Efficient custom mapping from data model to ViewModel/Dto or vice versa with LINQ support

How many lines of codes we waste on mapping from one source to a destination like DataModel to ViewModel.  There are well-known Mappers available to do the same but nothing can come close in terms of performance by using manual mapping just that we need to write at too many places or takes a huge chunk of code lines in our main codes, may be under controllers, services or where ever you prefer projection in code.

The idea is to develop a proper way to deal with mappings. Some time back I had written code for manual mapping under LINQ queries. Manual model mapping - LINQ projection technique, this still works great. The idea is to create a cleaner solution as a whole new package for handling Mapping which can support general mappings between models and support of LINQ projection to select the limited number of rows as required.

Primary objectives of the implementation.
- A Data model to view model/DTO/or any other.
- view model/DTO/or any other to a data model.
- Support of LINQ projection without selecting all columns from a table.
- Support of IEnumerable.

The above items are compelling enough to create mapping structure with performance benefits.

Let's start with creations of interfaces to support One-way mapper, Two-way mapper, and support of LINQ queries in both case.

Basically, all we need is creation of interfaces and implementation of mapping classes based on models.

A simple IMapper implementation which simply converts from one model to other. It is simple enough not to have any kind of explanation.

 public interface IMapper<TSource, TResult>  
 {  
   /// <summary>  
   /// Maps the specified model.  
   /// </summary>  
   /// <param name="model">The model.</param>  
   /// <returns>Projection/Mapping from <paramref name="model"/> to a new type.</returns>  
   TResult Map(TSource model);  
 }  

IQueryMapper is for supporting LINQ projection, it is a kind of extension of above.

 public interface IQueryMapper<TSource, TResult>  
 {  
   /// <summary>  
   /// Gets the expression for projection/mapping.  
   /// </summary>  
   /// <value>  
   /// The expression for projection/mapping.  
   /// </value>  
   Expression<Func<TSource, TResult>> Expression { get; }  
 }  

ITwoWayMapper, It is opposite of  IMapper by inheriting the feature. Like if IMapper is for User to UserDto, this would allow having UserDto to User.

 /// <summary>  
 /// Two way mapping for models.  
 /// </summary>  
 /// <typeparam name="TFirst">The type of the first.</typeparam>  
 /// <typeparam name="TSecond">The type of the second.</typeparam>  
 /// <seealso cref="IMapper{TFirst, TSecond}" />  
 public interface ITwoWayMapper<TFirst, TSecond>  
     : IMapper<TFirst, TSecond>  
 {  
   /// <summary>  
   /// Maps the specified model.  
   /// </summary>  
   /// <param name="model">The model.</param>  
   /// <returns>Projection/Mapping from <paramref name="model"/> to a new type.</returns>  
   TFirst Map(TSecond model);  
 }  

IQueryTwoWayMapper, is similar to ITwoWayMapper but with LINQ support.

 public interface IQueryTwoWayMapper<TFirst, TSecond>  
   : IQueryMapper<TFirst, TSecond>  
 {  
   /// <summary>  
   /// Gets the reverse projection/mapping expression.  
   /// </summary>  
   /// <value>  
   /// The reverse projection/mapping expression.  
   /// </value>  
   Expression<Func<TSecond, TFirst>> ReverseExpression { get; }  
 }  

All these above implementations use Interface Segregation Principle principle, we would see advantages for them soon.

Based on above interfaces, we can create extension methods to map it to expressions or System.Func implementation based on need. I have not entirely tested extension methods based on above interfaces but it is really simple to extend based on a need to do something else. This is one of the major parts of Mapping Structure.

