Forms and controls

How a WinForms application starts, the controls you will reach for first, and the designer files that are generated for you.

How the application starts

A WinForms app is a Windows message loop. Application.Run starts that loop and returns when the main form closes. The [STAThread] attribute on Main is required: Windows common dialogs and the clipboard need a single-threaded apartment.

// Program.cs
using System;
using System.Windows.Forms;

internal static class Program
{
    [STAThread]
    private static void Main()
    {
        ApplicationConfiguration.Initialize();   // sets DPI mode and visual styles
        Application.Run(new MainForm());
    }
}

// MainForm.cs - code only, no designer
using System.Windows.Forms;

public sealed class MainForm : Form
{
    public MainForm()
    {
        Text = "Inventory";
        Width = 900;
        Height = 600;
        StartPosition = FormStartPosition.CenterScreen;

        var label = new Label { Text = "Search", AutoSize = true, Location = new System.Drawing.Point(12, 15) };
        var box   = new TextBox { Location = new System.Drawing.Point(70, 12), Width = 240 };
        var grid  = new DataGridView { Dock = DockStyle.Bottom, Height = 420 };

        AcceptButton = new Button { Text = "Go", DialogResult = DialogResult.OK };

        Controls.Add(label);
        Controls.Add(box);
        Controls.Add(grid);
    }
}

• ApplicationConfiguration.Initialize() replaces the older Application.EnableVisualStyles() plus SetCompatibleTextRenderingDefault pair and also sets the high-DPI mode from the project file.
• Controls.Add order matters for Dock layout — the last control added docks first, so add the fill target last if you are not using a designer.
• Set AcceptButton and CancelButton so Enter and Escape close a dialog as users expect.

The control map

// Adding many rows without flicker: suspend layout and painting
listBox1.BeginUpdate();
try
{
    foreach (var item in LoadItems())
        listBox1.Items.Add(item);
}
finally
{
    listBox1.EndUpdate();
}

// A modal dialog returns a result instead of raising events
using var dialog = new SettingsForm();
if (dialog.ShowDialog(this) == DialogResult.OK)
    ApplySettings(dialog.Settings);

The generated designer file

The Visual Studio designer splits a form into MainForm.cs (your handler code) and MainForm.Designer.cs (control declarations and layout). The generated file is rewritten on every designer save, so any edit you make there is lost.

// MainForm.Designer.cs - generated, do not hand-edit
partial class MainForm
{
    private System.ComponentModel.IContainer components = null;
    private System.Windows.Forms.Button saveButton;
    private System.Windows.Forms.DataGridView grid;

    protected override void Dispose(bool disposing)
    {
        if (disposing && components != null) components.Dispose();
        base.Dispose(disposing);
    }

    private void InitializeComponent()
    {
        this.saveButton = new System.Windows.Forms.Button();
        this.grid = new System.Windows.Forms.DataGridView();
        this.SuspendLayout();

        this.saveButton.Text = "Save";
        this.saveButton.Click += new System.EventHandler(this.SaveButton_Click);
        this.grid.Dock = System.Windows.Forms.DockStyle.Fill;

        this.Controls.Add(this.grid);
        this.Controls.Add(this.saveButton);
        this.ResumeLayout(false);
    }
}

• Put custom logic in the hand-written partial class, never in InitializeComponent.
• The designer does not understand loops or conditionals in layout; if you need dynamic controls, build them in code at runtime.
• Review designer diffs in a pull request. A stray drag can silently re-parent a control or change a Dock value.
• Deleting a control in the designer leaves its event handler method behind as dead code — clean it up.

FAQ

Related

Events and layout Data binding and when not to use WinForms

Last refreshed 2026-09-18.