C# public variable hatası nasıl çözülür?

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;

namespace WindowsFormsApp2
{
    public partial class Form1 : Form
    {
        private int moveborder;
        public Form1()
        {
            InitializeComponent();
            moveborder = (Screen.PrimaryScreen.Bounds.Width - this.Size.Width) / 2 + button1.Width / 2;
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            if (0 < Control.MousePosition.X - moveborder && Control.MousePosition.X - moveborder < 920)
            {
                button1.Location = new Point(Control.MousePosition.X - moveborder, button1.Location.Y);
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            timer1.Start();
        }

}
}