Existing target object

If an existing object instance should be used as target, you can define the mapping method as void with the target as second parameter:

Mapper declaration

[Mapper]
public partial class CarMapper
{
    public partial void CarToCarDto(Car car, CarDto dto);
}

Mapper usage

var mapper = new CarMapper();
var car = new Car { NumberOfSeats = 10, ... };
var dto = new CarDto();

mapper.CarToCarDto(car, dto);
dto.NumberOfSeats.Should().Be(10);

Merge objects

To merge two objects together, AllowNullPropertyAssignment can be set to false. This ignores all properties on the source with a null value.

Declaration

[Mapper(AllowNullPropertyAssignment = false)]
static partial class FruitMapper
{
    public static partial void ApplyUpdate(FruitUpdate update, Fruit fruit);
}

class Fruit { public required string Name { get; set; } public required string Color { get; set; } }
record FruitUpdate(string? Name, string? Color);

Generated code

static partial class FruitMapper
{
    public static partial void Update(global::FruitUpdate update, global::Fruit fruit)
    {
        if (update.Name != null)
        {
            fruit.Name = update.Name;
        }
        if (update.Color != null)
        {
            fruit.Color = update.Color;
        }
    }
}

See also null value handling.