How to draw a rounded rectangle in .NETCF

In this short article I'd like to demonstrate how to draw rounded rectangles by P/Invoking the GDI function RoundRect. Let's create an extension method called FillRoundedRectangle to the Graphics class.

In order to use the function we need to create a few GDI objects: a Pen to draw the border, and a Brush to fill the rectangle. We will mostly use P/Invoke for creating and releasing GDI objects

const int PS_SOLID = 0;

const int PS_DASH = 1;

[DllImport("coredll.dll")]

static extern IntPtr CreatePen(int fnPenStyle, int nWidth, uint crColor);

[DllImport("coredll.dll")]

static extern int SetBrushOrgEx(IntPtr hdc, int nXOrg, int nYOrg, ref Point lppt);

[DllImport("coredll.dll")]

static extern IntPtr CreateSolidBrush(uint color);

[DllImport("coredll.dll")]

static extern IntPtr SelectObject(IntPtr hdc, IntPtr hgdiobject);

[DllImport("coredll.dll")]

static extern bool DeleteObject(IntPtr hgdiobject);

[DllImport("coredll.dll")]

static extern bool RoundRect(

    IntPtr hdc, 

    int nLeftRect, 

    int nTopRect, 

    int nRightRect, 

    int nBottomRect, 

    int nWidth, 

    int nHeight);

The CreateSolidBrush function in native code actually takes a COLORREF parameter, and the developer would normally use the RGB macro to create it. We need to translate that macro into a .NET function

static uint GetColorRef(Color value)

{

    return 0x00000000 | ((uint)value.B << 16) | ((uint)value.G << 8) | (uint)value.R;

}

Now we have our P/Invoke definitions in place we can neatly wrap all P/Invoke operations in a single function and let's call that FillRoundedRectangle()

public static void FillRoundedRectangle(

    this Graphics graphics,

    Pen border,

    Color color,

    Rectangle rectangle,

    Size ellipseSize)

{

    var lppt = new Point();

    var hdc = graphics.GetHdc();

    var style = border.DashStyle == DashStyle.Solid ? PS_SOLID : PS_DASH;

    var hpen = CreatePen(style, (int)border.Width, GetColorRef(border.Color));

    var hbrush = CreateSolidBrush(GetColorRef(color));

    try

    {

        SetBrushOrgEx(hdc, rectangle.Left, rectangle.Top, ref lppt);

        SelectObject(hdc, hpen);

        SelectObject(hdc, hbrush);

        RoundRect(hdc, 

                  rectangle.Left, 

                  rectangle.Top, 

                  rectangle.Right, 

                  rectangle.Bottom, 

                  ellipseSize.Width, 

                  ellipseSize.Height);

    }

    finally

    {

        SetBrushOrgEx(hdc, lppt.Y, lppt.X, ref lppt);

        DeleteObject(hpen);

        DeleteObject(hbrush);

        graphics.ReleaseHdc(hdc);

    }

}

Using this extension method should be straight forward, but here's an example where "e" is an instance of PaintEventArgs:

using (var pen = new Pen(Color.Blue))

    e.Graphics.FillRoundedRectangle(

        pen, 

        Color.LightBlue, 

        new Rectangle(10, 10, 100, 100), 

        new Size(16, 16));

I hope you found this useful. If you're interested in the full source code then you can grab it here