How do I compile a program that can repeat a keystroke while the same key is held?

Question

I have been trying for a while to make a simple program that can send a key repeatedly while the same key is held. I've tried to use an example with "Windows Forms" down below but I get the error "The type or namespace "Key" could not be found" even though I have included System.ComponentModel.DataAnnotations which, as far as I understand Key should be a part of. I have never made a program in Windows forms before but as far I understand it's basically including a GUI with your code? Only reason I'm using windows forms is because the examples I saw used it but if it can be done in a regular C# program that would be great too.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.ComponentModel.DataAnnotations;

namespace spamkey2
{
    public partial class Form1 : Form
    {
        bool keyHold = false;

        public Form1()
        {
            InitializeComponent();
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            if (keyHold)
            {
                //Do stuff
            }
        }

        private void Key_up(object sender, KeyEventArgs e)
        {
            Key key = (Key)sender;
            if (key == Key.A) //Specify your key here !
            {
                keyHold = false;
            }
        }

        private void Key_down(object sender, KeyEventArgs e)
        {
            Key key = (Key)sender;
            if (key == Key.A) //Specify your key here !
            {
                keyHold = true;
            }
        }
    }
}

Answer 1

use this code

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.ComponentModel.DataAnnotations;

namespace spamkey2
{
    public partial class Form1 : Form
    {
        bool keyHold = false;
        public Form1()
        {
            InitializeComponent();
            this.KeyDown += Key_down;
            this.KeyUp += Key_up;
            System.Timers.Timer timer = new System.Timers.Timer();
            timer.Elapsed += Timer_Elapsed;
            timer.Interval = 100;
            timer.Start();
        }

        private void Timer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
        {
            if (keyHold)
            {
                //Do stuff
                Console.WriteLine("Key A is down");
            }
        }

        private void Key_up(object sender, KeyEventArgs e)
        {
            if (e.KeyData == Keys.A) //Specify your key here !
            {
                keyHold = false;
            }
        }

        private void Key_down(object sender, KeyEventArgs e)
        {
            if (e.KeyData == Keys.A) //Specify your key here !
            {
                keyHold = true;
            }
        }
    }
}