Question

I’m a bit stumped about how to perform the necessary cast in the following:

public IList<IMyClass> Foo()
{
    IList<IMyClass> foo = SomeQuery(); 

    var result = foo.GroupBy(x => x.bar).Select(x => new MyClass()).ToList();

    // So now I have a List<MyClass> which needs casting as IList<IMyClass>

    return result;
}

using an explicit cast fails at run time. The error message isn’t helpful but I’m guessing it maybe something to do with the GroupBy as I wouldn’t normally expect to have any problem with something like this.

Answer 1

It’s nothing to do with GroupBy – it’s because IList<T> is invariant. If you could cast a List<MyClass> to IList<IMyClass> then the caller could then add an instance of a type which implemented IMyClass but wasn’t a MyClass. (I usually think of this in terms of real world objects: a bunch of bananas isn’t a fruit bowl: you can add an apple to a fruit bowl, but not to a bunch of bananas.)

The simplest way would be to manually specify the type in the Select clause:

public IList<IMyClass> Foo()
{
    IList<IMyClass> foo = SomeQuery(); 

    var result = foo.GroupBy(x => x.bar)
        .Select<IGrouping<IMyClass, Bar>>, IMyClass>(x => new MyClass())
        .ToList();
    return result;
}

This means result will actually be a List<IMyClass>. I don’t know the type of item within foo which makes it tricky to give it in more detail – given your comment, I’ll assume that that’s IMyClass, and that x.bar is of type Bar.

Another alternative (which would be more useful if you wanted to use a query expression for the group/select part) would be to use Cast:

public IList<IMyClass> Foo()
{
    IList<IMyClass> foo = SomeQuery(); 

    var result = foo.GroupBy(x => x.bar)
                    .Select(x => new MyClass())
                    .Cast<IMyClass>()
                    .ToList();
    return result;
}

Or perform the cast within the Select lambda expression itself:

public IList<IMyClass> Foo()
{
    IList<IMyClass> foo = SomeQuery(); 

    var result = foo.GroupBy(x => x.bar)
                    .Select(x => (IMyClass) new MyClass())
                    .ToList();
    return result;
}

EDIT: Apologies for the Select failure. It’s somewhat annoying that you have to specify both the source and the result, but there’s no easy way of fixing that 🙁 Given the ugliness involved, you may well be best off with Cast.