Question

In C#, am able to draw an ellipse on a screen device context taken from the Windows Calculator (calc.exe) window using the .NET System.Graphics.DrawEllipse method.

I would like to be able to do the same thing with the GDI32 Ellipse method. How can I make Ellipse draw to the screen?

In the following code, this line works:
CalculatorGraphics.DrawEllipse(penRed, 50, 50, 50, 50);
But this line does not:
PlatformInvokeGDI32.Ellipse(hDC, 100, 100, 100, 100);
What is the problem?

        //In size variable we shall keep the size of the window.
        SIZE size;

        //Win32 API functions are imported in classes
        //PlatformInvokeGDI32
        //PlatformInvokeUSER32.cs

        //Get handle of calc.exe window.
        IntPtr hwnd = PlatformInvokeUSER32.FindWindow("SciCalc", "Calculator");

        //Get window dimensions
        PlatformInvokeUSER32.RECT rect;
        PlatformInvokeUSER32.GetWindowRect(hwnd, out rect);
        size.cx = rect._Right - rect._Left;
        size.cy = rect._Bottom - rect._Top;

        //Get the device context of Calculator.
        IntPtr hDC = PlatformInvokeUSER32.GetDC(hwnd);

        //Draw on the Calculator surface.
        Graphics CalculatorGraphics = Graphics.FromHdc(hDC);
        Color colorRed = Color.FromName("Red");
        Pen penRed = new Pen(colorRed);
        CalculatorGraphics.DrawEllipse(penRed, 50, 50, 50, 50);
        CalculatorGraphics.Save();

        PlatformInvokeGDI32.COLORREF cl;
        cl.R = 255;
        cl.G = 0;
        cl.B = 0;

        PlatformInvokeGDI32.SetDCBrushColor(hDC, cl);
        PlatformInvokeGDI32.SetDCPenColor(hDC, cl);
        //PlatformInvokeGDI32.SetBkColor(hDC, cl);
        PlatformInvokeGDI32.Ellipse(hDC, 100, 100, 100, 100);
        PlatformInvokeGDI32.SaveDC(hDC);


        //Here we make a compatible device context in memory for screen device context.
        IntPtr hMemDC = PlatformInvokeGDI32.CreateCompatibleDC(hDC);


        //Create a compatible bitmap of window size and using screen device context.
        m_HBitmap = PlatformInvokeGDI32.CreateCompatibleBitmap(hDC, size.cx, size.cy);

        //As m_HBitmap is IntPtr we can not check it against null. For this purspose IntPtr.Zero is used.
        if (m_HBitmap != IntPtr.Zero)
        {
            //Here we select the compatible bitmap in memeory device context and keeps the refrence to Old bitmap.
            IntPtr hOld = (IntPtr)PlatformInvokeGDI32.SelectObject(hMemDC, m_HBitmap);
            //We copy the Bitmap to the memory device context.
            PlatformInvokeGDI32.BitBlt(hMemDC, 0, 0, size.cx, size.cy, hDC, 0, 0, PlatformInvokeGDI32.SRCCOPY);
            //We select the old bitmap back to the memory device context.
            PlatformInvokeGDI32.SelectObject(hMemDC, hOld);
            //We delete the memory device context.
            PlatformInvokeGDI32.DeleteDC(hMemDC);
            //We release the screen device context.
            PlatformInvokeUSER32.ReleaseDC(hwnd, hDC);
            //Image is created by Image bitmap handle and returned.
            return System.Drawing.Image.FromHbitmap(m_HBitmap);
        }
        //If m_HBitmap is null retunrn null.
        return null;

Answer 1

I see how I misunderstood the definition of the Ellipse method.

The last two parameters of the Ellipse method to refer to the distance from the left-most point and right-most point. Instead, they refer to the position of the right-most point and bottom-most point.

The last two parameters of the DrawEllipse method do refer to the horizontal and vertical distance of the bottom-right point from the top-left point of the bounding rectangle.

Ellipse(hDC, 100, 100, 200, 200);

Means approximately the same thing as

DrawEllipse(penRed, 100, 100, 100, 100); 

Here is the MSDN documentation on the Ellipse method:

hdc [in]
A handle to the device context.

nLeftRect [in]
The x-coordinate, in logical coordinates, of the upper-left corner of the bounding rectangle.

nTopRect [in]
The y-coordinate, in logical coordinates, of the upper-left corner of the bounding rectangle.

nRightRect [in]
The x-coordinate, in logical coordinates, of the lower-right corner of the bounding rectangle.

nBottomRect [in]
The y-coordinate, in logical coordinates, of the lower-right corner of the bounding rectangle.