PictureBox pictureBox1;

public partial class Form1 : Form {
 
    Objekt nastaveni = new Objekt();
 
    public Form1() {
        InitializeComponent();
    }
 
    private void pictureBox1_MouseUp(object sender, MouseEventArgs e) {
        nastaveni.MuzuKreslit = false;
    }
 
    private void pictureBox1_MouseDown(object sender, MouseEventArgs e) {
        nastaveni.MuzuKreslit = true;
        nastaveni.X = e.X;
        nastaveni.Y = e.Y;
    }
 
    private void pictureBox1_MouseMove(object sender, MouseEventArgs e) {
        if (nastaveni.MuzuKreslit == true) {
            Graphics g = pictureBox1.CreateGraphics();
            Pen p1 = new Pen(nastaveni.barva, 5);
            g.DrawLine(p1, nastaveni.X, nastaveni.Y, e.X, e.Y);
            nastaveni.X = e.X;
            nastaveni.Y = e.Y;
        }
    }
}
 
class Objekt {
    public Boolean MuzuKreslit = false;
    public Int32 X;
    public Int32 Y;
    public Color barva = Color.Red;
}