private class Vonas
{
    public List<Point> Pontok = new List<Point>();
    public Color Szin = Color.Black;
    public float Vastagsag = 3;
    public bool Kijelolt = false;

    public Rectangle Hatar()
    {
        int minX = Pontok.Min(p => p.X), maxX = Pontok.Max(p => p.X);
        int minY = Pontok.Min(p => p.Y), maxY = Pontok.Max(p => p.Y);
        int m = (int)Math.Ceiling(Vastagsag / 2) + 2;

        return new Rectangle(minX - m, minY - m, maxX - minX + 2 * m, maxY - minY + 2 * m);
    }
}

private enum Allapot { Nincs, Rajzol, Gumi, Mozgat }

private List<Vonas> vonasok = new List<Vonas>();
private Allapot allapot = Allapot.Nincs;
private Vonas epulo = null;
private Point kezdo, aktualis, elozo;
private Color tollSzin = Color.Black;
private List<List<Point>> mozgatasElott = null;

private void rp_MouseDown(object sender, MouseEventArgs e)
{
    if (e.Button != MouseButtons.Left) { return; }

    kezdo = aktualis = elozo = e.Location;

    if (rbToll.Checked)
    {
        epulo = new Vonas { Szin = tollSzin, Vastagsag = (float)nudVastag.Value };
        epulo.Pontok.Add(e.Location);
        vonasok.Add(epulo);
        allapot = Allapot.Rajzol;
    }
    else
    {
        Vonas talalat = null;

        for (int i = vonasok.Count - 1; i >= 0; i--)
        {
            if (vonasok[i].Hatar().Contains(e.Location)) { talalat = vonasok[i]; break; }
        }

        if (talalat != null)
        {
            if (!talalat.Kijelolt && (Control.ModifierKeys & Keys.Control) != Keys.Control)
            {
                foreach (Vonas v in vonasok) { v.Kijelolt = false; }
            }

            talalat.Kijelolt = true;
            mozgatasElott = vonasok.Where(v => v.Kijelolt).Select(v => new List<Point>(v.Pontok)).ToList();
            allapot = Allapot.Mozgat;
            rp.Cursor = Cursors.SizeAll;
        }
        else
        {
            if ((Control.ModifierKeys & Keys.Control) != Keys.Control)
            {
                foreach (Vonas v in vonasok) { v.Kijelolt = false; }
            }

            allapot = Allapot.Gumi;
        }
    }

    Frissit();
}

private void rp_MouseMove(object sender, MouseEventArgs e)
{
    tsslEger.Text = e.X + ", " + e.Y;
    aktualis = e.Location;

    switch (allapot)
    {
        case Allapot.Rajzol:
            Point utolso = epulo.Pontok[epulo.Pontok.Count - 1];

            if (Math.Abs(e.X - utolso.X) + Math.Abs(e.Y - utolso.Y) >= 2) { epulo.Pontok.Add(e.Location); rp.Invalidate(); }
            break;

        case Allapot.Gumi:
            rp.Invalidate();
            break;

        case Allapot.Mozgat:
            int dx = e.X - elozo.X, dy = e.Y - elozo.Y;

            foreach (Vonas v in vonasok.Where(x => x.Kijelolt))
            {
                for (int i = 0; i < v.Pontok.Count; i++) { v.Pontok[i] = new Point(v.Pontok[i].X + dx, v.Pontok[i].Y + dy); }
            }

            elozo = e.Location;
            rp.Invalidate();
            break;
    }
}

private void rp_MouseUp(object sender, MouseEventArgs e)
{
    if (allapot == Allapot.Gumi)
    {
        Rectangle keret = PontokbolRect(kezdo, e.Location);

        foreach (Vonas v in vonasok)
        {
            if (keret.IntersectsWith(v.Hatar())) { v.Kijelolt = true; }
        }
    }

