How can I change Windows Border Style?

Here's a typical example of enabling both moving, and re-sizing a WinForm that has no TitleBar, FormBorderStyle set to 'None, etc.

The strategy used here is:

1. define two "zones:" a zone where the mouse held down and moving will move the Form, and a zone where the mouse held down and moving will re-size the Form.

2. check in the MouseDown EventHandler for a hit in either of those two zones, and set boolean flags.

3. in the MouseMove EventHandler take appropriate action.

4. if the Form has been re-sized, reset the re-size zone in the Form Resize EventHandler:

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

namespace ResizableMovableForm
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private Rectangle MoveZone;
        private Rectangle ResizeZone;

        private const int HitZoneSize = 12;

        private bool isMoving = false;
        private bool isResizing = false;
        private bool IsMouseUp = false;
        private int startWidth, startHeight, startMouseX, startMouseY;
        private int newWidth, newHeight;

        private void Form1_Load(object sender, EventArgs e)
        {
            MoveZone = new Rectangle(0, 0, HitZoneSize, HitZoneSize);
            ResizeZone = new Rectangle(Width - HitZoneSize, Height - HitZoneSize, HitZoneSize, HitZoneSize);
        }

        private void button1_Click(object sender, EventArgs e)
        {
        }

        private void Form1_MouseUp(object sender, MouseEventArgs e)
        {
            IsMouseUp = true;
            isMoving = false;
            isResizing = false;
        }

        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
            startMouseX = e.X;
            startMouseY = e.Y;
            startWidth = this.Width;
            startHeight = this.Height;

            if (MoveZone.Contains(e.Location))
            {
                isMoving = true;
                IsMouseUp = false;
                return;
            }

            if (ResizeZone.Contains(e.Location))
            {
                isResizing = true;
                IsMouseUp = false;
                return;
            }
        }

        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            if (IsMouseUp) return;

            if (isMoving)
            {
                this.Left += e.X - startMouseX;
                this.Top += e.Y - startMouseY;
                return;
            }
            else
            {
                if (isResizing)
                {
                    newWidth = startWidth + e.X - startMouseX;
                    if (newWidth < this.MinimumSize.Width || newWidth > this.MaximumSize.Width) return;
                    this.Width = newWidth;

                    newHeight = startHeight + e.Y - startMouseY;
                    if (newHeight < this.MinimumSize.Height || newHeight > this.MaximumSize.Height) return;
                    this.Height = newHeight;
                }
            }
        }

        private void Form1_Resize(object sender, EventArgs e)
        {
            ResizeZone = new Rectangle(Width - HitZoneSize, Height - HitZoneSize, HitZoneSize, HitZoneSize);
        }
    }
}

Notes:

1. I strongly encourage you to set appropriate Minimum and Maximum sizes for your Form at design-time. The code above (for re-size) depends on having some meaningful values for those properties.

2. Even with the Form's DoubleBuffer property set to 'true, there may be some graphic side-effects of re-sizing which cannot be controlled by using SuspendLayout and ResumeLayout.