app.cs

using System;
using System.Threading.Tasks;

namespace AppThatRunsItself {
	public class App {
		public static void Main(string[] args) {
			// we might instantiate a host and run an app with it 
			var host = new Hosting.Host();

			var dotNet40_thing = new Task(() => Console.WriteLine("Hello from task"));

			// we might ask the host to run our app ... but we'll just pass it .NET 4.0 stuff
			host.GimmeShit(dotNet40_thing);
		}
	}
}

host.cs

using System;
using System.Reflection;

namespace Hosting {
	public class Host {

		string CurrentRuntime {
			get { return Environment.Version.ToString(); }
		}

		string CompiledRuntime {
			get { return Assembly.GetExecutingAssembly().ImageRuntimeVersion; }
		}

		public void GimmeShit(object shit) {
			Console.WriteLine("Host.GimmeShit was passed a {0}.  Running on: {1}.  Compiled on: {2}", 
								shit, CurrentRuntime, CompiledRuntime);
		}
	}
}

output.txt

# compile host.cs against .NET 2.0
$ gmcs -t:library host.cs

# compile app.cs against .NET 4.0, referencing host.dll
dmcs app.cs -r:host.dll

# run app.exe in the .NET 4.0 runtime
$ mono-2.8 app.exe

Host.GimmeShit was passed a System.Threading.Tasks.Task.  Running on: 4.0.30319.1.  Compiled on: v2.0.50727