Discussion/Suggestion: Type matching with generic constraints

[weitzhandler]

Hi,

I came across a funny issue today, where I have an object of unknown type, and it's composed of several interfaces without having a concrete type or connection between them, to be passed as a single type. Let the code do the talking:

foreach(PropertyInfo property in GetProperties()
{ 
  var value = property.GetValue(obj);
    if(value is Blah)
    {
    }
    else if (value is INotifyCollectionChanged incc
      && value is IEnumerable<ITrackableObject> trackableCollection)
    /*****************************************************/


 //1
 void ProcessTrackableCollection<TCollection>(TCollection collection)
    where TCollection : INotifyCollectionChanged, IEnumerable<ITrackableObject> 
 {
   //perfect now how r u gonna call it without having a concrete type
 }
 //2
 void ProcessTrackableCollection(object collection)
 {
   //meh
   var incc = (INotifyCollectionChanged)collection;
   var tCol = (IEnumerable<ITrackableCollection>)collection;
 }
 //3
 void ProcessTrackableCollection(INotifyCollectionChanged collection)
 {
    // IEnumerable<ITrackableCollection> will really cry
    var tCol = (IEnumerable<ITrackableCollection>)collection;
 }
 //4
 void ProcessTrackableCollection(INotifyCollectionChanged incc, IEnumerable<T> collection) 
 { /* :P */ }

---

I hope the situation is clear by the code itself, so my suggestion is to be able to define aliases of composed type constraints and use them wherever:

using TCollection : INotifyCollectionChanged, IEnumerable<ITrackableObject>;
namesapace Hello
{

  public class Program
  {
    
    static void Main()
    {
       PropertyInfo property = GetPropertyInfo();
       object obj = (TCollection)property.GetValue(GetItem());
       Foo(obj);
    }

    public static Foo(TCollection obj) 
    {
       obj.CollectionChanged += OnCollectionChanged;
       _Items = obj.ToArray(); //no cast
    }       
  }
}

I hope I'm clear enough, should I hear rants and I'll expand.

[HaloFour]

I believe that this falls under union/intersection types, which is not something that C# or the CLR supports (outside of generic type constraints). See #228, #344 and #399.

In short, you'd probably want something like:

if (value is (INotifyCollectionChanged & IEnumerable<ITrackableObject>) collection) {
    // collection treated as both interfaces here
}

This is assuming that I am interpreting your "discussion" correctly.

[weitzhandler]

@HaloFour lol I know it was gross, looks like you understood me well, tho I was more about the aliasing part than the union part.
Thanks for the references.