Mapper.cs

public SomeAbstractDto ToDto(SomeAbstractRepository repository)
{
  if (repository == null) return null;
  
  return (SomeAbstractDto)ToDto((dynamic)repository);
}

public DerivedSomeDto1  ToDto(DerivedSomeRepository1 repository)
{
   return new DerivedSomeDto1
   {
      SomeRandomProperty1  = repository.SomeRandomProperty1,
      SomeRandomProperty2 = repository.SomeRandomProperty2  
   }
}

    .
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public DerivedSomeDto18  ToDto(DerivedSomeRepository18 repository)
{
   return new DerivedSomeDto18
   {
      SomeRandomProperty1  = repository.SomeRandomProperty1
   }
}