[MonoDevelop] "System.Net not found" using monodevelop

[enzo]

I' using monodvelop 0.9 and mono 1.1.10 ( via mono-1.1.10_1-installer.bin )
I get a strange problem with also a very simple program like this:
using System;
using System.Net;
namespace Arta.Examples
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
        }
    }
}
 

If I compile using mcs all ok
[enzo at enzo5 test1]$ mcs Main.cs
[enzo at enzo5 test1]$ mono Main.exe
Hello World!

If I try to compile inside the monodevelop I got the following errors:
[Task:File=/home/enzo/progetti/mono/test1/Main.cs, Line=3, Column=1, 
Type=Error, Description=The type or namespace name `System.Net' could 
not be found. Are you missing a using directive or an assembly 
reference?(CS0246)
[Task:File=/home/enzo/progetti/mono/test1/Main.cs, Line=3, Column=7, 
Type=Error, Description=The type or namespace name `Net' does not exist 
in the namespace `System'. Are you missing an assembly reference?(CS0234)

Just to clarify: in this example System.Net is useless, but I just use 
this to point out the problem with a very simple and standard example

What should be wrong with monodevelop, which prevent the recognition of 
the namespace System.Net

reagrds, Enzo