v10 no longer uses default values for missing constructor parameters

[Ergamon]

Considering these 2 classes:

public class Source
{
  public required string Value1 { get; init; }
}

public class Target
{
  public Target(string value1, string value2)
  {
    Value1 = value1;
    Value2 = value2;
  }

  public string Value1 { get; }

  public string Value2 { get; }
}

Running this code:

    var source = new Source
    {
      Value1 = "123",
    };

    var target = source.Adapt<Target>();

This works in v7.4.0 resulting in a Source instance with Value1 = "123" and Value2 = null.

In v10 I get this exception:
System.InvalidOperationException: No default constructor for type 'Target', please use 'ConstructUsing' or 'MapWith'

Sounds like a pretty huge breaking change in my opinion. I dont see anything about it in the changelog.

[DocSvartz]

Hi @Ergamon , it is related to the point that this class in v10 is not a mapster record type.

Hm. I thought that this case shouldn't have worked in version 7.4.0 either.
Because I remember an open issue about this after version 7.4.0 was released;

@andrerav @stagep @Ergamon

We can add automatic activation of the class in this case, but there are two problems:

1. Сlass mapping never supported this behavior;
2. These will start having problems with MapToTarget Critical regression introduced in Mapster 7.3.0 when mapping to existing objects #537 Since the class mapping pipeline does not support such mapping.

[DocSvartz]

@andrerav @stagep There seems to be an error with the implementation of [UseDestinationValue] for primitive value AutoProperty. in v10
Why does it work even without using it, which probably shouldn't 🤔

If we accept this behavior as a feature and not a bug😅. we could implement constructor activation for classes with a default value (although there could still be issues with Projection).

But it seems to me that mutating the value of a property without a setter, without explicitly indicating this, is not a very good idea.

update: No, this was done exactly for such cases by MapToTarget.

[TestMethod]
public void CustomRecordWithConfig_Working_()
{
    var patch = new Target833("Hello","John");
    var destination = new Target833("Sample text", "Name");

    patch.Adapt(destination);

    destination.Value1.ShouldBe("Hello");
    destination.Value2.ShouldBe("John");
}

public class Source
{
  public required string Value1 { get; init; }
}

public class Target
{
  public Target(string value1, string value2)
  {
    Value1 = value1;
    Value2 = value2;
  }

  public string Value1 { get; }
  public string Value2 { get; }
}