Messing with C# types. Making type1.FullName==type2.FullName, but not type1==type2!

Please find the updated version of this post here: https://piotr.westfalewicz.com/blog/2016/07/the-performance-of-setting-t-vs.-list-by-index/

Given the following method:

private static void CompareTypes(Type type1, Type type2)
{
    Console.WriteLine($"type1.FullName = {type1.FullName}");
    Console.WriteLine($"type2.FullName = {type2.FullName}");
    Console.WriteLine($"type1.FullName {(type1.FullName == type2.FullName ? '=' : '!')}= type2.Fullname");
    Console.WriteLine($"type1.AssemblyQualifiedName = {type1.AssemblyQualifiedName}");
    Console.WriteLine($"type2.AssemblyQualifiedName = {type2.AssemblyQualifiedName}");
    Console.WriteLine($"type1.AssemblyQualifiedName {(type1.AssemblyQualifiedName == type2.AssemblyQualifiedName ? '=' : '!')}= type2.AssemblyQualifiedName");
    Console.WriteLine($"type1.GUID = {type1.GUID}");
    Console.WriteLine($"type2.GUID = {type2.GUID}");
    Console.WriteLine($"type1.GUID {(type1.GUID == type2.GUID ? '=' : '!')}= type2.GUID");

    Console.WriteLine("o1 = Activator.CreateInstance(type1)");
    Console.WriteLine("o2 = Activator.CreateInstance(type2)");
    var o1 = Activator.CreateInstance(type1);
    var o2 = Activator.CreateInstance(type2);
    Console.WriteLine($"o1 == {o1}");
    Console.WriteLine($"o2 == {o2}");

    Console.WriteLine();
    Console.WriteLine($"but... type1 {(type1 == type2 ? '=' : '!')}= type2");
}

Is it possible to get the following result?

type1.FullName = MyLibrary.MyPrecious
type2.FullName = MyLibrary.MyPrecious
type1.FullName == type2.Fullname
type1.AssemblyQualifiedName = MyLibrary.MyPrecious, MyLibrary, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
type2.AssemblyQualifiedName = MyLibrary.MyPrecious, MyLibrary, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
type1.AssemblyQualifiedName == type2.AssemblyQualifiedName
type1.GUID = cacf8c0d-b903-3da6-808f-024a3070ab9d
type2.GUID = cacf8c0d-b903-3da6-808f-024a3070ab9d
type1.GUID == type2.GUID
o1 = Activator.CreateInstance(type1)
o2 = Activator.CreateInstance(type2)
o1 == MyLibrary.MyPrecious
o2 == MyLibrary.MyPrecious

but... type1 != type2

As it turns out, it is. Doing such a hell is relatively easy:

private static Assembly LoadAssemblyByName(string name)
{
    var myPreciousAssemblyLocation = Path.Combine(Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location), name);
    using (var fs = new FileStream(myPreciousAssemblyLocation, FileMode.Open, FileAccess.Read))
    {
        var data = new byte[fs.Length];
        fs.Read(data, 0, data.Length);
        fs.Close();
        var assembly = Assembly.Load(data);
        return assembly;
    }
}

static void Main()
{
    var type1 = typeof (MyPrecious);
    var myLibraryAssembly = LoadAssemblyByName("MyLibrary.dll");
    var type2 = myLibraryAssembly.GetType("MyLibrary.MyPrecious", true);

    CompareTypes(type1, type2);
}

The code above compares type1 from referenced project to type2 from the same assembly, but loaded again through Assembly.Load(byte[]). That makes the library loaded twice in the AppDomain. Now when a call to AppDomain.CurrentDomain.GetAssemblies() is made, the assemblies are:

AppDomain.CurrentDomain.GetAssemblies:
mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
Microsoft.VisualStudio.HostingProcess.Utilities, Version=14.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
System.Drawing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
Microsoft.VisualStudio.HostingProcess.Utilities.Sync, Version=14.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
Microsoft.VisualStudio.Debugger.Runtime, Version=14.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
vshost32, Version=14.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
ConsoleApplication1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
MyLibrary, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
MyLibrary, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
Accessibility, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3

Even in such a small, console application it is quite confusing. So, let's make it more confusing... What's the output of the following code?

var myPreciousAssemblyLocation = Path.Combine(Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location), "MyLibrary.dll");
var myLibraryAssemblyLoadFrom = Assembly.LoadFrom(myPreciousAssemblyLocation);
var type3 = myLibraryAssemblyLoadFrom.GetType("MyLibrary.MyPrecious", true);
CompareTypes(type1, type3);

Now, surprisingly, its:

type1.FullName = MyLibrary.MyPrecious
type2.FullName = MyLibrary.MyPrecious
type1.FullName == type2.Fullname
type1.AssemblyQualifiedName = MyLibrary.MyPrecious, MyLibrary, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
type2.AssemblyQualifiedName = MyLibrary.MyPrecious, MyLibrary, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
type1.AssemblyQualifiedName == type2.AssemblyQualifiedName
type1.GUID = cacf8c0d-b903-3da6-808f-024a3070ab9d
type2.GUID = cacf8c0d-b903-3da6-808f-024a3070ab9d
type1.GUID == type2.GUID
o1 = Activator.CreateInstance(type1)
o2 = Activator.CreateInstance(type2)
o1 == MyLibrary.MyPrecious
o2 == MyLibrary.MyPrecious

but... type1 == type2

Hint

A nice hint is shown, when you try to execute the following code:

var o1 = Activator.CreateInstance(type1);
var o2 = Activator.CreateInstance(type2);
MyPrecious p1 = (MyPrecious) o1;
try
{
    MyPrecious p2 = (MyPrecious)o2;
}
catch (Exception e)
{
    Console.WriteLine(e);
}

System.InvalidCastException: [A]MyLibrary.MyPrecious cannot be cast to [B]MyLibrary.MyPrecious. Type A originates from 'MyLibrary, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' in the context 'LoadNeither' in a byte array. Type B originates from 'MyLibrary, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' in the context 'Default' at location 'c:\users\pwdev\documents\visual studio 2015\Projects\ConsoleApplication1\ConsoleApplication1\bin\Debug\MyLibrary.dll'. at ConsoleApplication1.Program.Main() in c:\users\pwdev\documents\visual studio 2015\Projects\ConsoleApplication1\ConsoleApplication1\Program.cs:line 49

Explanation

Yes, it's all about the load contexts. There are three different assembly load contexts: Load, LoadFrom, Neither. Usually there is no need to load the same library twice and get the strange behavior written above, but sometimes there might be. There are many advantages and disadvantages of using different Assembly.Load(From/File) methods. Take a look: Choosing a Binding Context. Furthermore, consider what's happening to assembly dependencies when you load an assembly. There are best practices described on MDSN for loading assemblies: Best Practices for Assembly Loading. I have to say, in my whole career I've been loading assemblies by hand twice, and from time perspective, both two cases were wrong.

TypeHandle

Instead of comparing the types in the examples above by == operator, there is a possibility to compare them by the TypeHandle:

TypeHandle encapsulates a pointer to an internal data structure that represents the type. This handle is unique during the process lifetime. The handle is valid only in the application domain in which it was obtained.

Source: MDSN. Well, I can't think of an interesting usage for the TypeHandles for now, but it's good to know.