Use CastingEnumeratorAdapter in CastingListAdapter; optimize as readonly struct to reduce allocations

Author: paulirwin

src/Lucene.Net/Support/CastingCollectionAdapter.cs

@@ -56,10 +56,12 @@ public void CopyTo(U[] array, int arrayIndex)
             }
         }
 
-        public IEnumerator<U> GetEnumerator() => new CastingEnumeratorAdapter<T, U>(collection.GetEnumerator());
+        public CastingEnumeratorAdapter<T, U> GetEnumerator() => new CastingEnumeratorAdapter<T, U>(collection.GetEnumerator());
 
-        public bool Remove(U item) => collection.Remove((T)item);
+        IEnumerator<U> IEnumerable<U>.GetEnumerator() => GetEnumerator();
 
         IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+
+        public bool Remove(U item) => collection.Remove((T)item);
     }
 }

src/Lucene.Net/Support/CastingEnumeratorAdapter.cs

@@ -21,11 +21,11 @@ namespace Lucene.Net.Support
      */
 
     /// <summary>
-    /// LUCENENET specific class used to adapt an <see cref="IEnumerator{T}"/> to one with a different type parameter.
+    /// LUCENENET specific struct used to adapt an <see cref="IEnumerator{T}"/> to one with a different type parameter.
     /// </summary>
     /// <typeparam name="T">The type of elements in the original enumerator.</typeparam>
     /// <typeparam name="U">The type of elements in the adapted enumerator.</typeparam>
-    internal class CastingEnumeratorAdapter<T, U> : IEnumerator<U>
+    internal readonly struct CastingEnumeratorAdapter<T, U> : IEnumerator<U>
         where T : U
     {
         private readonly IEnumerator<T> enumerator;

src/Lucene.Net/Support/CastingListAdapter.cs

@@ -1,5 +1,5 @@
-using System.Collections.Generic;
-using System.Linq;
+using System.Collections;
+using System.Collections.Generic;
 
 namespace Lucene.Net.Support
 {
@@ -60,7 +60,11 @@ public void CopyTo(U[] array, int arrayIndex)
             }
         }
 
-        public IEnumerator<U> GetEnumerator() => list.Cast<U>().GetEnumerator();
+        public CastingEnumeratorAdapter<T, U> GetEnumerator() => new CastingEnumeratorAdapter<T, U>(list.GetEnumerator());
+
+        IEnumerator<U> IEnumerable<U>.GetEnumerator() => GetEnumerator();
+
+        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
 
         public int IndexOf(U item) => list.IndexOf((T)item);
 
@@ -69,7 +73,5 @@ public void CopyTo(U[] array, int arrayIndex)
         public bool Remove(U item) => list.Remove((T)item);
 
         public void RemoveAt(int index) => list.RemoveAt(index);
-
-        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() => GetEnumerator();
     }
 }

src/Lucene.Net/Support/CastingSetAdapter.cs

@@ -35,7 +35,9 @@ public CastingSetAdapter(ISet<T> set)
             this.set = set;
         }
 
-        public IEnumerator<U> GetEnumerator() => new CastingEnumeratorAdapter<T, U>(set.GetEnumerator());
+        public CastingEnumeratorAdapter<T, U> GetEnumerator() => new CastingEnumeratorAdapter<T, U>(set.GetEnumerator());
+
+        IEnumerator<U> IEnumerable<U>.GetEnumerator() => GetEnumerator();
 
         IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();