c#6 features work with net2

CSharp6andNET2.cs

using System;
using System.Collections.Generic;
using static System.Math;

namespace System.Runtime.CompilerServices
{
    [AttributeUsage( AttributeTargets.Method )]
    public class ExtensionAttribute : Attribute
    {
    }
}

namespace CSharp6andNET2
{
    internal class Program
    {
        public delegate bool Filter( ArgumentException ex, string argName );

        public bool DoFilter( Filter test, ArgumentException ex, string argName )
        {
            return test( ex, argName );
        }

        private static void Main( string[] args )
        {
            Filter x = ( ArgumentException ex, string argName ) => ex.ParamName == argName;
            for ( int count = 0; count < 2; count++ )
            {
                try
                {
                    ExceptionFilterTest( count == 0 );
                }
                catch ( ArgumentException e ) when (x( e, "argumentName" ))
                {
                    Console.WriteLine( "Logged filtered exception" );

                    var test = new TestClass();
                    Console.WriteLine(
                        $"{nameof( test.Count )}: {test.Count}, {nameof( test.Count2 )}: {test.Count2}{nameof( test.Count3 )}: {test.Count3}, {nameof( test.Count4 )}: {test.Count4( 1 )}" );

                    var list = new List<int>
                    {
                        "9",
                        "25",
                        "36",
                        16,
                        4,
                        64
                    };

                    Console.WriteLine( "List" );
                    foreach ( var n in list )
                    {
                        Console.WriteLine( Sqrt( n ) );
                    }

                    var dictionary = new Dictionary<int, string>
                    {
                        [ 4 ] = "Test",
                        [ 2 ] = null,
                        [ 45 ] = null,
                        [ 34 ] = null,
                        [ 200 ] = null,
                        [ 16 ] = "Another test"
                    };

                    foreach ( var k in dictionary )
                    {
                        Console.WriteLine( $"{k.Key}: {k.Value?.Substring( 0, 3 )}" );
                    }
                }
                catch ( Exception )
                {
                    Console.WriteLine( "Logged exception" );
                }
            }
        }

        private static void ExceptionFilterTest( bool filterable )
        {
            if ( filterable ) throw new ArgumentException( "Exception", "argumentName" );

            throw new Exception( "Exception" );
        }
    }

    internal static class Extensions
    {
        public static void Add( this List<int> list, string value )
        {
            list.Add( Int32.Parse( value ) );
        }
    }

    internal class TestClass
    {
        public int Count { get; } = 6;
        public int Count2 { get; set; } = 6;
        public int Count3 => 6;
        public int Count4( int x ) => x + 6;
    }
}