CodeContracts: false warning "Possibly unboxing a null reference"

Question

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In the image above, you can see a warning from Code Contracts. I don't think this is legit, as this can never be null.

Is this a bug or am I missing something?

This property is a member of the following class:

public class NHibernateIQueryableQueryBase<TEntity, TQuery, TQueryInterface>
    : IQuery<TEntity>, IFluentQueryInterface<TEntity, TQueryInterface>
    where TQuery : NHibernateIQueryableQueryBase<TEntity, TQuery,
                                                 TQueryInterface>,
                   TQueryInterface
    where TQueryInterface : IQuery<TEntity>

Update:
Changing the property to the following still shows the warning - on the line return result;:

public TQueryInterface And
{
    get
    {
        var result = this as TQuery;
        return result;
    }
}

Complete shot in the dark cause I am a noob and I'm sure you've thought of it but couldn't the cast of TQuery(this) return null? — Jesse Carter, Nov 07 '12 at 16:39
Please show us your type declaration. What constraints do you have on those generic parameters? — SLaks, Nov 07 '12 at 16:45
@JesseCarter: No. It could result in an InvalidCastException, so I'd say that is not the reason of the warning. — Daniel Hilgarth, Nov 07 '12 at 16:46
@SLaks: Updated the question. I am afraid that is rather complex, but basically, it is guaranteed, that the cast will always succeed. — Daniel Hilgarth, Nov 07 '12 at 16:48
I suspect that adding `: class` to the constraints will suppress the warning. — SLaks, Nov 07 '12 at 16:51
@SLaks: Indeed. Adding a `class` constraint to `TQueryInterface` removed the warning. Could you please post this as an answer? — Daniel Hilgarth, Nov 07 '12 at 16:57
@default.kramer: `TQuery` implements `TQueryInterface` but `this` doesn't. Casting `this` to `TQueryInterface` doesn't compile. — Daniel Hilgarth, Nov 07 '12 at 16:58

score 1 · Accepted Answer · answered Nov 08 '12 at 17:24

The analysis doesn't understand that this is guaranteed to implement TQuery.
Therefore, it's warning you that you might end up taking a null reference to the interface type, and returning it as a struct that implements that interface:

You need to add : class to the constraints of the TQueryInterface parameter.

score 0 · Answer 2 · answered Nov 07 '12 at 16:47

0

Based on this question, I'd say you need to specify what happens if the cast fails.

Try return this as TQuery since the as operator will return null if the cast fails, whereas the explicit cast will throw an error.

answered Nov 07 '12 at 16:47

Tyler Lee

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CodeContracts: false warning "Possibly unboxing a null reference"

2 Answers2