Automapper Mapping base classes with inheritence

Question

I'm having some trouble mapping my parent class using automapper. Given the following classes, I have created a mapping profile.

Mapping classes:

public class SourceClass
{
    public int SourceProperty1 { get; set; }
    public int SourceProperty2 { get; set; }
    public string SourceProperty3 { get; set; }
    public string SourceProperty4 { get; set; }
}

public class TargetBaseClass
{
    public int TargetProperty1 { get; set; }
    public int TargetProperty2 { get; set; }
}

public class TargetClass1: TargetBaseClass
{
    public string TargetProperty3 { get; set; }
}

public class TargetClass2: TargetBaseClass
{
    public string TargetProperty4 { get; set; }
}

Map:

public class MappingProfile: Profile
{
    protected override void Configure()
    {
        CreateMap<SourceClass, TargetBaseClass>()
            .Include<SourceClass, TargetClass1>()
            .Include<SourceClass, TargetClass2>()
            .ForMember(dst => dst.TargetProperty1, opt => opt.MapFrom(src => src.SourceProperty1))
            .ForMember(dst => dst.TargetProperty2, opt => opt.MapFrom(src => src.SourceProperty2));
        CreateMap<SourceClass, TargetClass1>()
            .ForMember(dst => dst.TargetProperty3, opt => opt.MapFrom(src => src.SourceProperty3));
        CreateMap<SourceClass, TargetClass2>()
            .ForMember(dst => dst.TargetProperty4, opt => opt.MapFrom(src => src.SourceProperty4));
    }
}

And finally my program:

static void Main(string[] args)
{
    Mapper.Initialize(x => x.AddProfile<MappingProfile>());
    var sourceClass = new SourceClass
        {
            SourceProperty1 = 1,
            SourceProperty2 = 2,
            SourceProperty3 = "3",
            SourceProperty4 = "4"
        };
    var targetBaseClass = Mapper.Map<TargetBaseClass>(sourceClass);
    var targetClass1 = Mapper.Map<TargetClass1>(sourceClass);
    var targetClass2 = Mapper.Map<TargetClass2>(sourceClass);

    Console.WriteLine("TargetBaseClass: {0} {1}", targetBaseClass.TargetProperty1,
                      targetBaseClass.TargetProperty2); //1 2
    Console.WriteLine("TargetClass1: {0} {1} {2}", targetClass1.TargetProperty1, targetClass1.TargetProperty2,
                      targetClass1.TargetProperty3);//0 0 3 ???
    Console.WriteLine("TargetClass2: {0} {1} {2}", targetClass2.TargetProperty1, targetClass2.TargetProperty2,
                      targetClass2.TargetProperty4);//1 2 4
}

The problem is, when I try mapping to the derived classes, my parent class' properties won't get mapped in case of TargetClass1 but it will for TargetClass2. Can anyone explain to me what it is I'm doing wrong, and why these 2 maps are acting differently? (Does the order in which I Include matter?)

Edit: On close inspection, the order does indeed matter. However, I still don't know why only the second Include would count.

Edit2: Based on the comment by @GruffBunny, I think I could "fix" this by using an Extension Method. However, I don't really get why they made it this way. Looking at the code for AutoMapper.TypeMap, I can clearly see this:

public void IncludeDerivedTypes(Type derivedSourceType, Type derivedDestinationType)
{
    _includedDerivedTypes[derivedSourceType] = derivedDestinationType;
}

Obviously this means you can only specify one destination per included source type. I don't see anything, that would prevent them from supporting more than one destinationtype, however.

Take look at this [question](http://stackoverflow.com/questions/13994081/automapper-inheritance-mapping-not-working-same-source-multiple-destination) which covers your scenario. — Gruff Bunny, Oct 17 '13 at 11:49
@GruffBunny Thanks, I had found something similar in [this](http://stackoverflow.com/questions/14705064/mapping-one-source-class-to-multiple-derived-classes-with-automapper) question. However, I'm not entirely sure whether this is a bug, or by design. — Kippie, Oct 17 '13 at 12:12

score 5 · Accepted Answer · edited May 23 '17 at 11:51

You can take a look at this custom extension method. The source code can also be found on Github here.

The extension method has tree downsides that I can think of right now. The first is that the Mapper.AssertConfigurationIsValid() will fail because it will not find the property mappings defined in the base map. The solution to this would be to ignore any of the provided member mappings defined in the base map.

