Check out example codes for "how to custom window in C#". It will help you in understanding the concepts better.

Code Example 1

public class Win32
{
	[DllImport("user32")]
  	public static extern bool ReleaseCapture();
  	
  	[DllImport("user32")]
  	public static extern int SendMessage(IntPtr hWnd, int Msg, int wp, int lp);
}

public class CustomWindow : ContainerControl
{
	private Color headerBack = Color.FromArgb(44, 52, 63);
    private Color containerBack = Color.FromArgb(36, 41, 49);
    private Color borderBack = Color.Silver;
    private string title = "CustomWindow";
    private bool supportsIcon = true;
    private Font titleFont = new Font("Segoe UI", 12F);
    private Color titleColr = Color.FromArgb(243, 243, 243);
    private Rectangle headerRect, containerRect, borderRect, titleRect;
    private Image icon;
    
    public Color HeaderBackground
    {
    	get => headerBack;
        set { headerBack = value; Invalidate(); }
    }
    public Color ContainerBackground
    {
    	get => containerBack;
        set { containerBack = value; Invalidate(); }
    }
    public Color Border
    {
    	get => borderBack;
        set { borderBack = value; Invalidate(); }
    }
    public string Title
    {
    	get => title;
        set { title = value; Invalidate(); }
    }
    public Font TitleFont
    {
    	get => titleFont;
        set { titleFont = value; Invalidate(); }
    }
    public Color TitleColor
    {
    	get => titleColor;
        set { titleColor = value; Invalidate(); }
    }
    public bool SupportsIcon
    {
    	get => supportsIcon;
        set { supportsIcon = value; Invalidate(); }
    }
    public Image Icon
    {
    	get => icon;
        set { icon = value; Invalidate(); }
    }
    
    public CustomWindow()
    {
    	SetStyle(ControlStyles.AllPaintingInWMPaint |
        		ControlStyles.OptimizedDoubleBuffer |
                ControlStyles.ResizeRedraw |
                ControlStyles.UserPaint |
                ControlStyles.SupportsTransparentBackColor, true);
        DoubleBuffered = true;
        if (Parent != null)
        	Parent.Size = new Size(500, 400);
        if (Parent is Form form)
        	form.FormBorderStyle = FormBorderStyle.None;
        Dock = DockStyle.Fill;
    }
    protected override void OnPaint(PaintEventArgs e)
    {
    	base.OnPaint(e);
        Graphics g = e.Graphics;
        Size titleSize = TextRenderer.MeasureText(title, titleFont);
        headerRect = new Rectangle(0, 0, Width, 40);
        containerRect = new Rectangle(0, 41, Width, Height - 41);
        borderRect = new Rectangle(0, 40, Width, 1);
        titleRect = new Rectangle(supportsIcon ? 35 : 10, 10, titleSize.Width, titleSize.Height);
        g.FillRectangle(new SolidBrush(headerBack), headerRect);
        g.FillRectangle(new SolidBrush(borderBack), borderRect);
        g.FillRectangle(new SolidBrush(containerBack), containerRect);
        g.DrawString(title, titleFont, new SolidBrush(titleColor), titleRect, new StringFormat
        {
        	Alignment = StringAlignment.Near,
            LineAlignment = StringAlignment.Center
        });
        if (supportsIcon == true)
        	g.DrawIamge(icon ?? Resources.white_black_round_box, 5, 5, 30, 30);
    }
    protected override void OnMouseDown(MouseEventArgs e)
    {
    	base.OnMouseDown(e);
        Point mpos = new Point(e.X, e.Y);
        if (e.Button == MouseButtons.Left)
        {
        	if (headerRect.Contains(mpos))
            {
            	Win32.ReleaseCapture();
                Win32.SendMessage(Parent.Handle, 161, 2, 0);
            }
        }
    }
}

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