Custom controls and owner-drawn rendering

Derive from UserControl or Control, expose properties and events properly, override OnPaint, and take over drawing for list and tree controls.

UserControl or custom Control

public class StatusPill : Control
{
    private string _text = "";
    private Color _accent = Color.SeaGreen;

    [Category("Appearance"), DefaultValue("")]
    public string Caption
    {
        get => _text;
        set { _text = value ?? ""; Invalidate(); }   // never repaint directly from the setter
    }

    [Category("Appearance")]
    public Color Accent
    {
        get => _accent;
        set { _accent = value; Invalidate(); }
    }

    protected override void OnPaint(PaintEventArgs e)
    {
        e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
        using var brush = new SolidBrush(_accent);
        e.Graphics.FillEllipse(brush, 0, 0, Height - 1, Height - 1);

        TextRenderer.DrawText(e.Graphics, _text, Font,
            new Point(Height + 4, (Height - Font.Height) / 2), ForeColor);
    }
}

• Invalidate() marks the control dirty; calling Refresh() from a property setter forces an immediate synchronous repaint and makes layout loops worse.
• Decorate public properties with [Category], [Description] and [DefaultValue] so the designer and the property grid behave sensibly.
• To publish an event, use EventHandler<T> or a custom EventArgs subclass - never a bare delegate, or the designer cannot wire it.

Painting well

public partial class GraphView : UserControl
{
    public GraphView()
    {
        InitializeComponent();
        SetStyle(ControlStyles.AllPaintingInWmPaint |
                 ControlStyles.UserPaint |
                 ControlStyles.OptimizedDoubleBuffer |
                 ControlStyles.ResizeRedraw, true);
        SetStyle(ControlStyles.Selectable, true);   // so it can take focus
    }

    protected override void OnResize(EventArgs e)
    {
        base.OnResize(e);
        Invalidate();          // ResizeRedraw also does this, pick one
    }
}

• DoubleBuffered removes flicker for the common case; the three ControlStyles above are the standard combination.
• Never allocate a Pen, Brush or Font in OnPaint without disposing it - GDI handles are a finite resource and the symptom is a global crash, not a leak warning.
• Cache expensive geometry and rebuild it in OnResize, not on every paint.

Owner-draw for lists and trees

combo.DrawMode = DrawMode.OwnerDrawFixed;
combo.ItemHeight = 24;
combo.DrawItem += (s, e) =>
{
    e.DrawBackground();
    if (e.Index < 0) return;

    var item = (Contact)combo.Items[e.Index];
    var colour = item.Online ? Color.SeaGreen : Color.Gray;

    using var dot = new SolidBrush(colour);
    e.Graphics.FillEllipse(dot, e.Bounds.Left + 4, e.Bounds.Top + 7, 10, 10);
    TextRenderer.DrawText(e.Graphics, item.Name, combo.Font,
        new Rectangle(e.Bounds.Left + 20, e.Bounds.Top, e.Bounds.Width - 24, e.Bounds.Height),
        e.ForeColor, TextFormatFlags.VerticalCenter | TextFormatFlags.EndEllipsis);

    e.DrawFocusRectangle();
};

DrawItem for a variable-height list, DrawSubItem for a details-view ListView, DrawNode plus DrawMode = TreeViewDrawMode.OwnerDrawText for a tree. In every case call e.DrawBackground() first and e.DrawFocusRectangle() last, or selection and focus disappear.

FAQ

Related

Drawing, images and printing with GDI+ Events and layout

Last refreshed 2026-09-18.