The second is that the extension method depends on the the static Mapper class. If that's the way you use AutoMapper then there is no problem. If you have multiple mapping engines and/or write code against interfaces of AutoMapper you can't use this extension method. To support both cases we need to add two optional arguments: IConfigurationProvider and IMappingEngine.

I try to avoid the use of the static Mapper class and inject the interfaces where I need them through an IoC container.

The third is that the extension method doesn't return the IMappingExpression<TSource, TDestination> and prohibits overriding the base member mappings. To fix this we return the IMappingExpression<TSource, TDestination> and remove the conditions on all members.

This results in the following code:

public enum WithBaseFor
{
    Source,
    Destination,
    Both
}

public static class AutoMapperExtensions
{
    public static IMappingExpression<TSource, TDestination> InheritMappingFromBaseType<TSource, TDestination>(
        this IMappingExpression<TSource, TDestination> mappingExpression,
        WithBaseFor baseFor = WithBaseFor.Both,
        IMappingEngine mappingEngine = null,
        IConfigurationProvider configurationProvider = null)
    {
        Type sourceType = typeof (TSource);
        Type destinationType = typeof (TDestination);

        Type sourceParentType = baseFor == WithBaseFor.Both || baseFor == WithBaseFor.Source
            ? sourceType.BaseType
            : sourceType;

        Type destinationParentType = baseFor == WithBaseFor.Both || baseFor == WithBaseFor.Destination
            ? destinationType.BaseType
            : destinationType;

        mappingExpression
            .BeforeMap((sourceObject, destObject) =>
            {
                if (mappingEngine != null)
                    mappingEngine.Map(sourceObject, destObject, sourceParentType, destinationParentType);
                else
                    Mapper.Map(sourceObject, destObject, sourceParentType, destinationParentType);
            });

        TypeMap baseTypeMap = configurationProvider != null
            ? configurationProvider.FindTypeMapFor(sourceParentType, destinationParentType)
            : Mapper.FindTypeMapFor(sourceParentType, destinationParentType);

        if (baseTypeMap == null)
        {
            throw new InvalidOperationException(
                string.Format("Missing map from {0} to {1}.", new object[]
                {
                    sourceParentType.Name,
                    destinationParentType.Name
                }));
        }

        foreach (PropertyMap propertyMap in baseTypeMap.GetPropertyMaps())
            mappingExpression.ForMember(propertyMap.DestinationProperty.Name, opt => opt.Ignore());

        return mappingExpression;
    }
}

Usage

CreateMap<SourceClass, TargetBaseClass>()
    .ForMember(dst => dst.TargetProperty1, opt => opt.MapFrom(src => src.SourceProperty1))
    .ForMember(dst => dst.TargetProperty2, opt => opt.MapFrom(src => src.SourceProperty2));

CreateMap<SourceClass, TargetClass1>()
    .ForMember(dst => dst.TargetProperty3, opt => opt.MapFrom(src => src.SourceProperty3))
    .InheritMappingFromBaseType(WithBaseFor.Destination)
    ;

CreateMap<SourceClass, TargetClass2>()
    .ForMember(dst => dst.TargetProperty4, opt => opt.MapFrom(src => src.SourceProperty4))
    .InheritMappingFromBaseType(WithBaseFor.Destination)
    ;

Maybe there are still some scenario's which aren't covered but it certainly fixes your problem and this way you don't need to write specific extension methods.

Nice and especially *reusable* sollution. This'll save me the trouble of writing a new extension class every time I need to do something along these lines. Code looks clean, though it took me a bit to fully understand. Thanks — Kippie, Oct 18 '13 at 22:08

score 3 · Answer 2 · edited May 23 '17 at 12:01

I ended up creating an extension method in the way described here and here.

public static class Extensions
{
    public static IMappingExpression<SourceClass, TDestination> MapBase<TDestination>(
        this IMappingExpression<Source, TDestination> mapping)
        where TDestination: TargetBaseClass
    {
        // all base class mappings goes here
        return mapping
            .ForMember(dst => dst.TargetProperty1, opt => opt.MapFrom(src => src.SourceProperty1))
            .ForMember(dst => dst.TargetProperty2, opt => opt.MapFrom(src => src.SourceProperty2));
    }
}

I'm still not sure why this line doesn't allow for multiple types to be specified inside a IDictionary<type, IEnumerable<type>> though. I'm sure the folks at AutoMapper have their reasons.

Automapper Mapping base classes with inheritence

2 Answers2