Creates custom controls and UserControls for modern WinForms (.NET 6+). Use when deriving from Control, UserControl, Button, TextBox, or other base controls, implementing custom rendering with OnPaint overrides, creating composite controls with child control composition, adding custom properties with [Browsable], [Category], or [DefaultValue] attributes for Designer support, implementing INotifyPropertyChanged for control properties, handling Layout/LayoutEngine for custom sizing, creating scrollable controls with AutoScrollMinSize, implementing dark mode theming, or building List/Grid/Tree-like controls. Also triggers for "custom UserControl", "derive from Control", "owner-drawn control", "Designer properties", "[Browsable] attribute", "custom WinForms control", "LayoutEngine", "AutoScroll control", and "themed control".
Creates custom controls and UserControls for modern WinForms (.NET 6+). Use when deriving from Control, UserControl, Button, TextBox, or other base controls, implementing custom rendering with OnPaint overrides, creating composite controls with child control composition, adding custom properties with [Browsable], [Category], or [DefaultValue] attributes for Designer support, implementing INotifyPropertyChanged for control properties, handling Layout/LayoutEngine for custom sizing, creating scrollable controls with AutoScrollMinSize, implementing dark mode theming, or building List/Grid/Tree-like controls. Also triggers for "custom UserControl", "derive from Control", "owner-drawn control", "Designer properties", "[Browsable] attribute", "custom WinForms control", "LayoutEngine", "AutoScroll control", and "themed control".
Authoring custom controls and UserControls for modern WinForms (.NET 6+, C# 13/14): base-class selection, clip-proof layout/sizing, owner-draw, dark-mode theming, Designer serialization, and List/Grid controls.
Use this skill when asked to create, derive, or modernize a WinForms control, UserControl, owner-drawn control, themed/dark-mode-aware control, scrollable Control-derived control, or a List/Grid-like control, and when wiring up Designer serialization for new control properties.
Use it for designing UserControl to offload complex UI from forms, or when a reusable control is needed across multiple forms/projects. Also use it for modernizing legacy controls to be dark-mode aware and DPI-aware.
var discipline — use var only when the right-hand side is a constructor call or cast that makes the type obvious (e.g., var cp = new CreateParams()). Use explicit types for int, bool, string, float, double, Color, Size, Point, and similar short type names.
Expression-bodied members — use them for single-expression getters, simple one-liner methods, and read-only properties. Use block bodies when the logic has side effects or exceeds ~100 characters.
Blank line before return — improves readability in multi-statement methods.
Be brief — the code carries the intent.
1. Pick the right base class
Decide generic reusable control vs composite/LOB control first.
Generic, no domain (a slider, badge, gauge, custom button): derive from Control, or the closest specialized base (ButtonBase, ScrollableControl, ListControl, Panel, Label, —¦). Control gives you a blank canvas with full paint/input control. TextBoxBase is not available — it is internal; if you need text-box-like behavior, derive from TextBox or compose one.
Composite / LOB / domain-specific: derive from UserControl and assemble constituent controls (see §5).
List- or grid-shaped data: derive from DataGridView, not Control from scratch (see §6).
Prefer the most specific base that already solves the hard parts — don't reimplement ButtonBase's focus/click mechanics on a raw Control.
2. Layout & sizing — make it clip-proof
A control that looks fine at 100% DPI with the default font but clips at 200% / large fonts is broken. Treat sizing as first-class wherever content needs a minimum real estate to render fonts/glyphs/images without clipping.
Implement these as a coherent set:
GetPreferredSize(Size proposedSize) — return the size genuinely needed for current content, font, padding, and DPI. Measure text with TextRenderer.MeasureText (it matches TextRenderer.DrawText). Always add Padding.Size. This is the single source of truth for "how big do I need to be."
AutoSize — expose and honor it. When true, the layout engine calls GetPreferredSize; your job is to return an honest number. Re-raise layout when size-affecting content changes (PerformLayout() / Invalidate()).
SetBounds(...) — override when you must clamp or adjust the bounds you're given (e.g. enforce a minimum). Respect the BoundsSpecified flags so you don't clobber a dimension the caller didn't set.
Padding — honor it in painting andGetPreferredSize. Content draws inside the padded rectangle.
Rule of thumb: an honest GetPreferredSize plus honored AutoSize/Padding survives DPI and font scaling automatically.
