Question

Is it somehow possible to extend a type, wich is defined in another assembly, to add an attribute on one of its properties?

Exemple I have in assembly FooBar:

public class Foo
{
   public string Bar { get; set; }
}

But in my UI assembly, I want to pass this type to a third party tool, and for this third party tool to work correctly I need the Bar property to have a specific attribute. This attribute is defined in the third party assembly, and I don’t want a reference to this assembly in my FooBar assembly, since FooBar contains my domain an this is a UI tool.

Answer 1

You can’t, if the thirdy-party tool uses standard reflection to get the attributes for your type.

You can, if the third-party tool uses the TypeDescriptor API to get the attributes for your type.

Sample code for the type descriptor case:

public class Foo
{
    public string Bar { get; set; }
}

class FooMetadata
{
    [Display(Name = "Bar")]
    public string Bar { get; set; }
}

static void Main(string[] args)
{
    PropertyDescriptorCollection properties;

    AssociatedMetadataTypeTypeDescriptionProvider typeDescriptionProvider;

    properties = TypeDescriptor.GetProperties(typeof(Foo));
    Console.WriteLine(properties[0].Attributes.Count); // Prints X

    typeDescriptionProvider = new AssociatedMetadataTypeTypeDescriptionProvider(
        typeof(Foo),
        typeof(FooMetadata));

    TypeDescriptor.AddProviderTransparent(typeDescriptionProvider, typeof(Foo));

    properties = TypeDescriptor.GetProperties(typeof(Foo));
    Console.WriteLine(properties[0].Attributes.Count); // Prints X+1
}

If you run this code you’ll see that last console write prints plus one attribute because the Display attribute is now also being considered.