diff --git a/Src/FluentAssertions/Common/ReadOnlyNonGenericCollectionWrapper.cs b/Src/FluentAssertions/Common/ReadOnlyNonGenericCollectionWrapper.cs
index fe476343c6..a16632ad0a 100644
--- a/Src/FluentAssertions/Common/ReadOnlyNonGenericCollectionWrapper.cs
+++ b/Src/FluentAssertions/Common/ReadOnlyNonGenericCollectionWrapper.cs
@@ -2,6 +2,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Data;
+using System.Diagnostics;
 using System.Linq;
 
 namespace FluentAssertions.Common;
@@ -33,7 +34,14 @@ internal static class ReadOnlyNonGenericCollectionWrapper
     }
 }
 
-internal class ReadOnlyNonGenericCollectionWrapper<TCollection, TItem> : ICollection<TItem>
+/// <summary>
+/// This type exists to serve internal functionality in FluentAssertions. It is marked <c>public</c> to allow it to be
+/// referenced from packages that extend FluentAssertion's functionality.
+/// </summary>
+/// <typeparam name="TCollection">Collection type.</typeparam>
+/// <typeparam name="TItem">Item type.</typeparam>
+[DebuggerNonUserCode]
+public class ReadOnlyNonGenericCollectionWrapper<TCollection, TItem> : ICollection<TItem>
     where TCollection : ICollection
 {
     public TCollection UnderlyingCollection { get; }
diff --git a/Tests/Approval.Tests/ApprovedApi/FluentAssertions/net47.verified.txt b/Tests/Approval.Tests/ApprovedApi/FluentAssertions/net47.verified.txt
index 8a2d1f8e14..6793cc6654 100644
--- a/Tests/Approval.Tests/ApprovedApi/FluentAssertions/net47.verified.txt
+++ b/Tests/Approval.Tests/ApprovedApi/FluentAssertions/net47.verified.txt
@@ -647,6 +647,20 @@ namespace FluentAssertions.Common
     {
         System.TimeSpan Elapsed { get; }
     }
+    public class ReadOnlyNonGenericCollectionWrapper<TCollection, TItem> : System.Collections.Generic.ICollection<TItem>, System.Collections.Generic.IEnumerable<TItem>, System.Collections.IEnumerable
+        where TCollection : System.Collections.ICollection
+    {
+        public ReadOnlyNonGenericCollectionWrapper(TCollection collection) { }
+        public int Count { get; }
+        public bool IsReadOnly { get; }
+        public TCollection UnderlyingCollection { get; }
+        public void Add(TItem item) { }
+        public void Clear() { }
+        public bool Contains(TItem item) { }
+        public void CopyTo(TItem[] array, int arrayIndex) { }
+        public System.Collections.Generic.IEnumerator<TItem> GetEnumerator() { }
+        public bool Remove(TItem item) { }
+    }
     public static class Services
     {
         public static FluentAssertions.Common.Configuration Configuration { get; }
diff --git a/Tests/Approval.Tests/ApprovedApi/FluentAssertions/net6.0.verified.txt b/Tests/Approval.Tests/ApprovedApi/FluentAssertions/net6.0.verified.txt
index 1b6faed215..1fb1b7aa9c 100644
--- a/Tests/Approval.Tests/ApprovedApi/FluentAssertions/net6.0.verified.txt
+++ b/Tests/Approval.Tests/ApprovedApi/FluentAssertions/net6.0.verified.txt
@@ -659,6 +659,20 @@ namespace FluentAssertions.Common
     {
         System.TimeSpan Elapsed { get; }
     }
+    public class ReadOnlyNonGenericCollectionWrapper<TCollection, TItem> : System.Collections.Generic.ICollection<TItem>, System.Collections.Generic.IEnumerable<TItem>, System.Collections.IEnumerable
+        where TCollection : System.Collections.ICollection
+    {
+        public ReadOnlyNonGenericCollectionWrapper(TCollection collection) { }
+        public int Count { get; }
+        public bool IsReadOnly { get; }
+        public TCollection UnderlyingCollection { get; }
+        public void Add(TItem item) { }
+        public void Clear() { }
+        public bool Contains(TItem item) { }
+        public void CopyTo(TItem[] array, int arrayIndex) { }
+        public System.Collections.Generic.IEnumerator<TItem> GetEnumerator() { }
+        public bool Remove(TItem item) { }
+    }
     public static class Services
     {
         public static FluentAssertions.Common.Configuration Configuration { get; }
diff --git a/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netcoreapp2.1.verified.txt b/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netcoreapp2.1.verified.txt
index a9f68eb68d..a8a05b409b 100644
--- a/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netcoreapp2.1.verified.txt
+++ b/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netcoreapp2.1.verified.txt
@@ -647,6 +647,20 @@ namespace FluentAssertions.Common
     {
         System.TimeSpan Elapsed { get; }
     }
+    public class ReadOnlyNonGenericCollectionWrapper<TCollection, TItem> : System.Collections.Generic.ICollection<TItem>, System.Collections.Generic.IEnumerable<TItem>, System.Collections.IEnumerable
+        where TCollection : System.Collections.ICollection
+    {
+        public ReadOnlyNonGenericCollectionWrapper(TCollection collection) { }
