C# borderless form without titlebar

I haven't done WinForms in a long time, but try FormBorderStyle.FixedSingle instead.

EDIT =============================

You can always use interop and use the Win32 API to set the window style. It's kind of involved, but it's entirely possible do do. Keep in mind that once you take off the titlebar, the user can't move the window unless you write code to allow it (if I recall correctly, the API method you're interested in is CreateWindowEx). Check out www.pinvoke.net - they have prototypes and constants available fot support most Windows API methods.

What you're trying to do does not comply with the Windows UI best practice, so expect to have to write a bunch of weird code to realize your dream. It has been my experience that a non-standard UI are wasted on your typical user, who simply want to get the job done and close the app. Just sayin...

About the best you can do is to set the ControlBox property to false, and the Text property to an empty string. It leaves a "short" bar at the top (about 5 pixels) but it remains usable and sizeable.

check this c# - How to move and resize a form without a border? - Stack Overflow[^]

tested it, working..
not sure it will fit your requirement. give a try.

Here's one solution that results in no Form Border, and renders the Form re-sizable:

1. set 'FormBorderStyle to 'FixedToolWindow, and hide the other usual stuff.

2. this example does not include code for moving the Form; that's left for you to write.

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

namespace YourNameSpace
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        
        const int MoveZoneSize = 16;
        const int ReSizeThreshold = 4;

        private Rectangle scrnBounds;
        private Rectangle moveZone;

        // for use when making the Form movable
        private int mdx, mdy, dx, dy;
        
        private bool mouseIsUp = true;

        private void Form1_Load(object sender, EventArgs e)
        {
            UpdateScreenSize();
        }

        private void UpdateScreenSize()
        {
            moveZone = new Rectangle(0,0,MoveZoneSize,MoveZoneSize);
            scrnBounds = this.DisplayRectangle;
            moveZone.Offset(scrnBounds.Right - MoveZoneSize, scrnBounds.Bottom - MoveZoneSize);    
        }

        private void Form1_KeyDown(object sender, KeyEventArgs e)
        {
            if (ModifierKeys == Keys.Alt && e.KeyCode == Keys.Escape)
            {
                this.MouseMove -= Form1_MouseMove;
                this.Close();
            }
        }

        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
            if (moveZone.Contains(e.Location))
            {
                mdx = e.X;
                mdy = e.Y;

                this.MouseMove += Form1_MouseMove;
            }
        }
        
        private void Form1_MouseUp(object sender, MouseEventArgs e)
        {
            this.MouseMove -= Form1_MouseMove;
            UpdateScreenSize();
        }

        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            dx = Math.Abs(e.X - mdx);
            dy = Math.Abs(e.Y - mdy);

            if (dx < ReSizeThreshold && dy < ReSizeThreshold) return;

            this.Width = e.X;
            this.Height = e.Y;
        }
    }
}

Notes:

1. a Rectangle (square) 'moveZone is created that encloses an area of the bottom-right of the Form of size 'MoveZoneSize.

2. a threshold for "dampening" mouse move events 'ReSizeThreshold is defined. this may make the resizing of the Form visually smoother.

3. mouse down is detected and if the mouse down location is in the 'moveZone bounds, the mouse move handler is wired up to the Form.

4. in the mouse move event, the delta of mouse movement is calculated, and if it's smaller than 'ReSizeThreshold the method exits. if the delta is larger, then the Form is resized, and the current bounds of the form and the bounds of the 'moveZone rectangle are re-calculated.

Other comments:

1. depending on computer, graphics card, memory, etc., and the possible complexity of Form elements and their use of Anchor and Dock, Padding, Margin, etc. Properties: the visual quality the Form resize by mouse at run-time ... may vary. Of course, you should set the Form's 'DoubleBuffered Property to 'true.