How to load a .dll that was build for another .net version in .Net 5?

Question

In previous versions of ASP.NET (till ver 4.6), we can load a *.dll that was build for another .net version by modifying the web.config as follow:

<configuration>
     <startup useLegacyV2RuntimeActivationPolicy="true" >   
     </startup>
</configuration>

But in ASP.NET 5, there is no web.config, but a completely different configuration system. So how can we get the same result with the new version?

Answer 1

This blog post shows how to set this policy at runtime (vs. the "design-time" - by editing web.config) http://reedcopsey.com/2011/09/15/setting-uselegacyv2runtimeactivationpolicy-at-runtime/ but I havent' tried it with ASP.NET 5 myself. Worked on earlier versions though.

Basically you create this static helper class

public static class RuntimePolicyHelper
{
    public static bool LegacyV2RuntimeEnabledSuccessfully { get; private set; }

    static RuntimePolicyHelper()
    {
        ICLRRuntimeInfo clrRuntimeInfo =
            (ICLRRuntimeInfo)RuntimeEnvironment.GetRuntimeInterfaceAsObject(
                Guid.Empty, 
                typeof(ICLRRuntimeInfo).GUID);
        try
        {
            clrRuntimeInfo.BindAsLegacyV2Runtime();
            LegacyV2RuntimeEnabledSuccessfully = true;
        }
        catch (COMException)
        {
            // This occurs with an HRESULT meaning 
            // "A different runtime was already bound to the legacy CLR version 2 activation policy."
            LegacyV2RuntimeEnabledSuccessfully = false;
        }
    }

    [ComImport]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    [Guid("BD39D1D2-BA2F-486A-89B0-B4B0CB466891")]
    private interface ICLRRuntimeInfo
    {
        void xGetVersionString();
        void xGetRuntimeDirectory();
        void xIsLoaded();
        void xIsLoadable();
        void xLoadErrorString();
        void xLoadLibrary();
        void xGetProcAddress();
        void xGetInterface();
        void xSetDefaultStartupFlags();
        void xGetDefaultStartupFlags();

        [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
        void BindAsLegacyV2Runtime();
    }
}

Usage:

// before calling the code from your legacy assembly - 
if (RuntimePolicyHelper.LegacyV2RuntimeEnabledSuccessfully)
{
    // your Legacy code
}