Mapster.Tool incorrect codegen map to existing object

[heggi]

Empty console project for .Net 6.0 with nullable enabled

public struct Source
{
    public string Name { get; set; }
}

public struct Dest
{
    public string Name { get; set; }
}

[Mapper]
public interface ITestMapping
{
    Dest AdaptTo(Source source, Dest dest);
}

Got next generated code:

public partial class TestMapping : ITestMapping
{
    public Dest AdaptTo(Source p1, Dest p2)
    {
        Dest result = new Dest();

        result.Name = p1.Name;
        return result;

    }
}

Parameter p2 doesn't used. But expected next code

public partial class TestMapping : ITestMapping
{
    public Dest AdaptTo(Source p1, Dest p2)
    {
        p2.Name = p1.Name;
        return p2;
    }
}

[heggi]

Same trouble with basic mapper

public struct Source
{
    public string Name { get; set; }
}
public struct Dest
{
    public string Name { get; set; }

    public string Ignore { get; set; }
}

TypeAdapterConfig<Source, Dest>
    .NewConfig()
    .Ignore(s => s.Ignore);

var source = new Source
{
    Name = "test",
};
var dest = new Dest
{
    Ignore = "ignore",
};

dest = source.Adapt(dest);

Console.WriteLine($"{dest.Name} {dest.Ignore}");

Got result test, but expected test ignore

If I change Dest from struct to class, mapper work right.

[andrerav]

Probably the same issue as #450. This should definitely work. Which version of Mapster are you using?

[andrerav]

After some quick testing, I found the following:

• v2.6.1 was the last version where this worked correctly
• v3.0.0 introduced a crash (Unhandled exception. Mapster.CompileException: Error while compiling, Coalesce used with type that cannot be null)
• v4.0.0 had the same crash as v3.0.0
• v4.1.0 (which came directly after v4.0.0) introduced the behaviour we are seeing now with silent failure

[andrerav]

Here's the 4.1.0 release: https://github.com/MapsterMapper/Mapster/releases/tag/4.1.0 "Fixed mapping to existing struct" is mentioned in the release notes.

[andrerav]

The crash introduced in v3.0.0 was reported in #195, and then this commit was added to fix the issue: https://github.com/MapsterMapper/Mapster/commit/2e3b235f5d83b4347da6515aca68149135b47261

So that looks like a good place to start hacking :)

[heggi]

Which version of Mapster are you using?

Latest release version 7.3.0

[andrerav]

@heggi Alright, thank you. This bug has existed since 2017 when it was broken in v3.0.0. There are only two unit tests related to this (in the WhenMappingStruct test class), and neither of them tests mapping of actual structs, only mapping of class properties that are structs.

[andrerav]

@heggi I believe I have found a fix for this. I haven't tested this with Mapster.Tool yet, but I think the fix should carry over there as well. When I have verified the fix in Mapster.Tool I'll push a prerelease package that you can test.

[andrerav]

@heggi I've verified that this fixes the issue for both basic mapping and code generation. Please try version 8.4.0-pre05 and let me know how it goes:

dotnet tool install --global Mapster.Tool --version 8.4.0-pre05

[heggi]

It works very well in my case

[andrerav]

Great, thank you! This fix will be included in Mapster.Tool 8.4.0 and Mapster 7.4.0 which should be released in a few weeks at the latest.