diff --git a/src/BizHawk.Common/Extensions/CollectionExtensions.cs b/src/BizHawk.Common/Extensions/CollectionExtensions.cs
index edc01b9534..a883a0f631 100644
--- a/src/BizHawk.Common/Extensions/CollectionExtensions.cs
+++ b/src/BizHawk.Common/Extensions/CollectionExtensions.cs
@@ -105,6 +105,21 @@ namespace BizHawk.Common.CollectionExtensions
 			throw new InvalidOperationException("Item not found");
 		}
 
+		/// <inheritdoc cref="List{T}.AddRange"/>
+		/// <remarks>
+		/// (This is an extension method which reimplements <see cref="List{T}.AddRange"/> for other <see cref="ICollection{T}">collections</see>.
+		/// It defers to the existing <see cref="List{T}.AddRange">AddRange</see> if the receiver's type is <see cref="List{T}"/> or a subclass.)
+		/// </remarks>
+		public static void AddRange<T>(this ICollection<T> list, IEnumerable<T> collection)
+		{
+			if (list is List<T> listImpl)
+			{
+				listImpl.AddRange(collection);
+				return;
+			}
+			foreach (var item in collection) list.Add(item);
+		}
+
 		public static T? FirstOrNull<T>(this IEnumerable<T> list, Func<T, bool> predicate)
 			where T : struct
 		{
@@ -113,5 +128,32 @@ namespace BizHawk.Common.CollectionExtensions
 					return t;
 			return null;
 		}
+
+		/// <inheritdoc cref="List{T}.RemoveAll"/>
+		/// <remarks>
+		/// (This is an extension method which reimplements <see cref="List{T}.RemoveAll"/> for other <see cref="ICollection{T}">collections</see>.
+		/// It defers to the existing <see cref="List{T}.RemoveAll">RemoveAll</see> if the receiver's type is <see cref="List{T}"/> or a subclass.)
+		/// </remarks>
+		public static int RemoveAll<T>(this ICollection<T> list, Predicate<T> match)
+		{
+			if (list is List<T> listImpl) return listImpl.RemoveAll(match);
+			var c = list.Count;
+			if (list is IList<T> iList)
+			{
+				for (var i = 0; i < iList.Count; i++)
+				{
+					if (match(iList[i])) iList.RemoveAt(i--);
+				}
+			}
+			else
+			{
+				foreach (var item in list.Where(item => match(item)) // can't simply cast to Func<T, bool>
+					.ToList()) // very important
+				{
+					list.Remove(item);
+				}
+			}
+			return c - list.Count;
+		}
 	}
 }
diff --git a/src/BizHawk.Tests/Common/CollectionExtensions/CollectionExtensionTests.cs b/src/BizHawk.Tests/Common/CollectionExtensions/CollectionExtensionTests.cs
new file mode 100644
index 0000000000..d545ab2e89
--- /dev/null
+++ b/src/BizHawk.Tests/Common/CollectionExtensions/CollectionExtensionTests.cs
@@ -0,0 +1,84 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+
+using BizHawk.Common.CollectionExtensions;
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+using CE = BizHawk.Common.CollectionExtensions.CollectionExtensions;
+
+namespace BizHawk.Tests.Common.CollectionExtensions
+{
+	[TestClass]
+	public class CollectionExtensionTests
+	{
+		/// <summary>there isn't really an <see cref="IList{T}"/> implementor which isn't <see cref="List{T}"/> and isn't immutable... so I made one</summary>
+		private readonly struct ListImpl : IList<int>
+		{
+			private readonly IList<int> _wrapped;
+
+			public int Count => _wrapped.Count;
+
+			public bool IsReadOnly => false;
+
+			public ListImpl(params int[] init) => _wrapped = new List<int>(init);
+
+			public readonly int this[int index]
+			{
+				get => _wrapped[index];
+				set => throw new NotImplementedException();
+			}
+
+			public readonly void Add(int item) => throw new NotImplementedException();
+
+			public readonly void Clear() => throw new NotImplementedException();
+
+			public readonly bool Contains(int item) => throw new NotImplementedException();
+
+			public readonly void CopyTo(int[] array, int arrayIndex) => throw new NotImplementedException();
+
+			public readonly IEnumerator<int> GetEnumerator() => throw new NotImplementedException();
+
+			IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+
+			public readonly int IndexOf(int item) => throw new NotImplementedException();
+
+			public readonly void Insert(int index, int item) => throw new NotImplementedException();
+
+			public readonly bool Remove(int item) => throw new NotImplementedException();
+
+			public readonly void RemoveAt(int index) => _wrapped.RemoveAt(index);
+		}
+
+		[TestMethod]
+		public void TestAddRange()
+		{
+			List<int> a = new(new[] { 1, 2 });
+			CE.AddRange(a, new[] { 3, 4 });
+			Assert.AreEqual(4, a.Count, nameof(CE.AddRange) + " failed on List<int>");
+
+			SortedSet<int> b = new(new[] { 1, 2 });
+			b.AddRange(new[] { 3, 4 });
+			Assert.AreEqual(4, b.Count, nameof(CE.AddRange) + " failed on (ICollection<int> not List<int>)");
+		}
+
+		[TestMethod]
+		public void TestRemoveAll()
+		{
+			static bool Predicate(int i) => 2 <= i && i <= 3;
+
+			List<int> a = new(new[] { 1, 2, 3, 4 });
+			CE.RemoveAll(a, Predicate);
+			Assert.AreEqual(2, a.Count, nameof(CE.RemoveAll) + " failed on List<int>");
+
+			ListImpl b = new(1, 2, 3, 4);
+			b.RemoveAll(Predicate);
+			Assert.AreEqual(2, b.Count, nameof(CE.RemoveAll) + " failed on (IList<int> not List<int>)");
+
+			SortedSet<int> c = new(new[] { 1, 2, 3, 4 });
+			c.RemoveAll(Predicate);
+			Assert.AreEqual(2, c.Count, nameof(CE.RemoveAll) + " failed on (ICollection<int> not IList<int>)");
+		}
+	}
+}