Dynamically load assembly from local file and run with restricted privileges

What I need to do is : Read a local C# text file, execute a method from that. This is what I'm doing.

1. Read all text from the file
2. Compile into a local x.dll with CSharpCodeProvider
3. Load the dll with Assembly.LoadFrom()
4. Then execute the method with GetType().GetMethod().Invoke()

It works fine. Now, I want to run this code securely, i.e. restrict this code from accessing the file system, network etc. Basically, I need to run this with minimal privileges.

I tried the code from Restrict plugin access to file system and network via appdomain (answer by @Babar), but still not working as expected. The code in the text file is still able to access file system.

What I'm missing here? Any other way to make it work?