You can't cause a .Net application to elevate its own rights. It's simply not allowed. The best you can do is to specify elevated rights when you spawn another process. In this case you would have a two-stage application launch.
Stage 1 does nothing but prepare an elevated spawn using the System.Diagnostics.ProcessStartInfo object and the Start() call.
Stage 2 is the application running in an elevated state.
As mentioned above, though, you very rarely want to do this. And you certainly don't want to do it just so you can write temporary files into %programfiles%. Use this method only when you need to perform administrative actions like service start/stop, etc. Write your temporary files into a better place, as indicated in other answers here.