AutoMapper using the wrong constructor

Question

Today I upgraded a fully functioning application using AutoMapper v1.1 to now use AutoMapper v2.1 and I am coming across some issues that I never encountered using the previous version.

Here is an example of my code mapping back from Dto to Domain object

public class TypeOne {    public TypeOne()    {    }     public TypeOne(TypeTwo two)    {       //throw ex if two is null    }     public TypeOne(TypeTwo two, TypeThree three)    {       //throw ex if two or three are null    }     public TypeTwo Two {get; private set;}     public TypeThree Three {get; private set;} }  public class TypeOneDto {    public TypeOneDto()    {    }     public TypeTwoDto Two {get; set;}     public TypeThreeDto Three {get; set;} } 

...

Mapper.CreateMap<TypeThreeDto, TypeThree>(); Mapper.CreateMap<TypeTwoDto, TypeTwo>(); Mapper.CreateMap<TypeOneDto, TypeOne>();  var typeOne = Mapper.Map<TypeOne>(typeOneDto); 

However the first problem I encountered with v2.1 was that AutoMapper was trying to use the constructor with 2 args when one of the args was null and should be using the 1 arg constructor.

I then tried to use

Mapper.CreateMap<TypeOneDto, TypeOne>().ConstructUsing(x => new TypeOne()); 

But I kept getting an 'Ambiguous Invocation' error that I couldn't resolve.

I then tried

Mapper.CreateMap<TypeOneDto, TypeOne>().ConvertUsing(x => new TypeOne()); 

and that did successfully create the TypeOne object using the parameterless constructor but then it failed to set the private setter properties.

I have looked for help on the AutoMapper website and downloaded the source code to have a good look but didn't get far with the little documentation about and there were not many unit tests for ConstructUsing.

Is there anything obvious I am missing that I should change with v2.1? I am surprised that it has changed so much from v1.1.

Answer 1

You just need to add explicit cast to

Func<ResolutionContext, TypeOne> 

Here is the code:

Mapper.CreateMap<TypeOneDto, TypeOne>().ConstructUsing(             (Func<ResolutionContext, TypeOne>) (r => new TypeOne())); 

Current version of AutoMapper works as described below:

1. Sorts destination type constructors by parameter count

   destTypeInfo.GetConstructors().OrderByDescending(ci => ci.GetParameters().Length); 

2. Takes first constructor which parameters match source properties (without any check for null value). In your case it is constructor with two parameters.

Answer 2

Here's an extension method...

    public static void CreateMapWithDefaultConstructor<T, TU>(this Profile profile)         where TU : class, new()     {         profile.CreateMap<T, TU>().ConstructUsing(source => new TU());     }