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);
    }
}