Skip to main content

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:

[Mapper]
public partial class CarMapper
{
    public partial CarDto CarToCarDto(Car car);

    private int TimeSpanToHours(TimeSpan t) => t.Hours;
}

Whenever Mapperly needs a mapping from TimeSpan to int inside the CarMapper implementation, it will use the provided implementation. The types of the user-implemented mapping method need to match the types to map exactly, including nullability.

If there are multiple user-implemented mapping methods suiting the given type-pair, by default, the first one is used. This can be customized by using automatic user-implemented mapping method discovery and default mapping method.

Automatic user-implemented mapping method discovery​

By default, user implemented mapping methods are discovered automatically. This can be disabled by setting AutoUserMappings to false. With AutoUserMappings disabled, only methods marked by the UserMappingAttribute are discovered by Mapperly.

[Mapper(AutoUserMappings = false)]
public partial class CarMapper
{
    public partial CarDto CarToCarDto(Car car);

    // mark as user-implemented mapping
    [UserMapping]
    private int TimeSpanToHours(TimeSpan t) => t.Hours;

    private int IgnoredTimeSpanToHours(TimeSpan t) => t.Minutes / 60;
}

The AutoUserMappings value also applies to the usage of external mappers: With AutoUserMappings enabled all methods with a mapping method signature are discovered. With AutoUserMappings disabled, only methods with a UserMappingAttribute and, if the containing class has a MapperAttribute, partial methods are discovered.

Ignoring a user-implemented mapping method​

To ignore a user-implemented mapping method with enabled AutoUserMappings, [UserMapping(Ignore = true)] can be applied.

[Mapper]
public partial class CarMapper
{
    public partial CarDto CarToCarDto(Car car);

    // discovered and used by default (AutoUserMappings is true by default)
    private int TimeSpanToHours(TimeSpan t) => t.Hours;

    // ignored user-implemented mapping
    [UserMapping(Ignore = true)]
    private int IgnoredTimeSpanToHours(TimeSpan t) => t.Minutes / 60;
}

Default mapping methods​

Whenever Mapperly will need a mapping for a given type-pair, it will use the default mapping. A user-implemented or user-defined mapping is considered the default mapping for a type-pair if Default = true is set on the UserMapping attribute. If no user-implemented mapping with Default = true exists and AutoUserMappings is enabled, the first user-implemented mapping which has an unspecified Default value is used. For each type-pair only one default mapping can exist.

If multiple mappings exist for the same type-pair without a default mapping, RMG060 is reported. By default RMG060 has a severity of warning, which can be changed using the .editorconfig.

Map properties using user-implemented mappings​

See user-implemented property conversions.

Use external mappings​

Mapperly can also consider mappings implemented in other classes. In order for Mapperly to find the mappings, they must be made known with UseMapper / UseStaticMapper.

For static mappings, UseStaticMapper can be used:

[Mapper]
[UseStaticMapper(typeof(BananaMapper))]
public static partial class BoxMapper
{
  public static partial BananaBox MapBananaBox(BananaBoxDto dto);
}

public static class BananaMapper
{
  public static Banana MapBanana(BananaDto dto)
    => new Banana(dto.Weigth);
}

Whenever Mapperly needs a mapping from BananaBox to BananaBoxDto inside the BoxMapper implementation, it will use the provided implementation by the BananaMapper.

Used mappers themselves can be Mapperly backed classes.
The AutoUserMappings value also applies to the usage of external mappers.