fix: IsEquivalentTo falls back to Equals() for types with no public members

TUnit.Assertions.Tests/Bugs/Tests5040.cs

@@ -0,0 +1,119 @@
+namespace TUnit.Assertions.Tests.Bugs;
+
+/// <summary>
+/// Tests for issue #5040: IsEquivalentTo throws NotSupportedException on IEquatable types with private state.
+/// When a type has no public members but implements IEquatable, structural comparison should fall back to Equals().
+/// </summary>
+public class Tests5040
+{
+    #region Test Types
+
+    private sealed class PrivateStatePath : IEquatable<PrivateStatePath>
+    {
+        private readonly string _value;
+
+        public PrivateStatePath(string value)
+        {
+            _value = value;
+        }
+
+        public bool Equals(PrivateStatePath? other)
+        {
+            if (other is null)
+            {
+                return false;
+            }
+
+            return _value == other._value;
+        }
+
+        public override bool Equals(object? obj) => Equals(obj as PrivateStatePath);
+
+        public override int GetHashCode() => _value.GetHashCode();
+
+        public override string ToString() => _value;
+    }
+
+    private class PublicStateMessage
+    {
+        public string Content { get; set; } = string.Empty;
+    }
+
+    #endregion
+
+    [Test]
+    public async Task IEquatable_Class_With_No_Public_Members_Equal_Values_Passes()
+    {
+        var path1 = new PrivateStatePath("/home/user/file.txt");
+        var path2 = new PrivateStatePath("/home/user/file.txt");
+
+        await Assert.That(path1).IsEquivalentTo(path2);
+    }
+
+    [Test]
+    public async Task IEquatable_Class_With_No_Public_Members_Different_Values_Fails()
+    {
+        var path1 = new PrivateStatePath("/home/user/file1.txt");
+        var path2 = new PrivateStatePath("/home/user/file2.txt");
+
+        var exception = await Assert.ThrowsAsync<TUnitAssertionException>(
+            async () => await Assert.That(path1).IsEquivalentTo(path2));
+
+        await Assert.That(exception).IsNotNull();
+    }
+
+    [Test]
+    public async Task IEquatable_Class_Collection_Unordered_Passes()
+    {
+        // Reproduces the exact scenario from issue #5040
+        var list = new List<PrivateStatePath>
+        {
+            new("/home/user/a.txt"),
+            new("/home/user/b.txt"),
+            new("/home/user/c.txt"),
+        };
+
+        var expected = new List<PrivateStatePath>
+        {
+            new("/home/user/c.txt"),
+            new("/home/user/a.txt"),
+            new("/home/user/b.txt"),
+        };
+
+        await Assert.That(list).IsEquivalentTo(expected);
+    }
+
+    [Test]
+    public async Task No_Public_Members_Vs_Public_Members_Fails()
+    {
+        // Expected has no public members, actual has public members — types are incompatible
+        var expected = new PrivateStatePath("/home/user/file.txt");
+        var actual = new PublicStateMessage { Content = "/home/user/file.txt" };
+
+        var exception = await Assert.ThrowsAsync<TUnitAssertionException>(
+            async () => await Assert.That(actual).IsEquivalentTo(expected));
+
+        await Assert.That(exception).IsNotNull();
+    }
+
+    [Test]
+    public async Task Partial_Equivalency_With_No_Public_Members_Equal_Values_Passes()
+    {
+        var path1 = new PrivateStatePath("/home/user/file.txt");
+        var path2 = new PrivateStatePath("/home/user/file.txt");
+
+        await Assert.That(path1).IsEquivalentTo(path2).WithPartialEquivalency();
+    }
+
+    [Test]
+    public async Task Partial_Equivalency_With_No_Public_Members_Different_Values_Fails()
+    {
+        var path1 = new PrivateStatePath("/home/user/file1.txt");
+        var path2 = new PrivateStatePath("/home/user/file2.txt");
+
+        var exception = await Assert.ThrowsAsync<TUnitAssertionException>(
+            async () => await Assert.That(path1).IsEquivalentTo(path2).WithPartialEquivalency());
+
+        await Assert.That(exception).IsNotNull();
+    }
+}

TUnit.Assertions/Conditions/Helpers/ReflectionHelper.cs

@@ -25,7 +25,7 @@ public static List<MemberInfo> GetMembersToCompare(
         var properties = type.GetProperties(BindingFlags.Public | BindingFlags.Instance);
         foreach (var prop in properties)
         {
-            if (prop.GetIndexParameters().Length == 0)
+            if (prop.GetIndexParameters().Length == 0 && prop.CanRead && prop.GetMethod?.IsPublic == true)
             {
                 members.Add(prop);
             }

TUnit.Assertions/Conditions/Helpers/StructuralEqualityComparer.cs

@@ -105,6 +105,14 @@ private bool CompareStructurally(object? x, object? y, HashSet<object> visited)
 
         var members = ReflectionHelper.GetMembersToCompare(xType);
 
+        // When there are no public members to compare structurally (e.g., types with only
+        // private state), fall back to Equals(). This respects IEquatable<T> implementations
+        // and avoids false positives from empty member lists.
+        if (members.Count == 0)
+        {
+            return Equals(x, y);
+        }
+
         foreach (var member in members)
         {
             var xValue = ReflectionHelper.GetMemberValue(x, member);

TUnit.Assertions/Conditions/StructuralEquivalencyAssertion.cs

@@ -193,6 +193,19 @@ internal AssertionResult CompareObjects(
         // Compare properties and fields
         var expectedMembers = ReflectionHelper.GetMembersToCompare(expectedType);
 
+        // When there are no public members to compare structurally (e.g., types with only
+        // private state), fall back to Equals(). This respects IEquatable<T> implementations
+        // and avoids false positives from empty member lists.
+        if (expectedMembers.Count == 0)
+        {
+            if (!Equals(actual, expected))
+            {
+                var label = string.IsNullOrEmpty(path) ? "Objects" : $"Property {path}";
+                return AssertionResult.Failed($"{label} did not match{Environment.NewLine}Expected: {FormatValue(expected)}{Environment.NewLine}Received: {FormatValue(actual)}");
+            }
+            return AssertionResult.Passed;
+        }
+
         foreach (var member in expectedMembers)
         {
             var memberPath = string.IsNullOrEmpty(path) ? member.Name : $"{path}.{member.Name}";