C# - how enumerate all classes with custom class attribute?

Question

Question based on MSDN example: http://msdn.microsoft.com/en-us/library/aa288454(VS.71).aspx

Let's say we have some C# classes with HelpAttribute in standalone desktop application. Is it possible to enumerate all classes with such attribute? Does it make sense to recognize classes this way? Custom attribute would be used to list possible menu options, selecting item will bring to screen instance of such class. Number of classes/items will grow slowly, but this way we can avoid enumerating them all elsewhere, I think.

Answer 1

Yes, absolutely. Using Reflection:

static IEnumerable<Type> GetTypesWithHelpAttribute(Assembly assembly) {
    foreach(Type type in assembly.GetTypes()) {
        if (type.GetCustomAttributes(typeof(HelpAttribute), true).Length > 0) {
            yield return type;
        }
    }
}

Answer 2

Well, you would have to enumerate through all the classes in all the assemblies that are loaded into the current app domain. To do that, you would call the GetAssemblies method on the AppDomain instance for the current app domain.

From there, you would call GetExportedTypes (if you only want public types) or GetTypes on each Assembly to get the types that are contained in the assembly.

Then, you would call the GetCustomAttributes method on each Type instance, passing the type of the attribute you wish to find.

You can use LINQ to simplify this for you:

var typesWithMyAttribute =
    from a in AppDomain.CurrentDomain.GetAssemblies()
    from t in a.GetTypes()
    let attributes = t.GetCustomAttributes(typeof(HelpAttribute), true)
    where attributes != null && attributes.Length > 0
    select new { Type = t, Attributes = attributes.Cast<HelpAttribute>() };

The above query will get you each type with your attribute applied to it, along with the instance of the attribute(s) assigned to it.

Answer 3

As already stated, reflection is the way to go. If you are going to call this frequently, I highly suggest caching the results, as reflection, especially enumerating through every class, can be quite slow.

This is a snippet of my code that runs through all the types in all loaded assemblies:

   foreach (Assembly assembly in AppDomain.CurrentDomain.GetAssemblies()) // this is making the assumption that all assemblies we need are already loaded.
   {
    foreach (Type type in assembly.GetTypes())
    {
     var attribs = type.GetCustomAttributes(typeof(MyCustomAttribute), false);
     if (attribs != null && attribs.Length > 0)
     {
      // add to a cache.