    Veg();
}

private void rp_MouseCaptureChanged(object sender, EventArgs e)
{
    if (allapot == Allapot.Gumi || allapot == Allapot.Mozgat) { Megszakit(); }
}

private void Veg()
{
    allapot = Allapot.Nincs;
    epulo = null;
    mozgatasElott = null;
    rp.Cursor = Cursors.Cross;
    Frissit();
}

private void Megszakit()
{
    if (allapot == Allapot.Mozgat && mozgatasElott != null)
    {
        int i = 0;

        foreach (Vonas v in vonasok.Where(x => x.Kijelolt)) { v.Pontok = new List<Point>(mozgatasElott[i++]); }   // vissza a kezdő helyre
    }

    Veg();
}

protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
{
    if (keyData == Keys.Escape && (allapot == Allapot.Gumi || allapot == Allapot.Mozgat)) { Megszakit(); return true; }

    if (keyData == (Keys.Control | Keys.Z) && vonasok.Count > 0 && allapot == Allapot.Nincs) { vonasok.RemoveAt(vonasok.Count - 1); Frissit(); return true; }

    if (keyData == Keys.Delete && allapot == Allapot.Nincs) { vonasok.RemoveAll(v => v.Kijelolt); Frissit(); return true; }

    return base.ProcessCmdKey(ref msg, keyData);
}

private Rectangle PontokbolRect(Point p, Point q)
{
    return new Rectangle(Math.Min(p.X, q.X), Math.Min(p.Y, q.Y), Math.Abs(p.X - q.X), Math.Abs(p.Y - q.Y));
}

private void Frissit()
{
    tsslDb.Text = vonasok.Count + " vonal, " + vonasok.Count(v => v.Kijelolt) + " kijelölve";
    rp.Invalidate();
}

private void Rajzol(Graphics g)
{
    g.SmoothingMode = SmoothingMode.AntiAlias;

    foreach (Vonas v in vonasok)
    {
        using (Pen toll = new Pen(v.Szin, v.Vastagsag))
        using (Brush b = new SolidBrush(v.Szin))
        {
            toll.StartCap = toll.EndCap = LineCap.Round;
            toll.LineJoin = LineJoin.Round;

            if (v.Pontok.Count == 1) { g.FillEllipse(b, v.Pontok[0].X - v.Vastagsag / 2, v.Pontok[0].Y - v.Vastagsag / 2, v.Vastagsag, v.Vastagsag); }
            else { g.DrawLines(toll, v.Pontok.ToArray()); }
        }
    }
}

private void rp_Paint(object sender, PaintEventArgs e)
{
    Graphics g = e.Graphics;
    g.Clear(Color.White);
    Rajzol(g);

    foreach (Vonas v in vonasok.Where(x => x.Kijelolt))
    {
        using (Pen szagg = new Pen(Color.RoyalBlue) { DashStyle = DashStyle.Dash })
        {
            Rectangle h = v.Hatar();
            g.DrawRectangle(szagg, h.X, h.Y, h.Width, h.Height);
        }
    }

    if (allapot == Allapot.Gumi)
    {
        Rectangle keret = PontokbolRect(kezdo, aktualis);

        using (Pen szagg = new Pen(Color.RoyalBlue) { DashStyle = DashStyle.Dash })
        using (Brush halvany = new SolidBrush(Color.FromArgb(40, Color.RoyalBlue)))
        {
            g.FillRectangle(halvany, keret);
            g.DrawRectangle(szagg, keret.X, keret.Y, keret.Width, keret.Height);
        }
    }
}

private void btnMentes_Click(object sender, EventArgs e)
{
    sfd.Filter = "PNG (*.png)|*.png";

    if (sfd.ShowDialog(this) != DialogResult.OK) { return; }

    using (Bitmap kep = new Bitmap(rp.Width, rp.Height))
    {
        using (Graphics g = Graphics.FromImage(kep))
        {
            g.Clear(Color.White);
            Rajzol(g);
        }

        kep.Save(sfd.FileName, ImageFormat.Png);
    }
}