FSX What .NET version should I use for fsx simconnect?

I never needed know the particular .NET version when creating SimConnect application - I don't even know which version I should need. What I do have is:

I know this answer is a bit late to the party, but here is my version in case someone arrives here from google having the same question.

FSX SimConnect uses .NET 2 CLR
P3Dv3 SimConnect uses .NET 4 CLR

You can create a .NET 4.5 application (or 4.5.2) and reference the FSX SimConnect assembly because .NET 4 supports side loading of .NET 2 assemblies.

What you have to do is enable loading of .NET 2 assemblies in your app.config file.

<startup useLegacyV2RuntimeActivationPolicy="true">
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5.2"/>