Validation of .NET assemblies in C#?

Question

How do you check if a assembly loaded is a valid .NET assembly? I currently have this code but unmanaged DLL's throw a BadImageFormatException.

 string[] filepaths = Directory.GetFiles(Directory.GetCurrentDirectory(), "*.dll", SearchOption.AllDirectories);
        List<Type> potentialEffects = new List<Type>();
        foreach (string filepath in filepaths)
        {
            Assembly a = Assembly.LoadFile(filepath);
            potentialEffects.AddRange(a.GetTypes());
        }

Answer 1

You could simply catch this exception?

Also see this article here: http://blogs.msdn.com/suzcook/archive/2004/03/17/determining-whether-a-file-is-an-assembly.aspx Just catch the exception. Checking would be a lot more costly than just assuming that a file is an valid assembly.

Answer 2

The exception is your best way of validating the assembly.

Remember, it's not just the manager/unmanaged checks that are going on when you load an assembly. There may be signature checks, platform checks (32bit vs 64bit). These things can also cause exceptions to be thrown.

You're better off letting the framework do it's job.

If you're loading managed code that forms part of an add-in system (hence you're looking at all the dlls in a folder) then I would suggest you pick a naming convention, or use a seperate addin folder where you don't expect to find unmanaged code.

Answer 3

An unmanaged DLL is not a valid .NET assembly. Try to load the "assembly" and catch exceptions on failure. If an exception is not thrown, it's okay to go.

Answer 4

If you want to check it from C# you can add a Try Catch.

Or you can use the peverify.exe tool provided with the .net framework to know if a dll is a valid assembly.

Answer 5

I've seen BadImageFormatException thrown when trying to load x86 asms from x64 asms and vice versa.

http://blogs.msdn.com/arvindsh/archive/2009/06/21/tip-of-the-day-an-attempt-was-made-to-load-a-program-with-an-incorrect-format-net-p-invoke-issue.aspx

Answer 6

Take a look at the Activator class.

Wrapping this technique in a try/catch block can help you trap all manner of invalid types, not just .NET assemblies but many other runtime violations as well.

Answer 7

I have this piece of code to check whether a file is a PE file and a managed assembly. It's quite easy to use:

var files = Directory.GetFiles (
    Directory.GetCurrentDirectory (),
    "*.dll", SearchOption.AllDirectories);

var potentialEffects = new List<Type> ();

foreach (var file in files) {
    if (!Image.IsAssembly (file))
     continue;

    var assembly = Assembly.LoadFile (filepath);
    potentialEffects.AddRange (assembly.GetTypes ());
}

For what it's worth, a little benchmark shows that it's twice as fast on my machine compared to the exception Assembly.LoadFrom will trigger. On the other hand, it will lose a bit of time in the valid case. So it's a matter of timing the average case.

Answer 8

See MSDN: How to: Determine If a File Is an Assembly (C# Programming Guide)