Fun with string interpolation

Program.cs

using System;
using System.Linq;
using System.Runtime.CompilerServices;

namespace Scratch
{
    class Program
    {
        static void Main(string[] args)
        {
            // some miscellaneous variables to interpolate. It is a requirement that the
            // booleans values must be lowercased, but the non-boolean values should remain as is.
            bool isCool = true;
            bool isMagic = true;
            string uppercase = "HELLO";

            // interpolate and implicitly convert to string (what normally happens)
            string badString = $"http://www.example.com/?isCool={isCool}&isMagic={isMagic}&uppercase={uppercase}";
            // interpolate, but pass to function which will do some stuff before explicitly converting to a string
            string goodString = LowercaseBooleans($"http://www.example.com/?isCool={isCool}&isMagic={isMagic}&uppercase={uppercase}");
            
            Console.WriteLine($"badString: {badString}");    // prints badString: http://www.example.com/?isCool=True&isMagic=True&uppercase=HELLO
            Console.WriteLine($"goodString: {goodString}"); // prints goodString: http://www.example.com/?isCool=true&isMagic=true&uppercase=HELLO

            Console.ReadKey();
        }

        static string LowercaseBooleans(FormattableString str)
        {
            // for boolean arguments, lowercase them.
            var args = str.GetArguments()
                .Select(arg =>
                    arg is bool booleanArg
                    ? (booleanArg ? "true" : "false")
                    : arg)
                .ToArray();

            // create a new string based on the provided FormattableString
            return FormattableStringFactory
                .Create(str.Format, args)
                .ToString();
        }
    }
}