Rounded Corners in C# WinForms | Modern UI

Want to give your Windows Forms app a modern and stylish look? One simple tweak can make your form stand out - rounded corners!
In this guide, you’ll learn how to create smooth, rounded edges for any C# WinForms window using the GDI32 API.

Why Add Rounded Corners?​

Rounded corners make your app look cleaner and more polished - similar to modern Windows 11 apps.
Instead of using the default rectangular edges, you can easily customize your form’s region with just a few lines of code.

Full C# Code Example​

Here’s the complete C# code to create a borderless form with rounded corners

using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        [DllImport("Gdi32.dll", EntryPoint = "CreateRoundRectRgn")]
        private static extern IntPtr CreateRoundRectRgn
        (
            int nLeftRect,     // x-coordinate of upper-left corner
            int nTopRect,      // y-coordinate of upper-left corner
            int nRightRect,    // x-coordinate of lower-right corner
            int nBottomRect,   // y-coordinate of lower-right corner
            int nWidthEllipse, // width of ellipse
            int nHeightEllipse // height of ellipse
        );

        public Form1()
        {
            InitializeComponent();
            this.FormBorderStyle = FormBorderStyle.None;
            Region = System.Drawing.Region.FromHrgn(CreateRoundRectRgn(0, 0, Width, Height, 20, 20));
        }
    }
}

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Code Explanation​

Let’s break down what’s happening step by step:

Importing the Windows API​

The [DllImport] attribute lets you use functions from native DLLs.
Here, CreateRoundRectRgn is imported from Gdi32.dll, part of the Windows GDI (Graphics Device Interface).

Defining the Function​

This function creates a region in the shape of a rounded rectangle.
You provide:

• Form dimensions (Width, Height)
• Corner radius (20, 20 in this example)

Removing Default Borders​

this.FormBorderStyle = FormBorderStyle.None;
This removes the default rectangular window frame so your custom rounded region can take over.

Applying the Rounded Region​

Finally,

Region = System.Drawing.Region.FromHrgn(CreateRoundRectRgn(...));

assigns the new region to your form, making it display with smooth rounded edges

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Customize the Corner Radius​

You can change the last two parameters in CreateRoundRectRgn to adjust the curve:

Region = Region.FromHrgn(CreateRoundRectRgn(0, 0, Width, Height, 40, 40));

Higher numbers → more rounded corners
Lower numbers → subtle curve

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Pro Tip: Add a Drop Shadow​

If you’re removing borders, your window can look flat.
To fix that, add a drop shadow effect using the Windows API or form painting methods for a sleek floating look

Wrap-Up​

That’s it! With a few lines of code, you can give your old-school WinForms UI a modern refresh. Rounded corners make your C# applications look more elegant, especially when combined with custom colors or gradient backgrounds
Now you’re one step closer to building professional-looking desktop apps with C# and .NET!