AllowNullPropertyAssignment and ICollection

[sraedler]

Imagine the following tobe mapped.

public partial class Class1
{
    public int Id { get; set; }
    public virtual ICollection<Class2> prop1 { get; set; } = new List<Class2>();
}

public partial class Class2
{
    public string name { get; set; }
}

The mapping:

[Mapper(EnumMappingStrategy = EnumMappingStrategy.ByName, UseReferenceHandling = true, AllowNullPropertyAssignment = false)]
public static partial class DisableNullMapper
{
    public static partial void MapClass1([MappingTarget] this Class1 target, Class1 input);
}

What I would like to have the following behaviour on prop1 with pseudocode:

var input  = new Class1(){prop1 = new List<Class2>()};
var target = new Class1(){prop1 = new List<Class2>()};
target.prop1.Add(new Class2());

//mapping
if(input.prop1.Any())
{
target.prop1 = input.prop1;
}
else
{
//do nothing
}

In other words, I would like to only map from the source to the target, if the source collection property is not empty. I tried AllowNullPropertyAssignment but it is not working, because the input is not null, it is empty.

Can anyone please help me out?

Thanks a lot!

[latonz]

This behavior—only assigning the collection if it is not empty—is currently not supported by Mapperly.
What’s the use case or motivation behind wanting to skip the assignment if the collection is empty? Understanding your scenario better could help us think about potential workarounds or future feature ideas.

[sraedler]

Ah ok, thanks for the fast reply!

Basically, I think my design might be wrong but the intention is the following:

There is a parent object with n children (up to 2000). I want to create a deep copy of the object with all children.
Before creating the copy, the user can change properties of the parent.

The process is that I pass the changed parent without the children to a method, querying the original object, exchanging the parent with the changed one, but keeping the children and then insert it.

I hope this helps. Truely, I can map with the existing Mapperly properties and afterwards bind the children but I wanted to know whether it is possible to use Mapperly for this use case.

Thanks again!

[latonz]

Mapperly currently doesn’t support merging two objects in the way you described (i.e., mapping a modified parent while preserving or injecting existing children from another instance).

As you already mentioned, the recommended approach here would be to map the parent using Mapperly and then manually attach the original children in an after-map.