on an `IEnumerable`, for example for testing. It's not designed to query a SQL database since anything `IEnumerable` based, even when converted to an `IQueryable` needs to iterate over everything instead of translating the query to SQL. – CodesInChaos Jan 24 '14 at 08:26

score 1 · Answer 2 · answered Jul 20 '11 at 16:49

1

Try using base.AsQueryable() instead of this.AsQueryable().

answered Jul 20 '11 at 16:49

Daniel A. White

187,200
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Base class doesn't have such an extension – Sly Jul 20 '11 at 16:53
Wouldn't help anyways since the check if `IQueryable` is already implemented uses the runtime type. This is no virtual method. – CodesInChaos Jul 20 '11 at 16:56

List to implement IQueryable

2 Answers2