Get all derived types of a type

I'm pretty sure the method you suggested is going to be the easier way to find all derived types. Parent classes don't store any information about what their sub-classes are (it would be quite silly if they did), which means there's no avoiding a search through all the types here.

Only recommendation is to use the Type.IsSubclassOf method instead of Type.IsAssignable in order to check whether a particular type is derived from another. Still, perhaps there is a reason you need to use Type.IsAssignable (it works with interfaces, for example).

The only optimization you can squeeze out of this is to use Assembly.GetExportedTypes() to retrieve only publicly visible types if that's the case. Other than that, there's no way to speed things up. LINQ may help with readability side of things, but not performance-wise.

You can do some short-circuiting to avoid unnecessary calls to IsAssignableFrom which is, according to Reflector, quite expensive one, by first testing whether the type in question is of required "class". That is, you're searching for classes only, there's no point in testing enums or arrays for "assignability".

I think there is no better or direct way.

Better: Use IsSubclassOf instead of IsAssignable.

If you're just interested in browsing, then .NET Reflector has the ability to do this. However, it isn't something that's really feasible. Would you want all types that are in the currently loaded assemblies? Assemblies referenced by the executing assembly? There are many different ways to obtain a list of Types, and writing something that would account for (and provide options for) would be a pretty big cost with relatively low benefit.

What are you trying to do? There's likely a better (or at least more efficient) way to accomplish it.

I ended up using the code that the top answer gave. Only thing is I wanted the code to be more readable so I wrote basically the same thing but like this instead:

var derived_types = new List<Type>();
foreach (var domain_assembly in AppDomain.CurrentDomain.GetAssemblies())
{
  var assembly_types = domain_assembly.GetTypes()
    .Where(type => type.IsSubclassOf(typeof(MyType)) && !type.IsAbstract);

  derived_types.AddRange(assembly_types);
}

In my case I used type.IsSubClassOf(MyType) because I only wanted derived types excluding the base class like mentioned above. I also needed the derived types not to be abstract (!type.IsAbstract) so I am also excluding derived abstract classes.

Just create a static dictionary of derived types on start and do lookup with that. Eg. public static Dictionay<Type, Type[]> DerivedTypes { get;set; } where Type is any type you want to include in the search and Type[] is a list of derived types. Fill the dictionary up when app starts and use it during the whole life of the app.