IList<IWhatever> as a method parameter

Question

I have two IList<ICat> and I'm trying to create a method which takes an IList<ICat> and does some work. I'm having problems trying to pass either an IList<PussyCat> or IList<OtherCat> to it, both PussyCat and OtherCat implement ICat.

I've tried:

List<PussyCat> cats = ...
DoWork((IList<ICat>)cats);

and just

DoWork(cats);

But neither compile. Any ideas?

Answer 1

C# generics are invariant. It means List<string> is not a List<object>.

C# 4.0 introduces safe covariance/contravariance but still, you wouldn't be able to pass List<string> as List<object>. The reason is:

List<string> x = new List<string>();
List<object> o = x; // assume this statement is valid
o.Add(5); // Adding an integer to a list of strings. Unsafe. Will throw.

Arrays, on the other hand are covariant. You can pass a string[] to a method that expects object[].

Answer 2

Till C# 4.0 arrives which has support for co and contra variance you might be able to get away with something like this:

public void DoWork(IEnumerable<ICat> cats)
{
  //do something 
}
 List<PussyCat> pussyCats = new List<PussyCat>;
 List<OtherCat> otherCats = new List<OtherCat>;
 DoWork(pussyCats.OfType<ICat>);
 DoWork(otherCats.OfType<ICat>);

Answer 3

There are two alternatives:

1. Make your method like this:

   public void DoWork< T > (IList< T > cats_) where T : ICat { //Do work; }

2. The other possibility is to have a method like

   public void DoWork(IList< ICat > cats_) { //Do work; }

and call it in the following manner:

{
    //....Assuming: IList<PussyCat> iListOfPussyCats
    List<PussyCat> pussyCats = new List<PussyCats>(iListOfPussyCats);
    DoWork(pussyCats.ConvertAll<ICat>( c => c as ICat);
}

Answer 4

If the method doesn't truly require direct indexing (IList<T>) and doesn't require adding/removing items (ICollection<T>), then pass an IEnumerable<T>. The Cast<T>() extension methods allow casting any IList of [insert ICat-derived type] to be passed as an IEnumerable<ICat>.