.Net using 2.0 dll in 4.0

Using a dll compiled in 2.0 in a 4.0 project and your into trouble.
You will get the following error:
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.

The fix:
In your App.Config file add following at the end but before </configuration >

<startup uselegacyv2runtimeactivationpolicy="true" >

    <supportedruntime version="v4.0" >

    <requiredruntime version="v4.0.20506" >

  </requiredruntime >

</supportedruntime >

</startup >

App Inventor

I just discovered a great tool to make Android Apps App Inventor
It uses graphical programming ala Scratch.

Nice tool if you want to try out sesnsor etc on your phone without going through the pain of writing java code.