test: document #938 MapToTarget with UseDestinationValue

src/Mapster.Tests/WhenSameTypeMapToTargetWithUseDestinationValue.cs

@@ -0,0 +1,80 @@
+using MapsterMapper;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Shouldly;
+
+namespace Mapster.Tests
+{
+    /// <summary>
+    /// https://github.com/MapsterMapper/Mapster/issues/938
+    /// Same-type MapToTarget with ctor-init nested objects: use UseDestinationValue on members,
+    /// not per-type ShallowCopyForSameType(false). See maintainer guidance on #974.
+    /// </summary>
+    [TestClass]
+    public class WhenSameTypeMapToTargetWithUseDestinationValue
+    {
+        [TestMethod]
+        public void MapToTarget_ShouldRestoreNestedMembers_WhenGlobalShallowCopyEnabled()
+        {
+            var cfg = new TypeAdapterConfig();
+            cfg.RequireExplicitMapping = true;
+            cfg.Default.AvoidInlineMapping(true);
+
+            cfg.NewConfig<RandomObject2, RandomObject2>();
+            cfg.NewConfig<RandomObject1, RandomObject1>()
+                .Include<RandomObject2, RandomObject2>();
+
+            cfg.NewConfig<MyFailStuff, MyFailStuff>()
+                .UseDestinationValue(x => x.Item1)
+                .UseDestinationValue(x => x.Item2);
+
+            cfg.Compile();
+
+            var mapper = new Mapper(cfg);
+
+            var dynamicStuff = new MyFailStuff();
+            dynamicStuff.Item1 = new RandomObject1();
+            dynamicStuff.Item1.SampleName = "SN1";
+            dynamicStuff.Item2 = new RandomObject2 { SampleNumber = 2 };
+
+            var originalStuff = mapper.Map<MyFailStuff, MyFailStuff>(dynamicStuff);
+
+            dynamicStuff.Item1.SampleName = "SN1CHANGED";
+            dynamicStuff.Item2.SampleNumber = 3;
+
+            mapper.Map(originalStuff, dynamicStuff);
+
+            dynamicStuff.Item1.SampleName.ShouldBe("SN1");
+            dynamicStuff.Item2.SampleNumber.ShouldBe(2);
+            ReferenceEquals(dynamicStuff.Item1, originalStuff.Item1).ShouldBeFalse();
+            ReferenceEquals(dynamicStuff.Item2, originalStuff.Item2).ShouldBeFalse();
+        }
+
+        public class RandomObject1
+        {
+            public string? SampleName { get; set; }
+        }
+
+        public class RandomObject2 : RandomObject1
+        {
+            public int SampleNumber { get; set; }
+        }
+
+        public class MyFailStuff
+        {
+            public MyFailStuff()
+            {
+                CreateEmptyEntities();
+            }
+
+            public void CreateEmptyEntities()
+            {
+                Item1 ??= new RandomObject1();
+                Item2 ??= new RandomObject2();
+            }
+
+            public RandomObject1? Item1 { get; set; }
+
+            public RandomObject2? Item2 { get; set; }
+        }
+    }
+}