.net 3.5 application on .net 4.0 runtime

Question

If I have an application built to target .net 3.5, will it run if all the user has installed is .net 4.0? A link to official msdn documentation stating this would put my worries to rest.

Answer 1

Yes, it can. You can say that you support .NET 4 in your app.config file:

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

Note that changes in the framework may break your app. For details, see MSDN's list of .NET 4 migration issues, which includes recommended changes.

If you still want to support users with only .NET 3.5 installed, you also need to add a <supportedRuntime version="v2.0.50727" /> element:

<configuration>
  <startup>
    <supportedRuntime version="v4.0" /> 
    <supportedRuntime version="v2.0.50727" /> 
  </startup>
</configuration>

Note that the above configuration prefers .NET 4.0 if that is installed. Changing the order above makes the application prefer .NET 3.5 if that is installed. A complete table of possible configurations and effects can be found in MSDN's how-to on configuring your app to support .NET 4.0 or 4.5.