Custom Mapping with AutoMapper

Question

I have two very simple objects:

public class CategoryDto
{
    public string Id { get; set; }

    public string MyValueProperty { get; set; }
}

public class Category
{
    public string Id { get; set; }

    [MapTo("MyValueProperty")]
    public string Key { get; set; }
}

When mapping a Category to a CategoryDto with AutoMapper, I would like the following behavior:

The properties should be mapped as usual, except for those that have the MapTo attribute. In this case, I have to read the value of the Attribute to find the target property. The value of the source property is used to find the value to inject in the destination property (with the help of a dictionary). An example is always better that 1000 words...

Example:

Dictionary<string, string> keys = 
    new Dictionary<string, string> { { "MyKey", "MyValue" } };

Category category = new Category();
category.Id = "3";
category.Key = "MyKey";

CategoryDto result = Map<Category, CategoryDto>(category);
result.Id               // Expected : "3"
result.MyValueProperty  // Expected : "MyValue"

The Key property is mapped to the MyValueProperty (via the MapTo Attribute), and the assigned value is "MyValue", because the source property value is "MyKey" which is mapped (via dictionary) to "MyValue".

Is this possible using AutoMapper ? I need of course a solution that works on every object, not just on Category/CategoryDto.

Answer 1

I finally (after so many hours !!!!) found a solution. I share this with the community; hopefully it will help someone else...

Edit: Note that it's now much simpler (AutoMapper 5.0+), you can do like I answered in this post: How to make AutoMapper truncate strings according to MaxLength attribute?

public static class Extensions
{
    public static IMappingExpression<TSource, TDestination> MapTo<TSource, TDestination>(this IMappingExpression<TSource, TDestination> expression)
    {
        Type sourceType = typeof(TSource);
        Type destinationType = typeof(TDestination);

        TypeMap existingMaps = Mapper.GetAllTypeMaps().First(b => b.SourceType == sourceType && b.DestinationType == destinationType);
        string[] missingMappings = existingMaps.GetUnmappedPropertyNames();

        if (missingMappings.Any())
        {
            PropertyInfo[] sourceProperties = sourceType.GetProperties();
            foreach (string property in missingMappings)
            {
                foreach (PropertyInfo propertyInfo in sourceProperties)
                {
                    MapToAttribute attr = propertyInfo.GetCustomAttribute<MapToAttribute>();
                    if (attr != null && attr.Name == property)
                    {
                        expression.ForMember(property, opt => opt.ResolveUsing(new MyValueResolve(propertyInfo)));
                    }
                }
            }
        }

        return expression;
    }
}

public class MyValueResolve : IValueResolver
{
    private readonly PropertyInfo pInfo = null;

    public MyValueResolve(PropertyInfo pInfo)
    {
        this.pInfo = pInfo;
    }

    public ResolutionResult Resolve(ResolutionResult source)
    {
        string key = pInfo.GetValue(source.Value) as string;
        string value = dictonary[key];
        return source.New(value);
    }
}

Answer 2

This should be fairly straightforward using an implementation of IValueResolver and the ResolveUsing() method. You basically just need to have a constructor for the resolver that takes in the property info (or if you wanna be fancy that takes in a lambda expression and resolves the property info similar to How to get the PropertyInfo of a specific property?. Though I haven't tested it myself I imagine the following would work:

public class PropertyBasedResolver : IValueResolver
{
     public PropertyInfo Property { get; set; }

     public PropertyBasedResolver(PropertyInfo property)
     {
          this.Property = property;
     }

     public ResolutionResult Resolve(ResolutionResult source)
     {
           var result = GetValueFromKey(property, source.Value); // gets from some static cache or dictionary elsewhere in your code by reading the prop info and then using that to look up the value based on the key as appropriate
           return source.New(result)
     }
}

Then to set up that mapping you need to do:

AutoMapper.Mapper.CreateMap<Category, CategoryDto>()
    .ForMember(
         dest => dest.Value, 
         opt => opt.ResolveUsing(
              src => 
                   new PropertyBasedResolver(typeof(Category.Key) as PropertyInfo).Resolve(src)));

Of course that is a pretty gross lamda and I would suggest that you clean it up by having your property resolver determine the property on the source object it should look at based on the attribute/property info so you can just pass a clean new PropertyBasedResolver(property) into the ResolveUsing() but hopefully this explains enough to put you on the right track.

Answer 3

Lets assume I have the following classes

public class foo
{
  public string Value;
}
public class bar
{
    public string Value1;
    public string Value2;
}

You can pass a lambda to ResolveUsing:

.ForMember(f => f.Value, o => o.ResolveUsing(b =>
{
    if (b.Value1.StartsWith("A"));)
    {
        return b.Value1;
    }
    return b.Value2;
}


 ));