How do I compile for .NET 2.0 with C# 6.0?

Question

Visual Studio 2015 has no problem compiling for the older CLR with new c# compiler. It seems that it uses VBCSCompiler.exe for this under the hood, but I cannot find any doc

Answer 1

You can specify old .NET assemblies with /r:

 /reference:<alias>=<file>     Reference metadata from the specified assembly
                               file using the given alias (Short form: /r)
 /reference:<file list>        Reference metadata from the specified assembly
                               files (Short form: /r)

You will also need to suppress the automatic inclusion of the modern mscorlib with /nostdlib:

 /nostdlib[+|-]                Do not reference standard library (mscorlib.dll)

Together, these make it so that you can build .NET 2.0 apps with the C# 6 compiler.

csc.exe /r:"C:\Windows\Microsoft.NET\Framework\v2.0.50727\mscorlib.dll" /nostdlib Program.cs

You can even use C# 6 features in your app! (as long as they are compiler-only features that don't involve the .NET runtime)

public static string MyProp { get; } = "Hello!";
static void Main(string[] args)
{
    Console.WriteLine(MyProp);
    // prints "Hello!"

    var assembly = Assembly.GetAssembly(typeof(Program));
    Console.WriteLine(assembly.ImageRuntimeVersion);
    // prints "v2.0.50727"
}