+        public int Count { get; }
+        public bool IsReadOnly { get; }
+        public TCollection UnderlyingCollection { get; }
+        public void Add(TItem item) { }
+        public void Clear() { }
+        public bool Contains(TItem item) { }
+        public void CopyTo(TItem[] array, int arrayIndex) { }
+        public System.Collections.Generic.IEnumerator<TItem> GetEnumerator() { }
+        public bool Remove(TItem item) { }
+    }
     public static class Services
     {
         public static FluentAssertions.Common.Configuration Configuration { get; }
diff --git a/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netcoreapp3.0.verified.txt b/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netcoreapp3.0.verified.txt
index 8c7f8490bc..8567bce23d 100644
--- a/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netcoreapp3.0.verified.txt
+++ b/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netcoreapp3.0.verified.txt
@@ -647,6 +647,20 @@ namespace FluentAssertions.Common
     {
         System.TimeSpan Elapsed { get; }
     }
+    public class ReadOnlyNonGenericCollectionWrapper<TCollection, TItem> : System.Collections.Generic.ICollection<TItem>, System.Collections.Generic.IEnumerable<TItem>, System.Collections.IEnumerable
+        where TCollection : System.Collections.ICollection
+    {
+        public ReadOnlyNonGenericCollectionWrapper(TCollection collection) { }
+        public int Count { get; }
+        public bool IsReadOnly { get; }
+        public TCollection UnderlyingCollection { get; }
+        public void Add(TItem item) { }
+        public void Clear() { }
+        public bool Contains(TItem item) { }
+        public void CopyTo(TItem[] array, int arrayIndex) { }
+        public System.Collections.Generic.IEnumerator<TItem> GetEnumerator() { }
+        public bool Remove(TItem item) { }
+    }
     public static class Services
     {
         public static FluentAssertions.Common.Configuration Configuration { get; }
diff --git a/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netstandard2.0.verified.txt b/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netstandard2.0.verified.txt
index 1fc893363c..ad9b85f839 100644
--- a/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netstandard2.0.verified.txt
+++ b/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netstandard2.0.verified.txt
@@ -640,6 +640,20 @@ namespace FluentAssertions.Common
     {
         System.TimeSpan Elapsed { get; }
     }
+    public class ReadOnlyNonGenericCollectionWrapper<TCollection, TItem> : System.Collections.Generic.ICollection<TItem>, System.Collections.Generic.IEnumerable<TItem>, System.Collections.IEnumerable
+        where TCollection : System.Collections.ICollection
+    {
+        public ReadOnlyNonGenericCollectionWrapper(TCollection collection) { }
+        public int Count { get; }
+        public bool IsReadOnly { get; }
+        public TCollection UnderlyingCollection { get; }
+        public void Add(TItem item) { }
+        public void Clear() { }
+        public bool Contains(TItem item) { }
+        public void CopyTo(TItem[] array, int arrayIndex) { }
+        public System.Collections.Generic.IEnumerator<TItem> GetEnumerator() { }
+        public bool Remove(TItem item) { }
+    }
     public static class Services
     {
         public static FluentAssertions.Common.Configuration Configuration { get; }
diff --git a/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netstandard2.1.verified.txt b/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netstandard2.1.verified.txt
index dedd54bad1..7717c40c4a 100644
--- a/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netstandard2.1.verified.txt
+++ b/Tests/Approval.Tests/ApprovedApi/FluentAssertions/netstandard2.1.verified.txt
@@ -647,6 +647,20 @@ namespace FluentAssertions.Common
     {
         System.TimeSpan Elapsed { get; }
     }
+    public class ReadOnlyNonGenericCollectionWrapper<TCollection, TItem> : System.Collections.Generic.ICollection<TItem>, System.Collections.Generic.IEnumerable<TItem>, System.Collections.IEnumerable
+        where TCollection : System.Collections.ICollection
+    {
+        public ReadOnlyNonGenericCollectionWrapper(TCollection collection) { }
+        public int Count { get; }
+        public bool IsReadOnly { get; }
+        public TCollection UnderlyingCollection { get; }
+        public void Add(TItem item) { }
+        public void Clear() { }
+        public bool Contains(TItem item) { }
+        public void CopyTo(TItem[] array, int arrayIndex) { }
+        public System.Collections.Generic.IEnumerator<TItem> GetEnumerator() { }
+        public bool Remove(TItem item) { }
+    }
     public static class Services
     {
         public static FluentAssertions.Common.Configuration Configuration { get; }