InvalidCastException for two Objects of the same type

I have this weird problem that I cannot handle myself. A class in the model of my mvp-project designed as singleton causes an InvalidCastException.

The source of error is found in this code line where the deserialised object is assigned to the instance variable of the class: engineObject = (ENGINE)xSerializer.Deserialize(str);. It occurs whenever I try to add one of my UserControls to a Form or to a different UC. All of my UCs have a special presenter that accesses the above mentioned instance variable of the singleton class.

This is what I get when trying to add a UC somewhere:

'System.TypeInitializationException: The type initializer for 'MVP.Model.EngineData' threw an exception. ----> 

System.InvalidCastException: [A]Engine cannot be cast to [B]Engine. Type A originates from 'MVP.Model, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' in the context 'LoadNeither' 
   at location '[...]\AppData\Roaming\Microsoft\VisualStudio\9.0\ProjectAssemblies\uankw1hh01\MVP.Model.dll'. 
Type B originates from 'MVP.Model, Version=1.0.0.0, Cultur开发者_如何学Ce=neutral, PublicKeyToken=null' in the context 'LoadNeither' 
   at location '[...]\AppData\Roaming\Microsoft\VisualStudio\9.0\ProjectAssemblies\u_hge2de01\MVP.Model.dll'
...

So I somehow have two assemblies and they are not accessed from my project folder, but from a VS temp folder? I googled a lot and only found this: IronPython Exception: [A]Person cannot be cast to [B]Person. There is a solution offered, but it concerns IronPhyton and I don't know where to use it within my project.

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Types are per-assembly; if you have "the same" assembly loaded twice, then types in each "copy" of the assembly are not considered to be the same type.

These issues normally crop up when the two assemblies are in the Load and LoadFrom contexts. See

Difference between LoadFile and LoadFrom with .NET Assemblies?

and the link to suzcook's blog for details on that issue.

Also, consider using the fusion log viewer to help diagnose the problem.

http://msdn.microsoft.com/en-us/library/e74a18c4%28VS.71%29.aspx

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Judging by the context in which the assembly is getting loaded (the context is "LoadNeither"), some developers may be doing something like loading an assembly that has been internally packaged as a resource with your application. If you do this, you will be using the AppDomain.CurrentDomain.AssemblyResolve event handler, so that your application can specify where .NET should get any particular assembly that it needs.

My answer will not explain the machinations of how to do that - but I'm mentioning it because this process led directly to the same exact error encountered by the original poster. As Eric Lippert mentions, types are per-assembly. So if you load an individual assembly more than once, the same defined class will appear as different classes - even though visual inspection shows that they appear to be the same.

We've seen instances in which .NET will call the ResolveEventHandler more than once for the same DLL. I'm not sure why .NET sometimes does this (it happened on some machines, but not all machines). But to resolve the problem, we needed keep a global list of handles to loaded assemblies, so that if .NET wanted to load the assembly again, we returned a handle to the same Assembly that was originally loaded, instead of loading another copy into memory.

I have included the code that caused the issue for us, and notes about how to handle it properly.

    public void AppStartup (object sender, StartupEventArgs e)
    {
        AppDomain.CurrentDomain.AssemblyResolve += new ResolveEventHandler(CurrentDomain_AssemblyResolve);
    }

    public System.Reflection.Assembly CurrentDomain_AssemblyResolve(object sender, ResolveEventArgs args)
    {
        string dllName = args.Name.Contains(',') ? args.Name.Substring(0, args.Name.IndexOf(',')) : args.Name.Replace(".dll", "");
        dllName = dllName.Replace(".", "_");

        if (dllName.EndsWith("_resources")) return null;
        System.Resources.ResourceManager rm = new System.Resources.ResourceManager(GetType().Namespace + ".Properties.Resources", System.Reflection.Assembly.GetExecutingAssembly());
        byte[] bytes = null;
        try
        {
            bytes = (byte[])rm.GetObject(dllName);
        }
        catch (Exception ex)
        {
        }
        if (bytes != null)
        {
            // the following call will return a newly loaded assembly
            //   every time it is called
            // if this function is called more than once for the same
            //   assembly, you'll load more than one copy into memory
            // this can cause the InvalidCastException
            // instead of doing this, you keep a global list of loaded
            //   assemblies, and return the previously loaded assembly
            //   handle, instead of loading it again
            return System.Reflection.Assembly.Load(bytes);
        }
        return null;
    }

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My situation involved two copies of the same dll. One was in the bin folder and one was in a sub-folder of the same bin folder. Both were loaded, amazingly some things worked fine, but some things didn't and that's when this error message appeared:

System.InvalidOperationException; There was an error generating the XML document.; Source: System.Xml; TargetSite: Void Serialize(System.Xml.XmlWriter, System.Object, System.Xml.Serialization.XmlSerializerNamespaces, System.String, System.String);

Hidden in this was the following inner exception (this was to do with Microsoft Dynamics CRM 4.0, but could relate to anything)

System.InvalidCastException; [A]XXX.CRMCustomCode.YYY.CreateCompanyRequest cannot be cast to [B]XXX.CRMCustomCode.YYY.CreateCompanyRequest. Type A originates from 'XXX.CRMCustomCode, Version=2.0.0.0, Culture=neutral, PublicKeyToken=null' in the context 'LoadFrom' at location 'C:\Program Files\Microsoft CRM\Server\bin\assembly\XXX.CRMCustomCode.dll'. Type B originates from 'XXX.CRMCustomCode, Version=2.0.0.0, Culture=neutral, PublicKeyToken=null' in the context 'Default' at location 'C:\Program Files\Microsoft CRM\Server\bin\XXX.CRMCustomCode.dll'.; 

I simply deleted the duplicate dll (in C:\Program Files\Microsoft CRM\Server\bin) and the error went away.

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My particular case - class library referenced in web application was renamed and rebuilt. Older version of library was still in bin folder under old name. The framework loads whatever in bin folder (both libraries) and emits this error. So it happens not only when assemblies are loaded explicitly. The obvious solution in my case is to clean bin folder.

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I got it working when I tried changing this cast:

var  t = (TeacherWebPages)Session["TeachersAD"];

To this:

var  t = Session["TeachersAD"] as TeacherWebPages;

However the assembly/session/memcache was different and no data got back but no error occurred. So later I still had to delete specific temporary files every time source page was changed from the folder it was complaining about which would require me to kill IIS process from task manager. Or in my case I can just logout and it clears the session state.