C#•2y ago

❔ ✅ I can't convert List<BaseClass> to List<T> where T: BaseClass.

I have a List<List<Component>>. Component is a base class, and each item in this outer list is actually a List with different sub-classes of Component. At run-time I expect it to look like this:

List<List<Component>>:
  List<SubComponentA>
  List<SubComponentB>
  List<SubComponentC>

List<List<Component>>:
  List<SubComponentA>
  List<SubComponentB>
  List<SubComponentC>

Everything works nicely besides this - I can't find a way to get for example List<SubComponentA> from the outer list. Here's my current attempt:

32 Replies

lukkasz323OP•2y ago

Visual Studio propses this fix:

lukkasz323OP•2y ago

Which gets rid of the error, but still throws an exception at runtime

lukkasz323OP•2y ago

vdvman1•2y ago

You can't do that because it would allow a List<SubClass> to be converted to a List<BaseClass> and then an instance of BaseClass added to the list, which is invalid since the List<SubClass> is expecting all instances to be a SubClass However, if you just want to loop over the list, you can cast it to an IEnumerable<T> instead of a List<T>

lukkasz323OP•2y ago

I would like to be able to use .Add()

lukkasz323OP•2y ago

vdvman1•2y ago

Then you need to make sure that when you originally create the list that it is of the correct type, and store it as just an IList in the outer list Also, using a dictionary is probably better than a list here

lukkasz323OP•2y ago

btw this still throws an exception, unless you meant something else?

lukkasz323OP•2y ago

Yeah but then I wouldn't be able to use generics I think Here's how the method is used

lukkasz323OP•2y ago

vdvman1•2y ago

private Dictionary<Type, IList> _componentCollections = new();

public List<T> GetComponentCollection<T>()
  where T : Component
{
  var type = typeof(T);
  if (!_componentCollections.TryGetValue(type, out IList? untypedList))
  {
    untypedList = new List<T>();
    _componentCollections[type] = untypedList;
  }

  return (List<T>)untypedList;
}

private Dictionary<Type, IList> _componentCollections = new();

public List<T> GetComponentCollection<T>()
  where T : Component
{
  var type = typeof(T);
  if (!_componentCollections.TryGetValue(type, out IList? untypedList))
  {
    untypedList = new List<T>();
    _componentCollections[type] = untypedList;
  }

  return (List<T>)untypedList;
}

lukkasz323OP•2y ago

so Types as a key? I did that before switching to generics

vdvman1•2y ago

Yup, using the type as the key, and making sure that the new List<T>() has the correct type Unfortunately C# doesn't have a way to do this completely type safely, but as long as you implement your methods correctly it should work fine

lukkasz323OP•2y ago

okay thank you I'm wondering if im not overengineering this, all I actually need is - a few collections for each subclass of Component, each collection auto-generated for a different subcomponent. Then a way to get these collections by passing Type or preferably generic T to Read and Add to them. Everything besides "Get collection" and "Add to collection" is private too.

lukkasz323OP•2y ago

I ended up with something like this, I'd appreciate anything that can be done to simplify or improve this.

lukkasz323OP•2y ago

_componentCollections is a Dictionary btw

jalepi•2y ago

You can check this: https://learn.microsoft.com/en-us/dotnet/standard/generics/covariance-and-contravariance

Covariance and Contravariance in Generics

Learn about covariance, which allows you to use a more derived type, and contravariance, which allows you to use a less derived type, in .NET generics.

vdvman1•2y ago

That wouldn't let the caller add to that list

lukkasz323OP•2y ago

Now I see something. Am I right that this code doesn't preserve the same object reference? I think that's a problem, I wonder if there is a way to preserve that. I think this doesn't work, no matter what I try

vdvman1•2y ago

Can you show what you tried?

lukkasz323OP•2y ago

yeah 1 minute the problem is that it's a List<BaseClass> to List<T> conversion.

vdvman1•2y ago

That shouldn't be happening with the code I showed

lukkasz323OP•2y ago

you have IList right? Is that a IList or IList<>?

vdvman1•2y ago

The dictionary stores IList yes, but the new List that adds to the dictionary uses the actual type

lukkasz323OP•2y ago

I assumed before that you meant the generic IList<>, i'll try this instead. thanks, this works so far. Are there any downsides to this?

jalepi•2y ago

Why don't you use one list containing a mix of all components?

static void Main(string[] args)
{
    var components = new List<Component>();
    components.Add(new SubComponentA());
    components.Add(new SubComponentA());
    components.Add(new SubComponentB());
    components.Add(new SubComponentC());

    var subComponentAs = components.OfType<SubComponentA>().ToList();
    Console.WriteLine($"SubComponentA count: {subComponentAs.Count}");
}

abstract class Component { }
class SubComponentA : Component { }
class SubComponentB : Component { }
class SubComponentC : Component { }

static void Main(string[] args)
{
    var components = new List<Component>();
    components.Add(new SubComponentA());
    components.Add(new SubComponentA());
    components.Add(new SubComponentB());
    components.Add(new SubComponentC());

    var subComponentAs = components.OfType<SubComponentA>().ToList();
    Console.WriteLine($"SubComponentA count: {subComponentAs.Count}");
}

abstract class Component { }
class SubComponentA : Component { }
class SubComponentB : Component { }
class SubComponentC : Component { }

lukkasz323OP•2y ago

I think there was one problem here, i'll test this ToList(), isn't that getting rid of the original reference?

jalepi•2y ago

.ToList() returns a new list, with the same references inside.

lukkasz323OP•2y ago

yeah references inside are the same but I wanted to be able to use .Add() on that list so now changes are only preserved in one of the lists i think

jalepi•2y ago

not sure exactly what your use case is about, why having a list of lists? you could use an object with different lists as its properties.

lukkasz323OP•2y ago

Something like this. Yeah at this moment I'm sure a simple object would be better, but originally I didn't think it'd be this complicated. I just wanted a way to auto-add all components with reflection. I guess this could be marked as solved as I have a few different ways to do this now

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ ✅ I can't convert List<BaseClass> to List<T> where T: BaseClass.