Question

I am at chapter 9 of Jon Skeet’s CSharp in Depth at a section which explains the improvements to type inference in 3.0

There is code snippet on Pg.247 that ‘shouldn’t compile with 2.0’ – However I can’t find a reason why it should not. Tried the problem with VS2008 C# Express Edition on a project with target framework as 2.0 – all three of the below calls compile and run too.

2.0 introduced the ability to infer the right type of delegate.

new List<ThreadStart>().Add( delegate { Console.WriteLine("New Thread!"); } );   // works

Of course Jon can’t be wrong ( 😉 ‘Sql is broken’ + there is no mention of this on the errata page for the book.) So my prime suspicion would be that it’s still using 3.0 type inference – What am I missing ?

    delegate int MyDelegate(string s);
    delegate TOutput MyConverter<TInput, TOutput>(TInput input);
    static void MyParse(MyDelegate del)
    {
        Console.WriteLine(del("100"));
    }
    static void MyConvert<TInput, TOutput>(MyConverter<TInput, TOutput> del, TInput input)
    {
        Console.WriteLine(del(input));
    }

    // Jon's code snippet begin
    delegate T MyFunc<T>();
    static void WriteResult<T>(MyFunc<T> function)
    {
        Console.WriteLine(function());
    }
    // end

    static void Main(string[] args)
    {
        MyParse(delegate(string s)
            {
                return Int32.Parse(s);
            }
        );
        MyConvert(delegate(string s)
            {
                return Int32.Parse(s);
            },
            "100");
        WriteResult(delegate { return 5; });    // Jon: Shouldn't work.
    }

Answer 1

What the ‘target framework’ setting does, is disable libraries and methods that have been added in newer framework versions. Most of the new language features don’t require special runtime support or new libraries. So if you use them, you need the newer compiler, but your app will still work on 2.0.