Show HN: Net source generator for object mapping

Show HN: Net source generator for object mapping


Mapperly is a .NET source generator for generating object mappings. Inspired by MapStruct.

Because Mapperly creates the mapping code at build time, there is minimal overhead at runtime.
Even better, the generated code is perfectly readable, allowing you to verify the generated mapping code easily.

Mapperly is the fastet .NET object mapper out there, surpassing even the naive manual mapping approach! The benchmark was generated with

Method Mean Error StdDev Gen 0 Allocated
AgileMapper 1,523.8 ns 3.90 ns 3.25 ns 1.5106 3,160 B
TinyMapper 4,094.3 ns 3.90 ns 3.05 ns 1.0300 2,160 B
ExpressMapper 2,595.8 ns 5.49 ns 5.14 ns 2.3422 4,904 B
AutoMapper 1,203.9 ns 2.30 ns 2.15 ns 0.9098 1,904 B
ManualMapping 529.6 ns 0.52 ns 0.44 ns 0.5541 1,160 B
Mapster 562.1 ns 1.14 ns 0.89 ns 0.9098 1,904 B
Mapperly 338.5 ns 0.95 ns 0.84 ns 0.4396 920 B



Add the NuGet Package to your project:

dotnet add package Riok.Mapperly

Create your first mapper

Create a mapper declaration as a partial class
and apply the Riok.Mapperly.Abstractions.MapperAttribute attribute.
Mapperly generates mapping method implementations for the defined mapping methods in the mapper.

// Mapper declaration
public partial class DtoMapper

    public partial CarDto CarToCarDto(Car car);

// Mapper usage
var mapper = new DtoMapper();
var car = new Car  NumberOfSeats = 10, ... ;
var dto = mapper.CarToCarDto(car);


The attributes defined in Riok.Mapperly.Abstractions can be used to customize different aspects of the generated mapper.


The MapperAttribute provides options to customize the generated mapper class.
The default enum mapping and null handling strategy are adjustable.

Copy behaviour

By default, Mapperly does not create deep copies of objects to improve performance.
If an object can be directly assigned to the target, it will do so
(eg. if the source and target type are both Car[], the array and its entries will not be cloned).
To create deep copies, set the UseDeepCloning property on the MapperAttribute to true.


On each mapping method declaration, property mappings can be customized.
If a property on the target has a different name than on the source, the MapPropertyAttribute can be applied.
If a property should be ignored, the MapperIgnoreAttribute can be used.

Flattening and unflattening

It is pretty common to flatten objects during mapping, eg. Car.Make.Id=> CarDto.MakeId.
Mapperly tries to figure out flattenings automatically by making use of the pascal case C# notation.
If Mapperly can’t resolve the target or source property correctly, it is possible to manually configure it by applying the MapPropertyAttribute
by either using the source and target property path names as arrays or using a dot separated property access path string

[MapProperty(Source = new[] { nameof(Car), nameof(Car.Make), nameof(Car.Make.Id) }, Target = new[] { nameof(CarDto), nameof(CarDto.MakeId) })]
// Or alternatively
[MapProperty(Source = "Car.Make.Id", Target = "CarDto.MakeId")]

Note: Unflattening is not yet automatically configured by Mapperly and needs to be configured manually via MapPropertyAttribute.


The enum mapping can be customized by setting the strategy to use.
Apply the MapEnumAttribute and pass the strategy to be used for this enum.
It is also possible to set the strategy for the entire mapper via the MapperAttribute.
Available strategies:

Name Description
ByValue Matches enum entries by their values
ByName Matches enum entries by their exact names

The IgnoreCase property allows to opt in for case insensitive mappings.

User implemented mapping methods

If Mapperly cannot generate a mapping, one can implement it manually simply by providing a method body in the mapper declaration:

public partial class DtoMapper

    public partial CarDto CarToCarDto(Car car);

    private DateOnly DateTimeToDateOnly(DateTime dt) => DateOnly.FromDateTime(dt);

Whenever Mapperly needs a mapping from DateTime to DateOnly inside the DtoMapper implementation, it will use the provided implementation.

Before / after map

To run custom code before or after a mapping, the generated map method can be wrapped in a custom method:

public partial class DtoMapper

    public CarDto MapCarToCarDto(Car car)
        // custom before map code...
        var dto = CarToCarDto(car);
        // custom after map code...
        return dto;

    private partial CarDto CarToCarDto(Car car);

Void mapping methods

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
public partial class DtoMapper

    public partial void CarToCarDto(Car car, CarDto dto);

// Mapper usage
var mapper = new DtoMapper();
var car = new Car  NumberOfSeats = 10, ... ;
var dto = new CarDto();
mapper.CarToCarDto(car, dto);

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