Background and motivation

Dictionary<TKey, TValue>.Keys returns a Dictionary<TKey, TValue>.KeysCollection. This type implements ICollection<TKey> and has an efficient implementation of ICollection<TKey>.Contains... but it's explicitly implemented. That means if you do:

Dictionary<TKey, TValue> d = ...;
...
if (d.Keys.Contains(...)) { ... }

you end up invoking Enumerable.Contains. This does a dynamic check for ICollection and delegates to it, but that's still unnecessary overhead. We can easily fix this by making KeysCollection.Contains public / implicitly implemented.

API Proposal

namespace System.Collections.Generic;

public class Dictionary<TKey, TValue>
{
    public sealed class KeyCollection
    {
-        bool ICollection<TKey>.Contains(TKey item)
+        public bool Contains(TKey item);
    }
}

public class SortedDictionary<TKey, TValue>
{
    public sealed class KeyCollection
    {
-        bool ICollection<TKey>.Contains(TKey item)
+        public bool Contains(TKey item);
    }
}

API Usage

if (dictionary.Keys.Contains(...))

Alternative Designs

No response

Risks

No response