The reason this is not legal is because it is not safe. Suppose it were legal:
List<Giraffe> giraffes = new List<Giraffe>();
List<Animal> animals = giraffes; // this is not legal; suppose it were.
animals.Add(new Tiger()); // it is always legal to put a tiger in a list of animals
But “animals” is actually a list of giraffes; you can’t put a tiger in a list of giraffes.
In C# this is, unfortunately, legal with arrays of reference type:
Giraffe[] giraffes = new Giraffe[10];
Animal[] animals = giraffes; // legal! But dangerous because...
animals[0] = new Tiger(); // ...this fails at runtime!
In C# 4 this is legal on IEnumerable but not IList:
List<Giraffe> giraffes = new List<Giraffe>();
IEnumerable<Animal> animals = giraffes; // Legal in C# 4
foreach(Animal animal in animals) { } // Every giraffe is an animal, so this is safe
It is safe because IEnumerable<T>
does not expose any method that takes in a T.
To solve your problem you can:
- Create a new list of objects out of the old list.
- Make the method take an object[] rather than a
List<object>
, and use unsafe array covariance. - Make the method generic, so it takes a
List<T>
- Make the method take IEnumerable
- Make the method take
IEnumerable<object>
and use C# 4.