Mixed mode assembly in .NET 4

I wrote a class library in .NET 2.0 for database access some 2 years ago and have been using it all along on .NET 2.0, 3.0 and 3.5.

In the current project I'm working on (which is a .NET 4.0 application), tried using the old faithful class library and I got the following exception:

System.InvalidOperationException was unhandled
  Message=An error occurred creating the form. See Exception.InnerException for details.
    The error is: Mixed mode assembly is built against version 'v2.0.50727' of the runtime
    and cannot be loaded in the 4.0 runtime without additional configuration information.
    Source=SchoolManager
  StackTrace:
       at SchoolManager.My.MyProject.MyForms.Create__Instance__[T](T Instance) in 17d14f5c-a337-4978-8281-53493378c1071.vb:line 190
       at SchoolManager.My.MyProject.MyForms.get_frmGeneric()
       at SchoolManager.My.MyApplication.OnCreateMainForm() in D:\Alex\Documents\Visual Studio 2008\Projects\School Manager\SchoolManager\My Project\Application.Designer.vb:line 35
       at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.OnRun()
       at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.DoApplicationModel()
       at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.Run(String[] commandLine)
       at SchoolManager.My.MyApplication.Main(String[] Args) in 17d14f5c-a337-4978-8281-53493378c1071.vb:line 81
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: System.IO.FileLoadException
       Message=Mixed mode assembly is built against version 'v2.0.50727' of 
           the runtime and cannot be loaded in the 4.0 runtime without additional
           configuration information.
       Source=Dinofage.Data.XpressData
       StackTrace:
            at Dinofage.Data.XpressData.ExecuteSelectInternal(String selectCommand)
            at Dinofage.Data.XpressData.ExecuteSelect(String selectCommand)
            at SchoolManager.Academics.GetNewAdmissionCode(String academicYear) in D:\Alex\Documents\Visual Studio 2008\Projects\School Manager\SchoolManager\Modules\Academics.vb:line 89
            at SchoolManager.StudentDetail..ctor() in D:\Alex\Documents\Visual Studio 2008\Projects\School Manager\SchoolManager\UserControls\StudentDetail.vb:line 20
            at SchoolManager.frmGeneric.InitializeComponent() in D:\Alex\Documents\Visual Studio 2008\Projects\School Manager\SchoolManager\frmGeneric.Designer.vb:line 25
            at SchoolManager.frmGeneric..ctor()
       InnerException:

What could be wrong and how do I fix it?

The best would probably be to recompile your class library for .NET 4.0 in Visual Studio 2010 (ie. opening up the project, converting it, and changing the target framework.)

If you can't, or won't, do that, then you can try adding the following to your app.config file for your .NET 4.0 application:

<startup useLegacyV2RuntimeActivationPolicy="true">
    <supportedRuntime version="v4.0"/>
</startup>

ie.

<?xml version ="1.0"?> 
<configuration> 
    <startup useLegacyV2RuntimeActivationPolicy="true">
        <supportedRuntime version="v4.0"/>
    </startup>
</configuration>

Since your assembly is mixed-mode, it potentially can call managed code from the unmanaged machine code in the assembly. With the new in-process side-by-side CLR version support in .NET 4.0, the runtime doesn't know which CLR version needs to be provided when that happens. You have to tell it that with an app.exe.config file that should look like this:

<configuration>
  <startup useLegacyV2RuntimeActivationPolicy="true">
    <supportedRuntime version="v4.0"/>
  </startup>
</configuration>

Mixed mode assembly in .NET 4

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