Question

In .Net/C# I have a class that’s derived from List. When I try to use the FindNextIndex member of that derived class, I get compile errors like below.

Error 3 Argument 1: cannot convert from ‘method group’ to ‘System.Predicate’

Error 2 The best overloaded method match for ‘System.Collections.Generic.List.FindIndex(System.Predicate)’ has some invalid arguments

Some simplified code is below.

class CTileBag:List<int>
{
    ...
}

Then later I try to use it in another class

CTileBag c = new CTileBag();
int idx = c.FindIndex(IsSwamp);

And IsSwamp is defined in the class I’m using CTile bag in as

private static bool IsSwamp(TerrainType type)
        {
            if (type == TerrainType.TT_FUNGUS_SWAMP)
                return true;
            return false;
        }  

Answer 1

Your predicate takes a TerrainType, but your list is a List<int>. The predicate type depends on the list type. It sounds like your tile bag should actually be a List<TerrainType>, at which point it should work.

However:

• I would generally suggest not deriving from List<int>. Prefer composition over inheritance – I suspect it would be better to make your tile bag type contain a List<T>.
• Follow .NET naming conventions: don’t prefix your classes with C, and make your enum values just PascalCased, like FungusSwamp instead of TT_FUNGUS_SWAMP.

• Your method body can be simplified to just:

  return type == TerrainType.FungusSwamp;