Question

Scenario
I have three IEnumerable lists – IEnumerable<Dog>, IEnumerable<Cat> and IEnumerable<Horse>.
I want them in all in a single IEnumerable<NameColor>.
I have a public static explicit operator NameColor defined on each of the Dog, Cat and Horse objects to allow me to cast them to NameColor

So:

IEnumerable<NameColor> list = dogs as IEnumerable<NameColor>;
list = list.Union(cats as IEnumerable<NameColor>);
list = list.Union(horses as IEnumerable<NameColor>);

However, this doesnt work. I get the error Value cannot be null.
Parameter name: first but I dont see a parameter called first even being an option?!

Answer 1

I suggest:

var list = dogs.Select(d => (NameColor)d)
               .Union(cats.Select(c => (NameColor)c))
               .Union(horses.Select(h => (NameColor)h));