CodeProject: Getting the printer's REAL margin bounds in .NET. Free source code and programming help

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Introduction

In this article, I present a very useful class that works around a serious limitation in the .NET printing classes. The class PrinterBounds retrieves the real printing bounds of a printed page. The .NET printing classes don't take into account the physical left and top margins of the printer.

Background

When you want to print a document using .NET, you create a PrintDocument object, and subscribe to the PrintPage event handler. Let's say you want set the page margins to 1 inches on each side, and draw a 2 inch rectangle in the top-left corner of the page (using these margins)

  private void printDoc_PrintPage(object sender, 
    System.Drawing.Printing.PrintPageEventArgs e)
  {
    Rectangle r = e.MarginBounds;

    e.Graphics.DrawRectangle(Pens.Black , r.Left , r.Top , 200 , 200);

    e.HasMorePages = false;
  }

Looks easy, right? But there's a serious problem when you look at the printed output (it looks fine in a print preview). You will notice that the rectangle is not at 1 inch from the left and top edge, but maybe 1.2 inches.

The reason is simple: The MarginBounds property doesn't know anything about the physical left and right margins for your printer (the non-printable region), and .NET provides no way to find out what these margins are. On the other hand, the width and height of the MarginBounds rectangle DOES take into account the hard margins of your printer, but the offset (Left and Top) is incorrect. (I have no idea why Microsoft didn't fix this in v1.1 of the Framework)

The PrinterBounds class presented here solves this problem

Implementation

To determine the printer's physical margins, we need to call a Win32 function from gdi32.dll : GetDeviceCaps()

[DllImport("gdi32.dll")] private static extern Int32 
  GetDeviceCaps(IntPtr hdc, Int32 capindex);

We can then use it like this:

HardMarginLeft = GetDeviceCaps(hDC , PHYSICALOFFSETX);
HardMarginTop  = GetDeviceCaps(hDC , PHYSICALOFFSETY);

Of course, we need the handle to the device context of the printer, which is not too hard, since we have a reference to the Graphics object in the PrintPageEventArgs parameter of the PrintPage event handler:

IntPtr hDC = e.Graphics.GetHdc(); // Get the device context handle


HardMarginLeft = GetDeviceCaps(hDC , PHYSICALOFFSETX);
HardMarginTop  = GetDeviceCaps(hDC , PHYSICALOFFSETY);

e.Graphics.ReleaseHdc(hDC); // Don't forget to release it again

Now we have the margins in device units. To convert these to printer units (1/100 inch), we have to get the DPI of the printer, which is available in the DpiX and DpiY properties of our Graphics object.

Remember I mentioned that the MarginBounds does have the correct width and height, so we just need to shift the rectangle to the correct position, using the hard margins we just retrieved. The final class looks like this:

public class PrinterBounds
{
  [DllImport("gdi32.dll")] private static extern Int32 
     GetDeviceCaps(IntPtr hdc, Int32 capindex);

  private const int PHYSICALOFFSETX = 112;
  private const int PHYSICALOFFSETY = 113;

  public readonly Rectangle Bounds;
  public readonly int       HardMarginLeft;
  public readonly int       HardMarginTop;

  public PrinterBounds(PrintPageEventArgs e)
  {
    IntPtr hDC = e.Graphics.GetHdc();

    HardMarginLeft = GetDeviceCaps(hDC , PHYSICALOFFSETX);
    HardMarginTop  = GetDeviceCaps(hDC , PHYSICALOFFSETY);

    e.Graphics.ReleaseHdc(hDC);

    HardMarginLeft = (int)(HardMarginLeft * 100.0 / e.Graphics.DpiX);
    HardMarginTop  = (int)(HardMarginTop  * 100.0 / e.Graphics.DpiY);

    Bounds = e.MarginBounds;

    Bounds.Offset(-HardMarginLeft , -HardMarginTop);
  }
}

Using the code

Using the class is very simple. In your PrintPage event handler, use an instance of PrinterBounds instead of the MarginBounds property of PrintPageEventArgs:

private void printDoc_PrintPage(object sender, 
   System.Drawing.Printing.PrintPageEventArgs e)
{
  PrinterBounds objBounds = new PrinterBounds(e);

  Rectangle r = objBounds.Bounds;  // Get the REAL Margin Bounds !

  
  e.Graphics.DrawRectangle(Pens.Black , r.Left , r.Top , 200 , 200);

  e.HasMorePages = false;
}

Points of Interest

Microsoft knew about this problem long before v1.1 of the .NET Framework was released, but they didn't fix this bug. I suppose they didn't want to break existing code. I haven't checked the v2.0 beta of the Framework, but I suspect they left it like this.

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