Added interface ICollectionWrapper<TCollection> to expose the Underly…

…ingCollection property of ReadOnlyNonGenericCollectionWrapper<TCollection, TItem>.

Updated the ReadOnlyNonGenericCollectionWrapper<TCollection, TItem> type to implement ICollectionWrapper<TCollection>.
Updated DataColumnCollectionAssertionExtensions.cs to access UnderlyingCollection via ICollectionWrapper instead of the concrete type ReadOnlyNonGenericCollectionWrapper.
Updated the exposed API surface files correspondingly.

Src/FluentAssertions/Common/ICollectionWrapper.cs

@@ -0,0 +1,15 @@
+using System.Collections;
+
+namespace FluentAssertions.Common
+{
+    /// <summary>
+    /// Used to provide access to the underlying <typeparamref name="TCollection"/> for an object that wraps an underlying
+    /// collection.
+    /// </summary>
+    /// <typeparam name="TCollection">Collection type.</typeparam>
+    public interface ICollectionWrapper<TCollection>
+        where TCollection : ICollection
+    {
+        TCollection UnderlyingCollection { get; }
+    }
+}

Src/FluentAssertions/Common/ReadOnlyNonGenericCollectionWrapper.cs

@@ -33,7 +33,7 @@ internal static class ReadOnlyNonGenericCollectionWrapper
     }
 }
 
-internal class ReadOnlyNonGenericCollectionWrapper<TCollection, TItem> : ICollection<TItem>
+internal class ReadOnlyNonGenericCollectionWrapper<TCollection, TItem> : ICollection<TItem>, ICollectionWrapper<TCollection>
     where TCollection : ICollection
 {
     public TCollection UnderlyingCollection { get; }

Src/FluentAssertions/DataColumnCollectionAssertionExtensions.cs

@@ -30,7 +30,7 @@ public static class DataColumnCollectionAssertionExtensions
         Guard.ThrowIfArgumentIsNull(
             expected, nameof(expected), "Cannot verify same reference against a <null> collection (use BeNull instead?).");
 
-        if (assertion.Subject is ReadOnlyNonGenericCollectionWrapper<DataColumnCollection, DataColumn> wrapper)
+        if (assertion.Subject is ICollectionWrapper<DataColumnCollection> wrapper)
         {
             var actualSubject = wrapper.UnderlyingCollection;
 

Tests/Approval.Tests/ApprovedApi/FluentAssertions/net47.verified.txt

@@ -635,6 +635,11 @@ namespace FluentAssertions.Common
         System.Threading.Tasks.Task DelayAsync(System.TimeSpan delay, System.Threading.CancellationToken cancellationToken);
         FluentAssertions.Common.ITimer StartTimer();
     }
+    public interface ICollectionWrapper<TCollection>
+        where TCollection : System.Collections.ICollection
+    {
+        TCollection UnderlyingCollection { get; }
+    }
     public interface IConfigurationStore
     {
         string GetSetting(string name);

Tests/Approval.Tests/ApprovedApi/FluentAssertions/net6.0.verified.txt

@@ -647,6 +647,11 @@ namespace FluentAssertions.Common
         System.Threading.Tasks.Task DelayAsync(System.TimeSpan delay, System.Threading.CancellationToken cancellationToken);
         FluentAssertions.Common.ITimer StartTimer();
     }
+    public interface ICollectionWrapper<TCollection>
+        where TCollection : System.Collections.ICollection
+    {
+        TCollection UnderlyingCollection { get; }
+    }
     public interface IConfigurationStore
     {
         string GetSetting(string name);

Tests/Approval.Tests/ApprovedApi/FluentAssertions/netcoreapp2.1.verified.txt

@@ -635,6 +635,11 @@ namespace FluentAssertions.Common
         System.Threading.Tasks.Task DelayAsync(System.TimeSpan delay, System.Threading.CancellationToken cancellationToken);
         FluentAssertions.Common.ITimer StartTimer();
     }
+    public interface ICollectionWrapper<TCollection>
+        where TCollection : System.Collections.ICollection
+    {
+        TCollection UnderlyingCollection { get; }
+    }
     public interface IConfigurationStore
     {
         string GetSetting(string name);

Tests/Approval.Tests/ApprovedApi/FluentAssertions/netcoreapp3.0.verified.txt

@@ -635,6 +635,11 @@ namespace FluentAssertions.Common
         System.Threading.Tasks.Task DelayAsync(System.TimeSpan delay, System.Threading.CancellationToken cancellationToken);
         FluentAssertions.Common.ITimer StartTimer();
     }
+    public interface ICollectionWrapper<TCollection>
+        where TCollection : System.Collections.ICollection
+    {
+        TCollection UnderlyingCollection { get; }
+    }
     public interface IConfigurationStore
     {
         string GetSetting(string name);

Tests/Approval.Tests/ApprovedApi/FluentAssertions/netstandard2.0.verified.txt

@@ -628,6 +628,11 @@ namespace FluentAssertions.Common
         System.Threading.Tasks.Task DelayAsync(System.TimeSpan delay, System.Threading.CancellationToken cancellationToken);
         FluentAssertions.Common.ITimer StartTimer();
     }
+    public interface ICollectionWrapper<TCollection>
+        where TCollection : System.Collections.ICollection
+    {
+        TCollection UnderlyingCollection { get; }
+    }
     public interface IConfigurationStore
     {
         string GetSetting(string name);

Tests/Approval.Tests/ApprovedApi/FluentAssertions/netstandard2.1.verified.txt

@@ -635,6 +635,11 @@ namespace FluentAssertions.Common
         System.Threading.Tasks.Task DelayAsync(System.TimeSpan delay, System.Threading.CancellationToken cancellationToken);
         FluentAssertions.Common.ITimer StartTimer();
     }
+    public interface ICollectionWrapper<TCollection>
+        where TCollection : System.Collections.ICollection
+    {
+        TCollection UnderlyingCollection { get; }
+    }
     public interface IConfigurationStore
     {
         string GetSetting(string name);