High DPI, accessibility and theming

Per-monitor DPI awareness, autoscaling pitfalls, screen reader names and tab order, keyboard-only operation, and the honest state of dark mode in WinForms.

Per-monitor DPI

<!-- App.manifest: opt in to per-monitor v2, not just system aware -->
<application xmlns="urn:schemas-microsoft-com:asm.v3">
  <windowsSettings>
    <dpiAwareness xmlns="http://schemas.microsoft.com/SMI/2016/WindowsSettings">PerMonitorV2</dpiAwareness>
    <dpiAware xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">true/pm</dpiAware>
  </windowsSettings>
</application>

public partial class MainForm : Form
{
    public MainForm()
    {
        InitializeComponent();
        AutoScaleMode = AutoScaleMode.Dpi;     // scale from the font/DPI, not pixels
        AutoScaleDimensions = new SizeF(96F, 96F);
    }

    protected override void OnDpiChangedAfterParent(EventArgs e)
    {
        base.OnDpiChangedAfterParent(e);
        // anything laid out by hand must be recomputed here
        RebuildToolbarIcons(DeviceDpi);
    }
}

• AutoScaleMode.Font is the default and is usually right; Dpi is the honest choice when you place things programmatically.
• Never hard-code pixel sizes and then also let autoscaling run - you get double scaling and clipped text.
• Anchoring and docking scale correctly; absolute coordinates do not. This is the main practical reason to prefer layout panels plus Dock and Anchor.
• DeviceDpi is the current monitor's value; LogicalToDeviceUnits converts a design-time measurement.

Accessibility essentials

// the accessible name a screen reader announces
nameBox.AccessibleName = "Customer name";
nameBox.AccessibleDescription = "The full name as it appears on the invoice";

// keep related controls in the reading order the eye sees
label.Tag = nameBox;
label.UseMnemonic = true;
label.Text = "&Customer name:";        // Alt+C focuses the box

// give the custom control a real role and a value
statusPill.AccessibleRole = AccessibleRole.StatusBar;
statusPill.AccessibleName = "Connection";

// name the form itself
Text = "Order tool";
AccessibleName = "Order tool";

Check three things on any form: every control reachable with Tab only, no control that steals focus back, and every icon-only button carrying an AccessibleName - a screen reader reads an unnamed button as just "button".

Theming and dark mode

WinForms has no theme system. The supported route is Application.SetColorMode on .NET 9 and later, which switches supported controls to the system dark theme; anything drawn by hand must follow it.

// Program.cs - .NET 9 and later
Application.SetColorMode(SystemColorMode.System);

// older targets: build a small palette and apply it in one pass
static void ApplyDark(Control root)
{
    root.BackColor = Color.FromArgb(32, 32, 32);
    root.ForeColor = Color.Gainsboro;
    foreach (Control child in root.Controls)
    {
        if (child is TextBoxBase or ListControl)
        {
            child.BackColor = Color.FromArgb(45, 45, 48);
            child.ForeColor = Color.Gainsboro;
        }
        ApplyDark(child);
    }
}

FAQ

Related

Drawing, images and printing with GDI+ Deployment: ClickOnce, MSIX and single-file

Last refreshed 2026-09-18.