Operation could destabilize the runtime?

Question

I'm having a little bit of trouble understanding what the problem is here. I have a bit of code that pulls records from a database using LINQ and puts them into an object which is cast into an interface. It looks a bit like this:

public IEnumerable<ISomeObject> query()
{
    return from a in dc.SomeTable
           select new SomeObject
           {
             //Assign various members here
           } as ISomeObject;
}

When I test this, I put the returned IEnumerable into a variable called results and run this line:

Assert.AreEqual(EXPECTED_COUNT, results.Count());

When this is run, I get a System.Security.VerificationException: "Operation could destabilize the runtime."

I found the solution here, which is this:

var results = from a in dc.SomeTable
              select new SomeObject
              {
                //Assign various members here
              } as ISomeTable;
return results.OfType<ISomeObject>();

This works, but I'm having trouble understanding what's happening here. Why did I get the exception in the first place and how did the lines of code above fix it? The MSDN documentation seems to suggest that this is an issue of type safety, but I'm not seeing where the previous code was type-unsafe.

UPDATE A little bit more information I found out. The first example works if I make the return type IQueryable. This sheds a little bit more light on what was going wrong, but I'm still confused about the why. Why didn't the compiler force me to cast the IEnumerable into an IQueryable?

Answer 1

Just a guess, but the as operator may return a null - so it may have to do with the actual implementation of the new SomeObject { ... } code, since it's syntactic sugar. The return results.OfType<ISomeTable>(); filters based on type, so your method's return statement will only return that type (ensuring type safety). I've run into a similar issue with returning generic types.

P.S. I love the "Operation could destabilize the runtime." exception. That's almost like the "You might blow up the internet" exception.

Answer 2

Does it still fail if you change this:

select new SomeObject { ... } as ISomeTable;

to this:

select (ISomeTable) new SomeObject { ... };

?

If so (as I see you've confirmed), perhaps this has to do with the fact that an interface implementation could be either a class or a struct? Does the problem still appear if you cast to an abstract class rather than an interface?

Answer 3

I believe it is an issue of covariance or contravariance as noted by this forum post.

See Covariance and Contravariance in C#, Part Two: Array Covariance and the rest of the Covariance and Contravariance series at Eric Lippert's blog.

Although he is dealing with Arrays in the article I linked, I believe a similar problem presents itself here. With your first example, you are returning an IEnumerable that could contain objects that implement an interface that is larger than ISomeTable (i.e. - you could put a Turtle into an Animals IEnumerable when that IEnumerable can only contain Giraffes). I think the reason it works when you return IQueryable is because that is larger/wider than anything you could return, so you're guaranteed that what you return you will be able to handle(?).

In the second example, OfType is ensuring that what gets returned is an object that stores all the information necessary to return only those elements that can be cast to Giraffe.

I'm pretty sure it has something to do with the issues of type safety outlined above, but as Eric Lippert says Higher Order Functions Hurt My Brain and I am having trouble expressing precisely why this is a co/contravariant issue.

Answer 4

I guess, Linq to Sql may not support casting when translate to sql statement.

Answer 5

I found that OfType had some nasty side effects when using linq to sql. For example, parts of the linq that were previously evaluated after the query was run against the db were instead translated to SQL. This failed as those sections had no SQL equivalent. I ended up using .Cast instead which seems to solve the problem as well.

Answer 6

I found this entry while looking for my own solution to "operation could destabilize the runtime". While the covariance/contra-variance advice above looks very interesting, in the end I found that I get the same error message by running my unit tests with code coverage turned on and the AllowPartiallyTrustedCallers assembly attribute set.

Removing the AllowPartiallyTrustedCallers attribute caused my tests to run fine. I could also turn off code coverage to make them run but that was not an acceptable solution.

Hopefully this helps someone else who makes it to this page trying to find a solution to this issue.