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Mapperly is an incremental .NET source generator implementation. Source generators are explained here and here.


  • benchmarks Benchmarks to analyze the performance of the generated code and the source generator itself
  • build Build scripts
  • docs Documentation of Mapperly
  • samples Sample implementations of Mappers using Mapperly
  • src Source code of Mapperly
    • Riok.Mapperly The source generator implementation
    • Riok.Mapperly.Abstractions Abstractions and attributes to be used by the application code to configure Mapperly. This is referenced by the source generator but is not needed at runtime.
    • Riok.Mapperly.Templates Templates of code files which are embedded as resources into Riok.Mapperly and may be emitted during source generation depending on enabled features.
  • test Unit- and integration tests of Mapperly


This describes the process implemented by Mapperly on a higher level. For each discovered MapperAttribute a new DescriptorBuilder is created. The DescriptorBuilder is responsible to build a MapperDescriptor which holds all the mappings. The DescriptorBuilder does this by following this process:

  1. Extracting the configuration from the attributes
  2. Extracting user implemented and user defined mapping methods. It instantiates a User*Mapping (eg. UserDefinedNewInstanceMethodMapping) for each discovered mapping method and adds it to the queue of mappings to work on.
  3. Extracting user implemented object factories
  4. Extracting user implemented format providers
  5. Extracting external mappings
  6. For each mapping in the queue the DescriptorBuilder tries to build its implementation bodies. This is done by a so called *MappingBodyBuilder. A mapping body builder tries to map each property from the source to the target. To do this, it asks the DescriptorBuilder to create mappings for the according types. To create a mapping from one type to another, the DescriptorBuilder loops through a set of *MappingBuilders. Each of the mapping builders try to create a mapping (an ITypeMapping implementation) for the asked type mapping by using one approach on how to map types (eg. an explicit cast is implemented by the ExplicitCastMappingBuilder). These mappings are queued in the queue of mappings which need the body to be built (currently body builders are only used for object to object (property-based) mappings).
  7. The SourceEmitter emits the code described by the MapperDescriptor and all its mappings. The syntax objects are created by using SyntaxFactory and SyntaxFactoryHelper. The SyntaxFactoryHelper tries to simplify creating formatted syntax trees. If indentation is needed, the SyntaxFactoryHelper instance of the SourceEmitterContext/TypeMappingBuildContext can be used.

Roslyn multi targeting

Mapperly targets multiple Roslyn versions by building multiple NuGet packages and merging them together into a single one. Multi-targeting is needed to support new language features, such as required members introduced in C# 11, while still supporting older compiler versions.

See build/ for details.

To introduce support for a new roslyn version see common tasks.

C# language features

The Mapperly source generator targets .netstandard2.0 but uses the latest C# language version and polyfills generated by Meziantou.Polyfill. Some newer C# language features require new runtime features and therefore cannot be used in Mapperly. A good C# language features requirements overview is available here.