How to pass default value to unmapped constructor parameter

[KUTlime]

Imagine this repository entity:

public record SomeEntity(long Id, Guid UniqueId, string SomeProperty);

and API model:

public class SomeAPIModel
{
   public Guid UniqueId {get; set;} // A public unique ID 

   public string SomeProperty {get; set;}
}

Using Mapperly, mapping SomeEntity -> SomeAPIModel is fine, because Mapperly will map 3 properties to 2 properties, not vice versa. From SomeAPIModel -> SomeEntity, we missing value for Id. The use of default would be OK, because the value will overridden anyway, but generation of mapping would fail on this.

Is there any way how to overcome this? How to pass a value for certain properties?

[latonz]

What compiler error is thrown?

You should be able to ignore the property by using the MapperIgnoreTarget attribute. See docs.

[KUTlime]

Dear @latonz if I image a parallel universe in which I can push trough a code review a repository entity with Id as optional parameter placed on the end with reasoning "It is because of 3rd party source code generated mapping.", it is a really good or ideal solution, right?

I think it shouldn't be a problem to pass a default value to the mapping from Mapperly (which is basically what do you suggest, just more versatile and clear architecture), not bend the entity that is being mapped.

[latonz]

I don't think the solution is to let Mapperly inject default arguments into unknown constructor parameters. Also I can't think of a deserialiser (eg. System.Text.Json) doing this. IMO this would not be reasonable.
You always have the option to implement a specific mapping yourself or make use of an object factory.

[KUTlime]

Why unknown?

What is wrong about having an option like this? (name proposals)

[SetValueForMember(nameof(SomeEntity.Id), 0)]
[PassValueToMember(nameof(SomeEntity.Id), 0)]
[SetMemberValue(nameof(SomeEntity.Id), 0)]
[InitiliazeMemberWithValue(nameof(SomeEntity.Id), 0)]

It would enable Mapperly to cover more scenarios That is what we want, right? To make it usable as much as possible.

[cremor]

A simpler fist step could be to have something like [MapperSetDefaultValue(nameof(SomeEntity.Id))]. The source generator could just always assign default to the property given in that attribute.

Or a possible solution that doesn't even need a new attribute: What if the existing MapperIgnoreTargetAttribute would just cause the source generator to assign default if it is a constructor parameter or required property?

[cremor]

I've created a feature request: #631

[cremor]

I would need something similar. Instead of missing constructor parameters, it's missing required members in my case (error message "RMG023 Required member A on mapping target type X was not found on the mapping source type Y").

I like the new attribute name proposals mentioned here: #335 (reply in thread)

[TimothyMakkison]

Have you tried using an object factory?

[cremor]

Yes, using an object factory is a workaround. But it's not really nice looking code because you need to initialize all properties in the object factory, even those that could be mapped from the source object. So this might result in many Xyz = default or (even worse) Xyz = default! assignments in the object factory.

[latonz]

Closing as there exists an issue now: #631