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fix: honor per-type ShallowCopyForSameType overrides (#938) #974

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fix: honor per-type ShallowCopyForSameType overrides (#938) #974

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45499ac
fix: honor per-type ShallowCopyForSameType overrides (#938)
May 25, 2026
44b1479
fix: keep shallow copy limited to implicit same-type mappings
May 26, 2026
669a53d
fix: preserve inherited ShallowCopyForSameType on nested same-type maps
May 27, 2026
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108 changes: 108 additions & 0 deletions src/Mapster.Tests/WhenPerTypeShallowCopyOverride.cs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,108 @@
using Mapster.Models;
using MapsterMapper;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Shouldly;

namespace Mapster.Tests
{
/// <summary>
/// https://github.com/MapsterMapper/Mapster/issues/938
/// </summary>
[TestClass]
public class WhenPerTypeShallowCopyOverride
{
[TestMethod]
public void GlobalShallowCopy_WithPerTypeDeepCopy_ShouldRestoreViaMapToTarget()
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@DocSvartz DocSvartz May 26, 2026 •
edited
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This test is not valid and fail ❌

To obtain the desired result, the issue author used an incorrect configuration.

This is what the author wanted to achieve.

[TestMethod]
public void GlobalShallowCopy_WithPerTypeDeepCopy_ShouldRestoreViaMapToTarget()
{
    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>()
       // .ShallowCopyForSameType(true)
        .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 };
    //dynamicStuff.Item2.SampleNumber = 2;

    var str = dynamicStuff.BuildAdapter(cfg).CreateMapToTargetExpression<MyFailStuff>();



    var originalStuff = mapper.Map<MyFailStuff, MyFailStuff>(dynamicStuff);

    dynamicStuff.Item1.SampleName = "SN1CHANGED";
    (dynamicStuff.Item2 as RandomObject2).SampleNumber = 3;

    mapper.Map(originalStuff, dynamicStuff);

    dynamicStuff.Item1.SampleName.ShouldBe("SN1");
    (dynamicStuff.Item2 as RandomObject2).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; }
  }

{
var cfg = new TypeAdapterConfig();
cfg.RequireExplicitMapping = true;
cfg.Default.ShallowCopyForSameType(true);
cfg.Default.AvoidInlineMapping(true);

cfg.NewConfig<RandomObject2, RandomObject2>();
cfg.NewConfig<RandomObject1, RandomObject1>();

cfg.NewConfig<MyFailStuff, MyFailStuff>()
.ShallowCopyForSameType(false);

cfg.GetMergedSettings(new TypeTuple(typeof(MyFailStuff), typeof(MyFailStuff)), MapType.Map)
.ShallowCopyForSameType.ShouldBe(false);

cfg.Compile();

var mapper = new Mapper(cfg);

var dynamicStuff = new MyFailStuff();
dynamicStuff.Item1 = new RandomObject1();
dynamicStuff.Item1.SampleName = "SN1";
dynamicStuff.Item2 = new RandomObject2();
dynamicStuff.Item2.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();
}

[TestMethod]
public void ParentDeepCopyOverride_ShouldNotDisableImplicitNestedShallowCopyForSameType()
{
var cfg = new TypeAdapterConfig();
cfg.Default.ShallowCopyForSameType(true);
cfg.NewConfig<Container, Container>()
.ShallowCopyForSameType(false);

cfg.Compile();

var src = new Container { Child = new NestedChild { Value = 1 } };
var dest = src.Adapt<Container>(cfg);

ReferenceEquals(src.Child, dest.Child).ShouldBeTrue();
}

public class Container
{
public NestedChild? Child { get; set; }
}

public class NestedChild
{
public int Value { 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; }
}

public class RandomObject1
{
public string? SampleName { get; set; }
}

public class RandomObject2
{
public int SampleNumber { get; set; }
}
}
}
8 changes: 6 additions & 2 deletions src/Mapster/Adapters/BaseAdapter.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -503,8 +503,12 @@ internal Expression CreateAdaptExpression(Expression source, Type destinationTyp
var notUsingDestinationValue = mapping is not { UseDestinationValue: true };
Expression exp;

if (_source.Type == destinationType && arg.Settings.ShallowCopyForSameType == true
&& notUsingDestinationValue && rule == null)
var shallowCopySettings = _source.Type == destinationType
? arg.Context.Config.GetMergedSettings(tuple, arg.MapType)
: arg.Settings;

if (_source.Type == destinationType && shallowCopySettings.ShallowCopyForSameType == true
&& notUsingDestinationValue)
exp = _source;
Comment on lines +515 to 517
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@DocSvartz DocSvartz May 26, 2026

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This is the wrong level of mapping.
This is where properties are mapped, not types.

A shallow copy creates a new top-level object but does not duplicate nested elements. Instead, it copies the memory references of nested objects and arrays. This means modifying a nested property in the copy will inadvertently change it in the original object.

If you consider this a new TopLevel, a new object must be created here in any case, and its properties must be copied by reference.
Or if this is a parsing of the properties of the original TopLevel (the one specified in arg).
Then searching for this setting is pointless, since all TopLevel properties must be copied by reference in any case.

else if (source is ConditionalExpression cond && mapping != null)
{
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