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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‚Äč

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

[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);

See also null value handling.