Casting the results of linq-to-sql

Question

Working with interfaces, we commonly have a var or an IQueryable that is going to return a set of data objects that we will then cast to the interface and return as a List or IList, like this:

var items =
from t in SomeTable
where t.condition == true
select;
return items.ToList( ).Cast<SomeInterface>( ).ToList( );

NOTE: items.Cast( ).ToList( ) will compile, but will throw an InvalidCastException at run time.

Is there a better way? (I put the ToList/Cast/ToList in an extension method, but that's not really any better...)

return items.CastToList<SomeClass, SomeInterface>( );

Thanks!

Answer 1

What's the point of the first ToList() call? Why not just items.Cast<SomeInterface>().ToList()?

Alternately, you could do this instead:

var items = from t in SomeTable
            where t.condition == true
            select (SomeInterface) t;
return items.ToList();

If t doesn't implement SomeInterface it will still fail at runtime, though.

This will do the same thing, but won't fail; rather it will give you null if the object doesn't implement the interface:

var items = from t in SomeTable
            where t.condition == true
            select t as SomeInterface;
return items.ToList();

Answer 2

What you're doing is correct (you can't call Cast directly on the IQueryable), but it's sub-optimal, because you're creating 2 lists when you really only need one. Use AsEnumerable instead of the first ToList :

return items.AsEnumerable().Cast<SomeInterface>().ToList();

Calling AsEnumerable will cause Cast to be evaluated on a IEnumerable, not IQueryable, so the cast will be done on the actual object returned from the DB, rather than directly in the DB.

Answer 3

A few solutions...

Instead of converting data after collecting it, you could consider make a projection while collecting it.

return items.Select(x=>x as SomeInterface).ToList();

If you do this frequently, an extension method may still be helpful. It doesn't have to be Tolist/Cast/ToList as you mentioned though. Cast will take an IEnumerable already, but you can create an overload that accepts an IQueryable, something like this (untested).

public IEnumerable<TResult> Cast<T, TResult>(this IQueryable<T> input)
{
    return input.AsEnumerable().Cast<TResult>();
}

This would whittle your code down to this:

return items.Cast<SomeClass, SomeInterface>.ToList();