AutoMapper mapping with generic extension methods

Question

I want to map my objects with generic extension methods.

public class Customer    
{    
    public string FirstName { get; set; }    
    public string LastName { get; set; }    
    public string Email { get; set; }    
    public Address HomeAddress { get; set; }    
    public string GetFullName()    
    {  
        return string.Format(“{0} {1}”, FirstName, LastName);
    }    
}

And this is viewmodel

public class CustomerListViewModel    
{    
    public string FullName { get; set; }
    public string Email { get; set; }    
    public string HomeAddressCountry { get; set; }    
}

So I am creating map, Mapper.CreateMap<Customer, CustomerListViewModel>();

And I want to create an extension method

public static class MapperHelper
{
    public static CustomerListViewModel ToViewModel(this Customer cust)
    {
        return AutoMapper.Mapper.Map<Customer, CustomerListViewModel>(cust);
    }
}

But I want to make generic this helper:

public static class MapperHelper<TSource, TDest>
{
    public static TDest ToViewModel(this TSource cust)
    {
        return AutoMapper.Mapper.Map<TSource, TDest>(cust);
    }
}

Gives error: Extension method can only be declared in non-generic, non-nested static class

If I can not make generic, I should create helper class for all mapping. Is there any way to solution?

Answer 1

Even better than these solutions is to use the non-generic Map method:

public static class MapperHelper
{
    public static TDest MapTo<TDest>(this object src)
    {
        return (TDest)AutoMapper.Mapper.Map(src, src.GetType(), typeof(TDest));
    }
}

In your code:

var model = customter.MapTo<CustomerViewModel>();

Now you don't need the superfluous Source type in your generic method.

Answer 2

Can't you just do this?:

public static class MapperHelper
{
    public static TDest ToViewModel<TSource, TDest>(this TSource cust)
    {
       return AutoMapper.Mapper.Map<TSource, TDest>(cust);
    }
}

Answer 3

You can't define extension methods in a generic class because there would be no way for you to specify the type parameters when you invoke it!

public static class MapperHelper<TSource, TTarget>
{
  public static TTarget ToViewModel(this TSource source)
  {
    ...
  }
}

...

// this is no problem if we invoke our method like a
// static method:-
var viewModel = MapperHelper<MyModel, MyViewModel>.ToViewModel(model);

// but if we use the extension method syntax then how
// does the compiler know what TSource and TTarget are?:-
var viewModel = model.ToViewModel();

You have to make the method generic instead:-

public static class MapperHelper
{
  public static TTarget ToViewModel<TSource, TTarget>(this TSource source)
  {
    ...
  }
}

...

var viewModel = model.ToViewModel<MyModel, MyViewModel>();