How to draw a rounded rectangle in c#

public static GraphicsPath RoundedRect(Rectangle bounds, int radius)
{
    int diameter = radius * 2;
    Size size = new Size(diameter, diameter);
    Rectangle arc = new Rectangle(bounds.Location, size);
    GraphicsPath path = new GraphicsPath();

    if (radius == 0)
    {
        path.AddRectangle(bounds);
        return path;
    }

    // top left arc  
    path.AddArc(arc, 180, 90);

    // top right arc  
    arc.X = bounds.Right - diameter;
    path.AddArc(arc, 270, 90);

    // bottom right arc  
    arc.Y = bounds.Bottom - diameter;
    path.AddArc(arc, 0, 90);

    // bottom left arc 
    arc.X = bounds.Left;
    path.AddArc(arc, 90, 90);

    path.CloseFigure();
    return path;
}

And you can make two extension methods for the Graphics type so you can use them as the usual Draw... and Fill... shape-drawing methods.

public static void DrawRoundedRectangle(this Graphics graphics, Pen pen, Rectangle bounds, int cornerRadius)
{
    if (graphics == null)
        throw new ArgumentNullException(nameof(graphics));
    if (pen == null)
        throw new ArgumentNullException(nameof(pen));

    using (GraphicsPath path = RoundedRect(bounds, cornerRadius))
    {
        graphics.DrawPath(pen, path);
    }
}

public static void FillRoundedRectangle(this Graphics graphics, Brush brush, Rectangle bounds, int cornerRadius)
{
    if (graphics == null)
        throw new ArgumentNullException(nameof(graphics));
    if (brush == null)
        throw new ArgumentNullException(nameof(brush));

    using(GraphicsPath path = RoundedRect(bounds, cornerRadius))
    {
        graphics.FillPath(brush, path);
    }
}

Update 2020:

Recently I made my drawing libraries publicly available (NuGet). Feel free to explore the GraphicsExtensions class for more overloads (custom corner radius for each corners), and for other goodies.