Get All Types in an Assembly

asp.net mvc, code comments edit

Sometimes, you need to scan all the types in an assembly for a certain reason. For example, ASP.NET MVC does this to look for potential controllers.

One naïve implementation is to simply call Assembly.GetTypes() and hope for the best. But there’s a problem with this. As Suzanne Cook points out,

If a type can’t be loaded for some reason during a call to Module.GetTypes(), ReflectionTypeLoadException will be thrown. Assembly.GetTypes() also throws this because it calls Module.GetTypes().

In other words, if any type can’t be loaded, the entire method call blows up and you get zilch.

There’s multiple reason why a type can’t be loaded. Here’s one example:

public class Foo : Bar // Bar defined in another unavailable assembly
{
}

The class Foo derives from a class Bar, but Bar is defined in another assembly. Here’s a non-exhaustive list of reasons why loading Foo might fail:

  • The assembly containing Bar does not exist on disk.
  • The current user does not have permission to load the assembly containing Bar.
  • The assembly containing Bar is corrupted and not a valid assembly.

Once again, for more details check out Suzanne’s blog post on Debugging Assembly Loading Failures.

Solution

As you might expect, being able to get a list of types, even if you don’t plan on instantiating instances of them, is a common and important task. Fortunately, the ReflectionTypeLoadException thrown when a type can’t be loaded contains all the information you need. Here’s an example of ASP.NET MVC taking advantage of this within the internal TypeCacheUtil class (there’s a lot of other great code nuggets if you look around the source code)

Type[] typesInAsm;
try
{
    typesInAsm = assembly.GetTypes();
}
catch (ReflectionTypeLoadException ex)
{
    typesInAsm = ex.Types;
}

This would be more useful as a generic extension method. Well the estimable Jon Skeet has you covered in this StackOverflow answer (slightly edited to add in parameter validation):

public static IEnumerable<Type> GetLoadableTypes(this Assembly assembly)
{
    if (assembly == null) throw new ArgumentNullException("assembly");
    try
    {
        return assembly.GetTypes();
    }
    catch (ReflectionTypeLoadException e)
    {
        return e.Types.Where(t => t != null);
    }
}

I’ve found this code to be extremely useful many times.

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