MergeMap to NewMap

If you only define an IMergeMap<TSource, TDestination> for two given types it can also be used when creating a new object instead of defining a separate INewMap<TSource, TDestination> for the same types.

You can specify via CompositeMapperOptions/AsyncCompositeMapperOptions (and CompositeMapperMappingOptions/AsyncCompositeMapperMappingOptions overrides) the desired behaviour: if you want to create an empty destination object automatically (the default behaviour - a parameterless constructor is required) or provide a default value.

CategoryDto? IMergeMap<Category, CategoryDto>.Map(Category? source, CategoryDto? destination, MappingContext context){
	if(source != null){
		destination ??= new CategoryDto();
		destination.Id = source.Id;
		...
	}
	return destination;
}


// Map to an existing object
mapper.Map<Category, CategoryDto>(myCategory, myCategoryDto);

// Create a new object (a destination object will be created automatically
// and provided to the map)
var myProductDto = mapper.Map<Category, CategoryDto>(myCategory);

// Create a new object (a default/null destination object will be provided to the map)
var myProductDto = mapper.Map<Category, CategoryDto>(myCategory, new MappingOptions{ MergeMapsHandling = MergeMapsHandling.DefaultDestination });

You can still create both maps for the given types if you need specific behaviour in one case or the other.

ProjectionMap to NewMap

If you only define an IProjectionMap<TSource, TDestination> for two given types it can also be used to map them: the generated expression can be compiled into a delegate, which can be used to create a new object from an existing one, instead of defining a separate INewMap<TSource, TDestination> for the same types.

C# has some limitations regarding expression trees, so you may not be able to use all of the features.

Expression<Func<Category, CategoryDto>> IProjectionMap<Category, CategoryDto>.Project(ProjectionContext context){
	return source => new CategoryDto { ... };
}


// Create a projection expression
var expr = projector.Project<Category, CategoryDto>();

// Or create a new object
var myCategoryDto = mapper.Map<Category, CategoryDto>(myCategory);

All projection maps can be compiled by default, if your maps are not suitable for compilation (for example they use methods which can only be converted to other languages by LINQ providers) you can check if your expression is about to be compiled by checking ProjectionCompilationContext inside MappingOptions of the ProjectionContext.

bool ICanProject<Category, CategoryDto>.CanProject(ProjectionContext context){
	// Refuse compilation
	return !context.MappingOptions.HasOptions<ProjectionCompilationContext>();
}

Expression<Func<Category, CategoryDto>> IProjectionMap<Category, CategoryDto>.Project(ProjectionContext context){
	return source => new CategoryDto { ... };
}