Add an attribute to another assembly's class

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.

what do you mean by attribute? Like [DataMember] (WCF) attribute? — Mark W
– Mark W, Commented Nov 29, 2011 at 19:58
You'll have to wait for .NET 4.5 and its CustomReflectionContext class. — Hans Passant
– Hans Passant, Commented Nov 29, 2011 at 20:11
@Hans Passant, if the third party tool uses the TypeDescriptor API this is already possible as explained in my answer. — João Angelo
– João Angelo, Commented Nov 29, 2011 at 20:13
If like me you're here because you want to specify a custom editor for a type in PropertyGrid, read this question instead stackoverflow.com/questions/849202/… — Colonel Panic
– Colonel Panic, Commented Sep 24, 2012 at 15:54

João Angelo · Accepted Answer · 2011-11-29 20:16:29Z

10

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.

edited Nov 29, 2011 at 20:16

answered Nov 29, 2011 at 19:58

João Angelo

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4 Comments

Johnny5 Over a year ago

Can you explain further about TypeDescriptor ?

João Angelo Over a year ago

I added sample code that can be used to extend a types attributes if the consumer relies on the TypeDescriptor API. You need to assert if the tool uses that API or not, you could do it with Reflector or just ask the vendor.

Johnny5 Over a year ago

I didn't think of that before asking, but is it the same as using the MetadataType attribute?

João Angelo Over a year ago

The use of the Display in the sample code was just for illustration purposes, it's applicable to other attributes.

Justin Niessner · Accepted Answer · 2011-11-29 19:58:08Z

1

No. It's not possible to add attributes to types from separate assemblies.

What you can do, though, is create your own type that wraps the third-party type. Since you have full control over your wrapper class, you can add the attributes there.

answered Nov 29, 2011 at 19:58

Justin Niessner

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Comments

Maess · Accepted Answer · 2011-11-29 19:59:38Z

0

What about:

public class Foo
{
    public virtual string Bar
}

public class MyFoo : Foo
{
     [yourcustomattribute]
     public overrides string Bar
}

answered Nov 29, 2011 at 19:59

Maess

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3 Comments

Johnny5 Over a year ago

Foo belongs to my domain. The custom attribute belongs to the third party tool.

jason Over a year ago

Foo could be sealed. Better to proxy.

Jesper Over a year ago

Class Foo might also not have its' field declared as Virtual

Jordão · Accepted Answer · 2011-12-02 00:34:16Z

0

What I think you need is some kind of adapter layer that will not let that infrastructure dependency leak into your domain logic. Maybe you can create an adapter class that will be like a data transfer object to the other technology. This class lives in an integration assembly that has a dependency on the 3rd party library:

public class FooDTO {
   [TheirAttribute]
   public string Bar { get; set; }
}

You can then use something like AutoMapper to ease the pain of changing representations.

The ideal solution, though, is if the 3rd party library supported additional ways to provide metadata about their operations. Maybe you can ask them for this feature.

edited Dec 2, 2011 at 0:34

answered Nov 29, 2011 at 20:25

Jordão

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