fix bug with abstract type

Author: pwelter34

src/EntityChange/EntityComparer.cs

@@ -115,7 +115,11 @@ private void CompareObject(Type type, object? original, object? current)
             var currentValue = accessor.GetValue(current);
 
             var propertyName = accessor.Name;
-            var propertyType = accessor.MemberType.GetUnderlyingType();
+
+            // using value type to support abstracts
+            var propertyType = originalValue?.GetType()
+                ?? currentValue?.GetType()
+                ?? accessor.MemberType.GetUnderlyingType();
 
             _memberStack.Push(memberMapping);
             _pathStack.PushProperty(propertyName);

test/EntityChange.Tests/EntityCompareTests.cs

@@ -3,6 +3,7 @@
 using System.Collections.ObjectModel;
 using System.Globalization;
 using System.Linq;
+using System.Threading.Channels;
 using System.Threading.Tasks;
 
 using EntityChange.Tests.Models;
@@ -1094,6 +1095,34 @@ public void CompareArrayRootElementsTest()
         changeRecord.Path.Should().Be("[0]");
     }
 
+    [Fact]
+    public async Task CompareAbstract()
+    {
+        var obj1 = new Consumer()
+        {
+            SomeProperty = new ConcreteClass()
+            {
+                Id = 1,
+                SomeString = "Test1"
+            }
+        };
+        var obj2 = new Consumer()
+        {
+            SomeProperty = new ConcreteClass()
+            {
+                Id = 2,
+                SomeString = "Test2"
+            }
+        };
+
+        var comparer = new EntityComparer();
+        var changes = comparer.Compare(obj1, obj2);
+
+        changes.Should().NotBeEmpty();
+
+        await Verifier.Verify(changes).UseDirectory("Snapshots");
+    }
+
     private void WriteMarkdown(IReadOnlyList<ChangeRecord> changes)
     {
         var formatter = new MarkdownFormatter();

test/EntityChange.Tests/Models/Consumer.cs

@@ -0,0 +1,16 @@
+namespace EntityChange.Tests.Models;
+
+public abstract class AbstractBase
+{
+    public int Id { get; set; }
+}
+
+public class ConcreteClass : AbstractBase
+{
+    public string SomeString { get; set; }
+}
+
+public class Consumer
+{
+    public AbstractBase SomeProperty { get; set; }
+}

test/EntityChange.Tests/Snapshots/EntityCompareTests.CompareAbstract.verified.txt

@@ -0,0 +1,22 @@
+[
+  {
+    PropertyName: SomeString,
+    DisplayName: Some String,
+    Path: SomeProperty.SomeString,
+    Operation: Replace,
+    OriginalValue: Test1,
+    CurrentValue: Test2,
+    OriginalFormatted: Test1,
+    CurrentFormatted: Test2
+  },
+  {
+    PropertyName: Id,
+    DisplayName: Id,
+    Path: SomeProperty.Id,
+    Operation: Replace,
+    OriginalValue: 1,
+    CurrentValue: 2,
+    OriginalFormatted: 1,
+    CurrentFormatted: 2
+  }
+]