diff --git a/src/Lucene.Net.Tests/Support/TestCollectionExtensions.cs b/src/Lucene.Net.Tests/Support/TestCollectionExtensions.cs
new file mode 100644
index 0000000000..16b9f25b18
--- /dev/null
+++ b/src/Lucene.Net.Tests/Support/TestCollectionExtensions.cs
@@ -0,0 +1,160 @@
+using Lucene.Net.Attributes;
+using Lucene.Net.Util;
+using NUnit.Framework;
+using System.Collections;
+using System.Collections.Generic;
+using JCG = J2N.Collections.Generic;
+#nullable enable
+
+namespace Lucene.Net.Support
+{
+    /*
+     * Licensed to the Apache Software Foundation (ASF) under one or more
+     * contributor license agreements.  See the NOTICE file distributed with
+     * this work for additional information regarding copyright ownership.
+     * The ASF licenses this file to You under the Apache License, Version 2.0
+     * (the "License"); you may not use this file except in compliance with
+     * the License.  You may obtain a copy of the License at
+     *
+     *     http://www.apache.org/licenses/LICENSE-2.0
+     *
+     * Unless required by applicable law or agreed to in writing, software
+     * distributed under the License is distributed on an "AS IS" BASIS,
+     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+     * See the License for the specific language governing permissions and
+     * limitations under the License.
+     */
+
+    /// <summary>
+    /// Tests for <see cref="CollectionExtensions"/>
+    /// </summary>
+    [TestFixture]
+    [LuceneNetSpecific]
+    public class TestCollectionExtensions : LuceneTestCase
+    {
+        [Test]
+        public void RetainAll_EmptySource()
+        {
+            ISet<int> set = new JCG.HashSet<int>(); // empty
+            bool result = set.RetainAll(new[] { 1, 2, 3 });
+            Assert.IsFalse(result);
+            Assert.AreEqual(0, set.Count);
+        }
+
+        [Test]
+        public void RetainAll_Set()
+        {
+            ISet<int> set = new JCG.HashSet<int> { 1, 2, 3, 4, 5 };
+            bool result = set.RetainAll(new[] { 1, 2, 3, 100, 101 });
+            Assert.IsTrue(result);
+            Assert.AreEqual(3, set.Count);
+            Assert.IsTrue(set.Contains(1));
+            Assert.IsTrue(set.Contains(2));
+            Assert.IsTrue(set.Contains(3));
+            Assert.IsFalse(set.Contains(4));
+            Assert.IsFalse(set.Contains(5));
+        }
+
+        [Test]
+        public void RetainAll_List()
+        {
+            IList<int> list = new JCG.List<int> { 1, 2, 3, 4, 5 };
+            bool result = list.RetainAll(new[] { 1, 2, 3, 100, 101 });
+            Assert.IsTrue(result);
+            Assert.AreEqual(3, list.Count);
+            Assert.AreEqual(1, list[0]);
+            Assert.AreEqual(2, list[1]);
+            Assert.AreEqual(3, list[2]);
+        }
+
+        [Test]
+        public void RetainAll_Collection()
+        {
+            ICollection<int> collection = new TestCollection<int> { 1, 2, 3, 4, 5 };
+            bool result = collection.RetainAll(new[] { 1, 2, 3, 100, 101 });
+            Assert.IsTrue(result);
+            Assert.AreEqual(3, collection.Count);
+            Assert.IsTrue(collection.Contains(1));
+            Assert.IsTrue(collection.Contains(2));
+            Assert.IsTrue(collection.Contains(3));
+            Assert.IsFalse(collection.Contains(4));
+            Assert.IsFalse(collection.Contains(5));
+        }
+
+        [Test]
+        public void RemoveAll_EmptySource()
+        {
+            ISet<int> set = new JCG.HashSet<int>(); // empty
+            bool result = set.RemoveAll(new[] { 1, 2, 3 });
+            Assert.IsFalse(result);
+            Assert.AreEqual(0, set.Count);
+        }
+
+        [Test]
+        public void RemoveAll_Set()
+        {
+            ISet<int> set = new JCG.HashSet<int> { 1, 2, 3, 4, 5 };
+            bool result = set.RemoveAll(new[] { 1, 2, 3, 100, 101 });
+            Assert.IsTrue(result);
+            Assert.AreEqual(2, set.Count);
+            Assert.IsFalse(set.Contains(1));
+            Assert.IsFalse(set.Contains(2));
+            Assert.IsFalse(set.Contains(3));
+            Assert.IsTrue(set.Contains(4));
+            Assert.IsTrue(set.Contains(5));
+        }
+
+        [Test]
+        public void RemoveAll_List()
+        {
+            IList<int> list = new JCG.List<int> { 1, 2, 3, 4, 5 };
+            bool result = list.RemoveAll(new[] { 1, 2, 3, 100, 101 });
+            Assert.IsTrue(result);
+            Assert.AreEqual(2, list.Count);
+            Assert.AreEqual(4, list[0]);
+            Assert.AreEqual(5, list[1]);
+        }
+
+        [Test]
+        public void RemoveAll_Collection()
+        {
+            ICollection<int> collection = new TestCollection<int> { 1, 2, 3, 4, 5 };
+            bool result = collection.RemoveAll(new[] { 1, 2, 3, 100, 101 });
+            Assert.IsTrue(result);
+            Assert.AreEqual(2, collection.Count);
+            Assert.IsFalse(collection.Contains(1));
+            Assert.IsFalse(collection.Contains(2));
+            Assert.IsFalse(collection.Contains(3));
+            Assert.IsTrue(collection.Contains(4));
+            Assert.IsTrue(collection.Contains(5));
+        }
+
+        /// <summary>
+        /// A simple implementation of <see cref="ICollection{T}"/> for testing purposes,
+        /// that intentionally does not implement <see cref="IList{T}"/> or <see cref="ISet{T}"/>.
+        /// </summary>
+        /// <typeparam name="T">The type of the elements of <see cref="ICollection{T}"/>.</typeparam>
+        private class TestCollection<T> : ICollection<T>
+        {
+            private readonly List<T> _items = new();
+
+            public IEnumerator<T> GetEnumerator() => _items.GetEnumerator();
+
+            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+
+            public void Add(T item) => _items.Add(item);
+
+            public void Clear() => _items.Clear();
+
+            public bool Contains(T item) => _items.Contains(item);
+
+            public void CopyTo(T[] array, int arrayIndex) => _items.CopyTo(array, arrayIndex);
+
+            public bool Remove(T item) => _items.Remove(item);
+
+            public int Count => _items.Count;
+
+            public bool IsReadOnly => false;
+        }
+    }
+}