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;
    }

并且您可以为 Graphics 类型创建两个扩展方法，这样您就可以将它们用作通常的 Draw... 和 Fill...代码> 形状绘制方法.

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("graphics");
        if (pen == null)
            throw new ArgumentNullException("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("graphics");
        if (brush == null)
            throw new ArgumentNullException("brush");

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

2020 年更新:

最近我公开了我的绘图库(NuGet).随意探索 GraphicsExtensions 用于更多重载(每个角的自定义角半径)和其他好东西的类.

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.