AutoMapper 5 Custom Value Resolver “Cannot convert expression type X to return type Y”

Question

I upgraded from an old version of AutoMapper and converted my custom resolvers, but I'm having a hard time.

public class ProductMappingProfile : Profile
{
    public ProductMappingProfile()
    {
        CreateMap<Product, ProductViewModel>()
            .ForMember(
                dest => dest.Model,
                opt => opt.ResolveUsing<ModelNameResolver>(src => src.ModelId));
        // won't compile
    }
}

Where Product has a int? ModelId int? ModelId property and ProductViewModel has a string Name property.

Custom Resolver

public class ModelNameResolver : IValueResolver<short?, string, string>
{
    private readonly InventoryService _inventoryService;
    public ModelNameResolver(InventoryService inventoryService)
    {
        _inventoryService = inventoryService;
    }

    public string Resolve(short? source, string destination, string destMember, ResolutionContext context)
    {
        if (!source.HasValue)
            return "n/a";

        return _inventoryService.GetModel(source.Value)
            .Name;
    }
}

Compilation Error:

The type 'MyNamespace.Web.Resolvers.ModelCodeResolver' cannot be used as type parameter 'TValueResolver' in the generic type or method `'AutoMapper.IMemberConfigurationExpression<TSource,TDestination,TMember>.ResolveUsing<TValueResolver>()'`.

There is no implicit reference conversion from `'MyNamespace.Web.Resolvers.ModelCodeResolver'` to `'AutoMapper.IValueResolver<Data.Models.Product,Web.ViewModels.ProductViewModel,string>'`.

What am I doing wrong? I suspect I am misunderstanding the new custom resolver interface.

Answer 1

IValueResolver interface should be parametrized with source object type , destination object type and type of destination member (that should be type of Resolve method result). In your case parameters should be

 IValueResolver<Product, ProductViewModel, string>

But you are creating resolver which is parametrized with

 IValueResolver<short?, string, string>

short? is not your source object type

string is not your destination object type

You should use something like:

public class ModelNameResolver : IValueResolver<Product, ProductViewModel, string>
{
    private readonly InventoryService _inventoryService;
    public ModelNameResolver(InventoryService inventoryService)
    {
        _inventoryService = inventoryService;
    }

    public string Resolve(Product source, ProductViewModel destination,
       string destMember, ResolutionContext context)
    {
        var modelId = source.ModelId;
        if (!modelId.HasValue)
            return "n/a";

        return _inventoryService.GetModel(modelId.Value).Name;
    }
}

And your mapping should look like

var inventoryService = new InventoryService();
var modelNameResolver = new ModelNameResolver(inventoryService);

Mapper.Initialize(c =>
{
    c.CreateMap<Product, ProductViewModel>()
        .ForMember(dest => dest.Model, opt => opt.ResolveUsing(modelNameResolver));
});

Of course, you can ask your IoC container for instance of model name resolver.

---

UPDATE: If you want resolver reusable between different source and destination data types, then you have two options:

If your resolver can have parameterless constructor, then you can use IMemberValueResolver :

public class ModelNameResolver : IMemberValueResolver<object, object, int?, string>
{
    // create or assign _inventoryService
    // also note objects as source and destination
    public string Resolve(object source, object destination,
       int? sourceMember, string destMember,
       ResolutionContext context)
    {
         if (!sourceMember.HasValue)
            return "n/a";

         return _inventoryService.GetModel(sourceMember.Value).Name;
    }
}

Usage

.ForMember(dest => dest.Model, 
           opt => opt.ResolveUsing<ModelNameResolver, int?>(src => src.ModelId)

Second option - do not use resolver. Just use MapFrom with your custom class which does mapping:

.ForMember(dest => dest.Model, 
           opt => opt.MapFrom(src => someClass.GetModelName(src.ModelId)));     

And someSlass should contain method for getting model name by id

public string GetModelName(int? modelId)
{
    if (!modelId.HasValue)
        return "n/a";

    return _inventoryService.GetModel(modelId.Value).Name;
}