Automapper: Formatter for Interface Property - interface

Question

If I have some classes that implement the same interface, so they all contain the same property. Is there a way to add a formatter to these properties? I only found the possibility to add a formatter to a specific property type.
Here's some code that should clarify what I mean:
public interface ITaggable
{
IList<string> Tags { get; set; }
}
public class Post : ITaggable
{
public IList<string> Tags { get; set; }
public IList<string> Categories { get; set; }
...
}
public class Page : ITaggable
{
public IList<string> Tags { get; set; }
....
}
I'd like to map these to view models that look something like this:
public class PostViewModel
{
public string Tags { get; set; }
public IList<string> Categories { get; set; }
...
}
public class PageViewModel
{
public string Tags { get; set; }
...
}
If a Post has the tags "foo" and "bar", then the PostViewModel's Tags property should contain the string "foo, bar". Categories should remain an IList<string>.
I could accomplish this by creating a custom formatter and then add it on every mapping, like this:
protected override void Configure()
{
CreateMap<Post, PostViewModel>()
.ForMember(x => x.Tags, opt => opt.AddFormatter<TagsFormatter>());
CreateMap<Page, PageViewModel>()
.ForMember(x => x.Tags, opt => opt.AddFormatter<TagsFormatter>());
}
But I'd like to do something like this (this code doesn't work ;-)
protected override void Configure()
{
ForSourceType<ITaggable>()
.ForMember(x => x.Tags, opt => opt.AddFormatter<TagsFormatter>());
CreateMap<Post, PostViewModel>();
CreateMap<Page, PageViewModel>();
}

Answer 1

It's not exactly an answer to the question I was asking, but a solution to the problem:
I built a TagsFormatter that checks the name of the property for "Tags":
public class TagsFormatter : IValueFormatter
{
public string FormatValue(ResolutionContext context)
{
if (context.MemberName.Equals("Tags", StringComparison.InvariantCultureIgnoreCase))
{
var tags = context.SourceValue as IList<string>;
if (tags != null)
return String.Join(", ", tags);
}
return context.SourceValue.ToString();
}
}
In the configuration I can then register this Formatter for all mappings:
public class ViewModelProfile : AutoMapper.Profile
{
protected override void Configure()
{
ForSourceType<IList<string>>().AddFormatter<TagsFormatter>();
CreateMap<Post, PostViewModel>();
CreateMap<Page, PageViewModel>();
}
}