Question

How can I determine by reflection if the type of an object is defined by a class in my own assembly or by the .NET Framework?

I dont want to supply the name of my own assembly in code, because it should work with any assembly and namespace.

Answer 1

Where would third-party types come in? You might want to differentiate between types which claim to be provided by Microsoft and types which don’t.

using System;
using System.Linq;
using System.Reflection;

class Test
{
    static void Main()
    {
        Console.WriteLine(IsMicrosoftType(typeof(string)));
        Console.WriteLine(IsMicrosoftType(typeof(Test)));
    }

    static bool IsMicrosoftType(Type type)
    {
        object[] attrs = type.Assembly.GetCustomAttributes
            (typeof(AssemblyCompanyAttribute), false);

        return attrs.OfType<AssemblyCompanyAttribute>()
                    .Any(attr => attr.Company == "Microsoft Corporation");
    }
}

Of course, any type could claim to be a Microsoft one given this scheme, but if you’re actually only going to call it on your own types and framework ones, I suspect this should work fine.

Alternatively, you could use the assembly’s public key token. This is likely to be harder to fake. It relies on Microsoft using a common public key for all their assemblies, which they don’t (according to Mehrdad’s comment below). However, you could easily adapt this solution for a set of accepted “this is from Microsoft” public keys. Perhaps combine the two approaches somehow and report any differences for further inspection…

static bool IsMicrosoftType(Type type)
{
    AssemblyName name = type.Assembly.GetName();
    byte[] publicKeyToken = name.GetPublicKeyToken();

    return publicKeyToken != null
        && publicKeyToken.Length == 8
        && publicKeyToken[0] == 0xb7
        && publicKeyToken[1] == 0x7a
        && publicKeyToken[2] == 0x5c
        && publicKeyToken[3] == 0x56
        && publicKeyToken[4] == 0x19
        && publicKeyToken[5] == 0x34
        && publicKeyToken[6] == 0xe0
        && publicKeyToken[7] == 0x89;
}