accessing .net2.0 dll in a .net 4.0 dll

Question

I want to load a .NET 2.0 dll in my.Net4.0 dll . I did search for the solution and found the 'useLegacyV2RuntimeActivationPolicy' attribute that I can add to my app.config as in this url: Mixed mode assembly is built against version 'v1.1.4322'

But the problem for me is that, in my case, I am trying to use the .net2.0 dll in my 4.0 DLL and not in an 4.0 exe . The reason is this 4.0 dll is then used in Excel as an automation add-in.

Currently, when I acccess my .NET 4.0 API (which loads the 2.0 dll for a feature ) from Excel, I get the 'mixed mode 2.0 dll cannot be loaded in 4.0 runtime' which is understandable since COM has loaded the 4.0 CLR and hence cannot load the 2.0 dll.

Somehow I need to make my 4.0 DLL to load the 2.0 runtime. I tried adding a app.config to the 4.0 dll but I guess only.exe reads the app.config by default to load the clr. This is what I have in my app.config in the project of 4.0 dll but no use.

    <?xml version="1.0"?>
<configuration>
  <startup useLegacyV2RuntimeActivationPolicy="true">
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/>
  </startup>
  <runtime>
    <NetFx40_LegacySecurityPolicy enabled="true"/>
  </runtime>
</configuration>

How do I solve this problem to enable my .NET 4.0 DLL to use .NET 2.0 dll?

Thanks, Mani

Answer 1

The answer is already done by Hans Passant but he has posted as a comment. Hence, answering this separately in here.

The config has to be called excel.exe.config and should be in the same office directory as excel.exe for this scenario to work.