strings-are-mutable.cs

class MainClass {
    static void Main () {
        var text = "abcdef";
        System.Console.WriteLine (text);
        Mutate (text);
        System.Console.WriteLine (text);
    }

    unsafe static void Mutate (string text) {
        fixed (char* t = text)
        for (int i = 0; i < text.Length; i++)
            t[i] = 'z';
    }
}

strings-are-mutable.fs

#nowarn "9"
let mutate (txt:string) : unit =
    use ch = fixed txt
    for i = 0 to txt.Length - 1 do
        Microsoft.FSharp.NativeInterop.NativePtr.set ch i 'z'
let text = "abcd"
printfn "%s" text
mutate text
printfn "%s" text