A typical GetPreferredSize for a text-bearing Control-derived control:
OnTextChanged / OnFontChanged overrides should call PerformLayout() (and Invalidate() if owner-drawn) so a new preferred size is picked up while AutoSize is on.
3. Owner-draw → always double-buffer
When you take over painting (OnPaint, owner-draw cells, custom rendering), enable double buffering to eliminate flicker:
Do all painting in OnPaint — don't scatter logic into OnPaintBackground. Call Invalidate() (not Refresh()) to request a repaint so it coalesces with other invalidations rather than forcing a synchronous redraw.
4. Dark mode & theming
Modern WinForms supports dark mode. Two things bite custom controls:
4.1 Win32 scrollbars on a Control-derived class are not themed automatically
If you add scrollbars by calling Win32 (WS_HSCROLL / WS_VSCROLL, ShowScrollBar, etc.) on a Control subclass, those scrollbars render in the classic (light) theme even in dark mode. To get them themed, override CreateParams and opt in:
protectedoverride CreateParams CreateParams
{
get
{
SetStyle(ControlStyles.ApplyThemingImplicitly, true); // opt-in; false = opt-out
CreateParams cp = base.CreateParams;
// add WS_HSCROLL / WS_VSCROLL here as neededreturn cp;
}
}
This cannot be done in the constructor: the base constructor reads CreateParamsbefore your constructor body runs. It must live in the CreateParams getter, with the SetStyle call placed beforebase.CreateParams is read.
4.2 SystemColors flip in dark mode — names become counter-intuitive
In dark mode, SystemColors are remapped to roughly complementary values. The name no longer matches the apparent brightness, and that is intentional: ControlLightLight is very bright in classic mode and correspondingly dark in dark mode. The remap is not always a strict mathematical complement — some colors are nudged to keep contrast adequate.
Consequences:
If you use SystemColors, accept that the name describes its classic-mode role, not its color.
For guaranteed contrast in both modes, don't use SystemColors for your control's palette. Define your own colors as explicit #AARRGGBB values with separate defaults for classic and dark mode.
Pick the active set at runtime via Application.IsDarkModeEnabled, which reflects the current mode:
Color faceColor = Application.IsDarkModeEnabled
? Color.FromArgb(unchecked((int)0xFF2D2D30))
: Color.FromArgb(unchecked((int)0xFFF0F0F0));
5. UserControls & composite custom controls
For a UserControl (or any control composed of constituent controls):
No design-time Font definitions. Don't set Font on the UserControl or its children at design time. Design-time fonts pollute the HighDPI AutoScaleDimensions code generation, so when the user later re-opens the control in the Designer the generated scaling code is wrong. Adjust font size only at runtime.
Layout in a TableLayoutPanel with AutoSize rows/columns. Let the table compute geometry.
The control itself exposes AutoSize and an honest GetPreferredSize big enough to render every child unclipped across all HighDPI and large-font scenarios (§2). The TableLayoutPanel with AutoSize cells does most of this for you, but verify the outer control reports it.
Very large UserControls with many containers/controls where a single clip-proof preferred size is impractical: host the content in a scrolling Panel (AutoScroll = true) rather than forcing an unreasonable minimum size.
6. List- / Grid-like controls
When asked for anything list- or grid-shaped, derive from DataGridView rather than building from scratch.
For image-rich or graphically rich data records, the best outcome is usually to render the entire data item in one cell via a custom DataGridViewCell — symbol fonts, multiple colors, multiple font sizes, multi-line layout, all in a single cell. Skip column headers in that case.
Keep column headers (and therefore per-field columns) only when the context genuinely needs a schema overview or per-column sort/filter. Even then, prefer custom cell rendering to get the rich graphical UI; don't fall back to plain text just because headers exist.
Whenever cells vary in height, compute row height from the tallest cell in the data row — measure each cell's required height and set the row to the max, or the content clips.
7. Designer serialization for new properties
Every public property you add must tell the Designer how to serialize it. Pick exactly one of these three mechanisms (they conflict if combined):
[DesignerSerializationVisibility(...)] — Hidden for runtime-only/derived properties the Designer must not write to InitializeComponent; Content for collections whose items serialize individually.
[DefaultValue(...)] — the property serializes only when its value differs from this constant. Use for simple properties with a fixed, compile-time-constant default.
ResetXxx() + ShouldSerializeXxx() pair — ShouldSerializeXxx returns whether the value should be written; ResetXxx restores the default. Use this for ambient properties, and whenever a default is not constant (depends on another property or the theme) — [DefaultValue] can't express that.