 /// <summary>  
 /// Extension methods for mapper  
 /// </summary>  
 public static class ExtensionMapper  
 {  
   /// <summary>  
   /// Maps the model from <typeparamref name="TSource"/> to <typeparamref name="TResult"/>.  
   /// </summary>  
   /// <typeparam name="TSource">The type of the source.</typeparam>  
   /// <typeparam name="TResult">The type of the result.</typeparam>  
   /// <param name="mapper">The mapper.</param>  
   /// <param name="source">The source.</param>  
   /// <returns>Projection from <typeparamref name="TSource"/> to <typeparamref name="TResult"/>.</returns>  
   public static IEnumerable<TResult> Map<TSource, TResult>(  
     this ITwoWayMapper<TSource, TResult> mapper,  
     IEnumerable<TSource> source)  
   {  
     return source.Select(val => mapper.Map(val));  
   }  
   /// <summary>  
   /// Maps the model from <typeparamref name="TSource"/> to <typeparamref name="TResult"/>.  
   /// </summary>  
   /// <typeparam name="TSource">The type of the source.</typeparam>  
   /// <typeparam name="TResult">The type of the result.</typeparam>  
   /// <param name="source">The source.</param>  
   /// <param name="mapper">The mapper.</param>  
   /// <returns>Projection from <typeparamref name="TSource"/> to <typeparamref name="TResult"/>.</returns>  
   public static IEnumerable<TResult> Map<TSource, TResult>(  
     this IQueryable<TSource> source,  
     IQueryTwoWayMapper<TSource, TResult> mapper)  
   {  
     return source.Select(mapper.Expression);  
   }  
   /// <summary>  
   /// Maps the model from <typeparamref name="TSource"/> to <typeparamref name="TResult"/>.  
   /// </summary>  
   /// <typeparam name="TSource">The type of the source.</typeparam>  
   /// <typeparam name="TResult">The type of the result.</typeparam>  
   /// <param name="mapper">The mapper.</param>  
   /// <param name="source">The source.</param>  
   /// <returns>Projection from <typeparamref name="TSource"/> to <typeparamref name="TResult"/>.</returns>  
   public static IEnumerable<TResult> Map<TSource, TResult>(  
     this ITwoWayMapper<TResult, TSource> mapper,  
     IEnumerable<TSource> source)  
   {  
     return source.Select(val => mapper.Map(val));  
   }  
   /// <summary>  
   /// Maps the model from <typeparamref name="TSource"/> to <typeparamref name="TResult"/>.  
   /// </summary>  
   /// <typeparam name="TSource">The type of the source.</typeparam>  
   /// <typeparam name="TResult">The type of the result.</typeparam>  
   /// <param name="source">The source.</param>  
   /// <param name="mapper">The mapper.</param>  
   /// <returns>Projection from <typeparamref name="TSource"/> to <typeparamref name="TResult"/>.</returns>  
   public static IEnumerable<TResult> Map<TSource, TResult>(  
     this IQueryable<TSource> source,  
     IQueryTwoWayMapper<TResult, TSource> mapper)  
   {  
     return source.Select(mapper.ReverseExpression);  
   }  
 }  

Implementation

Sample DTO

 public class Book  
 {  
   public string Name {get;set;}  
   public string Publisher {get;set;}  
 }  
 public class BookDto  
 {  
   public string BookName {get;set;}  
   public string BookPublisher {get;set;}  
 }  

A simple one-way mapper implementation of IMapper based on above models.

 public class BookMapper  
      : IMapper<BookDto, Book>  
 {  
      public Book Map(BookDto model)  
         => new Book  
         {  
           Name = model.BookName,  
           Publisher = model.BookPublisher  
         };  
 }  

Usage

 public ActionResult AddBook(BookDto model)  
 {  
      var bookMapper = new BookMapper();  
      var bookDataModel = bookMapper.Map(model);  
      // Code to insert book.  
 }  

Two-way with IQueryTwoWayMapper sample mapper implementation.

 public class BookMapper  
   : ITwoWayMapper<Book, BookDto>  
     , IQueryTwoWayMapper<Book, BookDto>  
 {  
   public Book Map(BookDto model)  
         => new Book  
         {  
           Name = model.BookName,  
           Publisher = model.BookPublisher  
         };  
   public BookDto Map(Book model)  
       => new BookDto  
       {  
         BookName = model.Name,  
         BookPublisher = model.Publisher  
       };  
   public Expression<Func<BookDto, Book>> ReverseExpression  
       => model  
       => new Book  
       {  
         Name = model.BookName,  
         Publisher = model.BookPublisher  
       };  
   public Expression<Func<Book, BookDto>> Expression  
       => model  
       => new BookDto  
       {  
         BookName = model.Name,  
         BookPublisher = model.Publisher  
       };  
 }  

With above codes, it is a bit confusing why similar type of codes are repeated for function and expression implementation. The thing is System.Func and general function cannot be truly converted to expression easily. If we directly try to pass Func than it would execute on CLR rather than SQL level which would result in the selection of all columns and then the projection. With Expressions, only required columns can be selected.

Also, Expression cannot be simply converted to Func since those can be really complex with multi-level of Expression Tree. If you still need a cleaner way and ready to compromise performance than this line can be used with expression compilation but I won't recommend going in that way.

 public BookDto Map(Book model) => Expression.Compile().Invoke(model);  

Implementation of above

The separation of interface allows us to use in any combination. Use, IMapper with IQueryMapper in a case of one-way mapping with LINQ queries. IMapper could be individually used like in the first example and in above we used all interface but just by inheriting two of them which depends upon their root inheritance.

 public ActionResult AddBook(BookDto model)  
 {  
      var bookMapper = new BookMapper();  
      var bookDataModel = bookMapper.Map(model);  
      // Projection from Context Book data to BookDto through Expression by using extension method.  
      var bookDtos = BookRepository.All().Select(bookMapper.Map());  
 }  

I have used the implementation as a concept but use as per your requirement, altering codes and further separation/removal of interface and extra implementation of Extension methods.