Table of Contents

Upcasting

Most generic collection interfaces in .NET are invariant (for example IList<T>, ICollection<T>, IDictionary<TKey, TValue>). Even if Derived : Base, an IList<Base> cannot be used as an IList<Derived>.

Garyon provides lightweight “upcasting” views for common collection shapes when you already know the runtime values are of a more specific type and want to work with that type without copying the collection.

List Upcasting

using Garyon.Extensions.Upcasting;

IList<BaseItem> items = [new DerivedItem("a"), new DerivedItem("b")];

IList<DerivedItem> derivedItems = items.UpcastList<BaseItem, DerivedItem>();
derivedItems.Add(new DerivedItem("c"));

Console.WriteLine(items.Count); // 3 (same backing list)

Other Collection Shapes

UpcastingCollectionExtensions also supports:

  • ICollection<T> / IReadOnlyCollection<T>
  • IDictionary<TKey, TValue> / IReadOnlyDictionary<TKey, TValue> (values are cast; keys are unchanged)
  • ISet<T> (and IReadOnlySet<T> on frameworks that provide it)

Safety Notes

  • These helpers do not validate that every element/value is of the requested type; mistyping elements may cause undefined behavior.
  • The returned objects are views over the original collection: changes are reflected immediately.
  • Upcasting is not the same as downcasting, where the opposite casting relationship would apply. Downcasting is not supported yet, but is planned for the future.

API Reference

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