The method pair, for a property whose default depends on the current theme:
///<summary>Gets or sets the color of the control's face.</summary>public Color FaceColor { get; set; } = DefaultFaceColor;
privatestatic Color DefaultFaceColor
=> Application.IsDarkModeEnabled
? Color.FromArgb(unchecked((int)0xFF2D2D30))
: Color.FromArgb(unchecked((int)0xFFF0F0F0));
// The Designer discovers these by naming convention — keep them private.privateboolShouldSerializeFaceColor()
=> FaceColor != DefaultFaceColor;
privatevoidResetFaceColor()
=> FaceColor = DefaultFaceColor;
Note the convention: the methods are private, named exactly ShouldSerialize<Property> / Reset<Property>, and take no parameters. Don't combine them with [DefaultValue] on the same property — the two mechanisms conflict.
Don't leave a property with none of these — the Designer will either over-serialize or fail to round-trip.
7.1 Property-window attributes
Beyond serialization, decorate every new public property so it behaves well in the Properties window and IntelliSense. These are four distinct concerns:
[Category("Appearance")] — the Properties-window group. Reuse standard names (Appearance, Behavior, Layout, Data, —¦) so your properties merge with the built-in ones.
[Description("...")] — help text in the Properties window's description pane. Write it for a control consumer.
[Browsable(false)] — hides the property from the Properties window. Use for runtime-only state. It almost always also needs [DesignerSerializationVisibility(Hidden)] — hiding from the grid does not stop serialization.
[EditorBrowsable(EditorBrowsableState.Never | Advanced)] — controls IntelliSense visibility, independent of the grid. Never hides from completion; Advanced shows only when "Hide advanced members" is off.
To fully suppress an inherited property that's meaningless on your control (e.g. shadowing Text), combine all three on the new/override member:
[Browsable(false)]
[EditorBrowsable(EditorBrowsableState.Never)]
[DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
publicoverridestring Text
{
get => base.Text;
set => base.Text = value;
}
A normal, visible property reads like this:
[Category("Appearance")]
[Description("The color used to paint the control's face.")]
public Color FaceColor { get; set; } = DefaultFaceColor;
8. Construction order — ISupportInitialize
During InitializeComponent, the Designer sets your properties one at a time in an order you don't control. If two properties are interdependent — or a setter does expensive work (re-layout, allocation, a Win32 round-trip) — the control repeats that work against a half-configured state, and may even throw because property B isn't set yet when property A's setter runs.
Implement ISupportInitialize to defer it. The Designer emits BeginInit() before the property block and EndInit() after; do nothing expensive in between:
Every interdependent setter checks _initializing and skips the expensive path while it's true. EndInit runs that path exactly once. At runtime (no Designer), code that sets properties directly without calling BeginInit/EndInit still works — _initializing is just false, so each setter does its work immediately.
9. Designer support code (.NET 6+)
Designer functionality in .NET 6+ requires the WinForms Designer SDK NuGet package.
Use the current preview: Microsoft.WinForms.Designer.SDK 1.13.0-preview.2.24575.3. The latest stable (1.6.0) is missing features you'll likely need — prefer the preview.
Without shipping a separate Designer NuGet package, you can put the following in the same .NET assembly as the control: custom CodeDomSerializers, custom TypeConverters, and custom DesignerActionList (smart-tag) actions.
Out of scope for this skill: Designers with a custom design-time UI. Those can't live in the control assembly — they need a dedicated multi-assembly NuGet package (client side on .NET Framework 4.7.2, server-side process on .NET, and a multi-targeted communication layer). If the user needs that, say so and stop.
Quick checklist
Most specific base class (Control generic, UserControl composite, DataGridView list/grid).
Owner-draw → OptimizedDoubleBuffer | AllPaintingInWmPaint | UserPaint; repaint via Invalidate().
Dark mode: own #AARRGGBB palette per mode, switched on Application.IsDarkModeEnabled; Win32 scrollbars themed via CreateParams + ApplyThemingImplicitly.
UserControl: no design-time fonts, TableLayoutPanel with AutoSize cells, scrolling Panel if too large.
Every new property: [DefaultValue], [DesignerSerializationVisibility], or ShouldSerializeXxx/ResetXxx; plus [Category]/[Description].
Interdependent or expensive property setters → ISupportInitialize with a _initializing guard.
Designer SDK preview 1.13.0-preview.2.24575.3 if design-time code is involved.
