C# tuple deconstructor inside another tuple constructor

Question

Is there C# feature to deconstruct tuple into subset fields of anoter tuple?

public (bool, int) f1() => (true, 1);
public (double, bool, int) f2 => (1.0, f1() /*!!here!!*/);

Thanks!

Answer 1

There is currently no such C# feature, so you will have to write f2() as

public (double, bool, int) f2()
{
    var tuple = f1();
    return (1.0, tuple.Item1, tuple.Item2);
}

However if you really wanted to, you could write a helper class to combine tuples:

public static class TupleCombiner
{
    public static (T1, T2, T3) Combine<T1, T2, T3>(T1 item, (T2, T3) tuple)
    {
        return (item, tuple.Item1, tuple.Item2);
    }

    public static (T1, T2, T3) Combine<T1, T2, T3>((T1, T2) tuple, T3 item)
    {
        return (tuple.Item1, tuple.Item2, item);
    }

    // Etc for other combinations if needed
}

Then if you put using static YourNamespace.TupleCombiner; at the top of the code file your code would look like this:

public static (bool, int)         f1() => (true, 1);
public static (double, bool, int) f2() => Combine(1.0, f1());
public static (bool, int, double) f3() => Combine(f1(), 1.0);

Of course, this also works for var declarations inside methods:

public static void Main()
{
    var tuple = Combine(1.0, f1());
}

I'm not convinced this is in any way worth it, but it's an option.

Also note that - as pointed out by /u/Ry- in the comments - There is a proposal for tuple "splatting" on Github, but that proposal is over three years old (and it doesn't really seem to be exactly what you're asking for).