Dialog has unrequested additional space

Question

I've designed a Windows Forms dialog that should be reusable in other applications, WPF and Windows Forms. This works fine when I use it in a Windows Forms application, but it causes some layout trouble when called in a WPF application. Dimensions and sizes are inconsistent when measured from the pixels on the screen, from what WinForms API says it is, and from Spy++. The window is 10 pixels wider and taller when run without a debugger than Spy++ says it is, and than I say it should be. What's the problem here? I can't find anything but say it's a badly broken .NET Framework.

Here's the Form class code:

using System;
using System.Drawing;
using System.Windows.Forms;

namespace DialogTestApp
{
    internal class MyDialog : Form
    {
        public MyDialog()
        {
            Text = "Title";
            Width = 500;   // -> actually 510 (Spy++ says 500)
            Height = 300;   // -> actually 310 (Spy++ says 300)
            Font = SystemFonts.MessageBoxFont;
            FormBorderStyle = FormBorderStyle.FixedDialog;
            MaximizeBox = false;
            MinimizeBox = false;
            ShowInTaskbar = false;
            StartPosition = FormStartPosition.CenterScreen;

            TableLayoutPanel mainLayout = new TableLayoutPanel();
            mainLayout.BackColor = Color.FromArgb(255, 171, 255);   // pink
            mainLayout.Dock = DockStyle.Fill;
            mainLayout.Margin = Padding.Empty;
            mainLayout.Padding = Padding.Empty;
            mainLayout.RowStyles.Add(new RowStyle(SizeType.AutoSize));   // Only use minimum required space
            mainLayout.RowStyles.Add(new RowStyle(SizeType.Percent, 100));
            Controls.Add(mainLayout);

            int row = 0;
            Label label = new Label();
            label.Text = "Hello world. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque suscipit vestibulum gravida.";
            label.Font = new Font(Font.FontFamily, 12);
            label.MaximumSize = new Size(mainLayout.Width, 0);
            label.AutoSize = true;
            label.Dock = DockStyle.Fill;
            label.Margin = Padding.Empty;
            label.BackColor = Color.FromArgb(58, 171, 58);   // green
            label.ForeColor = Color.White;
            mainLayout.Controls.Add(label, 0, row++);

            TextBox textBox = new TextBox();
            textBox.Dock = DockStyle.Fill;
            textBox.Margin = Padding.Empty;
            textBox.Multiline = true;
            textBox.ScrollBars = ScrollBars.Both;
            mainLayout.Controls.Add(textBox, 0, row++);
        }
    }
}

Just put this file in an empty WPF application project and call it from the application constructor:

public MainWindow()
{
    InitializeComponent();

    new MyDialog().ShowDialog();
    Application.Current.Shutdown();
}

Here's what it looks with a debugger:

And without:

The additional pink border is the 10 pixels that are not supposed to be there. The green label is set to fill up all space.

Answer 1

Even without the TableLayoutPanel, Label, and using System.Windows.Forms.Application.Run(new MyDialog()), the problem still happens. The line that is causing the issue is FormBorderStyle = FormBorderStyle.FixedDialog;

Seems to be the same issue as described here: Form tells wrong size on Windows 8 — how to get real size?

Workaround:

        mainLayout.SizeChanged += delegate {
            label.MaximumSize = new Size(mainLayout.Width, 0);
            //MessageBox.Show("hi"); // called when not ran in debugger
        };