Question

Can IL produced by C# 4.0 compiler run on CLR 2.0?

To clarify, I’m not asking about VS 2010 multi-targeting (where VS chooses the right compiler version), I want to know if csc.exe 4.0 supports multi-targeting…

Answer 1

If all else fails, try it. I’ve just tested the following in VS2010b2, compiled targeting 2.0:

using System;    
class Program {
    static void Main() {
        Write();
        Write(msg: "world");
        Console.ReadLine();
    }
    static void Write(string msg = "Hello") {
        Console.WriteLine(msg);
    }
}

This uses two new C# 4.0 language features, which use the meta-data that is also present in .NET 2.x/3.x/CLR 2; it executes fine on my regular (CLR 2) machine (my VS2010b2 is a VM). So I conclude “yes, for some features”. Obviously if you use a framework-dependent feature (dynamic, etc) it isn’t going to end so well.

Edit: re your comment; I’ve tried csc at the command-line, and by default this does target CLR 4; I’ll try to see if I can get it to target CLR 2 (like VS obviously can). Unfortunately, it no longer includes the (faked, btw) command line in the build output window…

Update: some “in the know” people came back with:

Pass /nostdlib and a reference to the 2.0 mscorlib.dll.

And sure enough:

C:\Windows\Microsoft.NET\Framework\v4.0.21006>csc /nostdlib /reference:%SystemRo
ot%\microsoft.net\framework\v2.0.50727\mscorlib.dll /out:c:\my.exe /target:exe "
C:\Users\Marc\Documents\Visual Studio 2010\Projects\ConsoleApplication6\ConsoleA
pplication6\program.cs"
Microsoft (R) Visual C# 2010 Compiler version 4.0.21006.1
Copyright (C) Microsoft Corporation. All rights reserved.

works fine (exe works on my non-4.0 machine). Credit: Kevin Pilch-Bisson