C# stackalloc using Span<T>; password not handled by the GC.

out.cs

static void Main(string[] args)
        {
            // Allocate the buffer on the stack (therefore not managed by the GC)
            Span<char> buffer = stackalloc char[20];
            
            // Ask for the password
            Console.Write("Type a password: ");
            TypePassword(buffer, 0);
            Console.WriteLine();

            // Unsafe since transformed in string; real usage would avoid that
            Console.WriteLine($"Your password: {buffer.ToString()}");

            // Flush the buffer
            Flush(buffer);

            // Unsafe since transformed in string; real usage would avoid that
            Console.WriteLine($"Flushed buffer: {buffer.ToString()}");

            Console.Read();
        }

        /// <remarks>
        /// Pray for tailrec optimization.
        /// </remarks>
        static Span<char> TypePassword(Span<char> buffer, int i)
        {
            if (i > buffer.Length) return buffer;

            switch (Console.ReadKey(intercept: true))
            {
                // Displayable character
                case var key when !char.IsControl(key.KeyChar):
                    buffer[i] = key.KeyChar;
                    return TypePassword(buffer, i + 1);

                // Backspace
                case var key when key.Key == ConsoleKey.Backspace && i > 0:
                    buffer[i] = (char)0;
                    return TypePassword(buffer, i - 1);

                // Validation
                case var key when key.Key == ConsoleKey.Enter:
                    return buffer;

                default:
                    return TypePassword(buffer, i);
            }
        }

        static void Flush<T>(Span<T> span) where T: new()
        {
            span.Fill(new T());
        }
    }