Help with parameter usage
I have the following classes defined (code redacted for brevity)
Using .NET8 c#12
public interface ICollectionItem
{}
Which in turn is inherited by:
public class CollectionItem : NotifyPropertyChanged, ICollectionItem
{}
And in turn is inherited by:
public class DeviceItem : CollectionItem, IDisposable, IAsyncDisposable
{
public DeviceItem(DeviceCollectionBase<ICollectionItem> deviceCollection)
{}
}
Which has a constructor so i can then create a DeviceItem instance from within yet another class:
public abstract class DeviceCollectionBase<T> : PacedItemCollection<T>, IDisposable, IAsyncDisposable where T : ICollectionItem
{
protected override async Task UpdateCollection()
{
var v = new DeviceItem(this)...
}
}
But the parameter this is flagged with the error CS1503 "Argument type DeviceCollectionBase<T>, is not assignable to parameter type DeviceCollectionBase<ICollectionItem>" , yet T is constrained to be an ICollectionItem!
My covariance and/or contravariance games are shite, so i dont quite see where im going wrong, any help?1 Reply
if i remember correctly the reaons you can't it's that you could pass to that a more derived class
DeviceCollectionBase<DerivedType1> which could have implementations that use specific DerivedType1 stuff to other DeviceCollectionBase<...> that can't use those specific implementations (you have to remember that DeviceCollectionBase<DerivedType1> is DerivedType1, or rather every implemented DeviceCollectionBase<> is a different type)
honestly this inheritance stuff sometimes is so convoluted that i avoid it
(EDIT no in fact the issue would be another i think: when you are using generic T in a derived class of DeviceCollectionBase you could be "fixing" that type to a more derived one; the types in DeviceCollectionBase<ICollectionItem> are not necessarily that specific T, so a DeviceCollectionBase<ICollectionItem> of Device1 items vs a Device2CollectionBase:DeviceCollectionBase<Device2>)