Mapping with constructor and default values for ignored properties?

[DumboJetEngine]

I try to map from class A to class B, where B is missing a property of A. Usually, to do this you have to use .Ignore(e => e.Property) but I use a non-default constructor for initialization of the destination class (because I need to validate the incoming data within the constructor) and this fails.

Apparently, Mapster removes the ignored parameter from the constructor invocation parameters instead of using the default parameter value. And this inevitably fails.

Here is some sample code:

using Mapster;
using System.Reflection;

var config = new TypeAdapterConfig()
{
    RequireExplicitMapping = false,
    RequireDestinationMemberSource = true,
};

config.Default
    .PreserveReference(true)
    .MapToConstructor(true)
    .Unflattening(false)
    .IgnoreNullValues(false)
    .NameMatchingStrategy(new NameMatchingStrategy
    {
        SourceMemberNameConverter = input => input.ToLowerInvariant(),
        DestinationMemberNameConverter = input => input.ToLowerInvariant(),
    })
    .ShallowCopyForSameType(false);

var ctorToUse = GetConstructor<B>();
config
    .NewConfig<A, B>()
    .Ignore(e => e.Id) // <--------------- The code fails with or without this invocation.
    .MapToConstructor(ctorToUse);

var mapper = new MapsterMapper.Mapper(config);

static ConstructorInfo? GetConstructor<TDestination>()
{
    var parameterlessCtorInfo = typeof(TDestination).GetConstructor(BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public, new Type[0]);

    var ctors = typeof(TDestination).GetConstructors(BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
    var validCandidateCtors = ctors.Except(new[] { parameterlessCtorInfo }).ToArray();
    var ctorToUse = validCandidateCtors.Length == 1
        ? validCandidateCtors.First()
        : validCandidateCtors.OrderByDescending(c => c.GetParameters().Length).First();

    return ctorToUse;
}

var a = new A
{
    Text = "test",
};

var docKind = mapper.Map<B>(a); // <------ This fails.

class A
{
    public string? Text { get; set; }
}

class B
{
    public int Id { get; private set; }
    public string Text { get; private set; }

    public B(int id, string text)
    {
        Id = id;
        Text = text;
    }
}

My humble opinion is that this code should work and invoke the constructor using the default value for Id, which is default(int), which is 0. Omitting it during the invocation, which is what currently appears to happen, makes little sense to me.

I would also fancy an .Ignore() overload method that allows providing the value for the property, like this one:

config
    .NewConfig<A, B>()
    .Ignore(e => e.Id, 0) // Use value of 0 for id
    .Ignore(e => e.Id2, 0) // Use value of 0 for id2
    .MapToConstructor(ctorToUse);

...but perhaps this is too much to ask.

P.S. : I know I could use .MapWith(a => new B(0, a.Text)) to manually fix the issue, but I try to keep the mapping as automated as possible, because I need to do this configuration for many types that contain a lot of constructor parameters and this would be messy. So, this is not an option for my case.

[DumboJetEngine]

I have tried out a fix that seems to work without breaking the existing tests. I am not an expert on the internals of Mapster, but this is the fix anyway: When you are supposed to use the target class constructor and the source property getter is null, then if the property is ignored I set the getter to return the default value instead of marking it as an unmapped member. Let me know if this makes any sense code-wise.

Note that, the problem above mostly applies for the case you use RequireDestinationMemberSource = true. I want to use this setting, because I need to know of any unmapped destination members and avoid surprises.