Background

Introduction
This article describes the implementation of an enhanced PrintPreviewDialog class.
Background
The PrintPreviewDialog is convenient and easy to use. All you need to do is create an
instance of the dialog class, assign your PrintDocument object to the Document property,
and call the ShowDialog method.
However, PrintPreviewDialog has some shortcomings, including the following:
• The entire document must be rendered before the preview appears. This is annoying
for long documents.
• There are no options for choosing the printer, adjusting the page layout, or selecting
specific pages to print.
• The dialog looks outdated. It hasn't changed since .NET 1.0, and it wasn't exactly
cutting-edge even back then.
• The dialog allows little or no customization.
• There is no option to export the document to other formats such as PDF.
• Page images are cached in the control, which limits the size of the documents that
can be previewed.
The CoolPrintPreviewDialog class presented here addresses these shortcomings. It is
just as easy to use as the standard PrintPreviewDialog, but has the following
enhancements:
• Pages can be previewed as soon as they are rendered. The first page is shown
almost instantly and subsequent pages become available while the user browses the
first ones.
• The "Print" button shows a dialog that allows users to select the printer and page
ranges to print. A "Page Layout" button is also available so users can change page
size, orientation, and margins.
• The dialog uses a ToolStrip control instead of the old toolbar.
• You have the source and can customize everything from appearance to behavior.
• The control creates a list of images which can be exported to other formats
including PDF (although the version presented here doesn't actually do that).
Using the Code

Using the CoolPrintPreviewDialog is as easy as using the traditional
PrintPreviewDialog. You instantiate the control, set the Document property to the
PrintDocument you want to preview, then call the dialog's Show method.
If you have code that uses the PrintPreviewDialog class, switching to the
CoolPrintPreviewDialog only requires changing one line of code. For example:
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// using a PrintPreviewDialog
using (var dlg = new PrintPreviewDialog())
{
dlg.Document = this.printDocument1;
dlg.ShowDialog(this);
}

// using a CoolPrintPreviewDialog
using (var dlg = new CoolPrintPreview.CoolPrintPreviewDialog())
{
dlg.Document = this.printDocument1;
dlg.ShowDialog(this);
}
Generating the Preview Images
The core of the CoolPrintPreviewDialog class is a CoolPreviewControl that generates
and shows the page previews.
The PrintDocument object has a PrintController property that specifies an object
responsible for creating the Graphics objects where the document is rendered. The default
print controller creates Graphics objects for the default printer and is not interesting in this
case. But .NET also defines a PreviewPrintController class that creates metafiles
instead. These remain available to the caller to be shown in the preview area.
The CoolPreviewControl works by temporarily replacing the document's original print
controller with a PreviewPrintController, calling the document's Print method, and
getting the page images while the document is rendered. The images represent pages in the

document, and are scaled and displayed in the control just like any regular Image object.
The code that creates the page previews looks like this (this code is simplified for clarity,
refer to the source for a better version):
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// list of page images
List<Image> _imgList = new List<Image>();

// generate page images
public void GeneratePreview(PrintDocument doc)
{
// save original print controller
PrintController savePC = doc.PrintController;

// replace it with a preview print controller
doc.PrintController = new PreviewPrintController();

// hook up event handlers
doc.PrintPage += _doc_PrintPage;
doc.EndPrint += _doc_EndPrint;

// render the document
_imgList.Clear();
doc.Print();

// disconnect event handlers
doc.PrintPage -= _doc_PrintPage;
doc.EndPrint -= _doc_EndPrint;

// restore original print controller
doc.PrintController = savePC;

}
The code installs the controller and hooks up the event handlers, then calls the Print
method to generate the pages, and cleans up when it's done.
When the Print method is invoked, the document starts firing events. The PrintPage and
EndPrint event handlers capture the pages as soon as they are rendered and add them to an
internal image list.
The event handlers also call the Application.DoEvents method to keep the dialog
responsive to user actions while the document renders. This allows users to switch pages,
adjust the zoom factor, or cancel the document generation process. Without this call, the
dialog would stop operating until the whole document finishes rendering.
This is the code that does all this:
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void _doc_PrintPage(object sender, PrintPageEventArgs e)
{
SyncPageImages();
if (_cancel)
{
e.Cancel = true;
}
}
void _doc_EndPrint(object sender, PrintEventArgs e)
{
SyncPageImages();
}
void SyncPageImages()
{
// get page previews from print controller
var pv = (PreviewPrintController)_doc.PrintController;
var pageInfo = pv.GetPreviewPageInfo();

// add whatever images are missing from our internal list
for (int i = _img.Count; i < pageInfo.Length; i++)
{
// add to internal list
_img.Add(pageInfo[i].Image);

// fire event to indicate we have more pages
OnPageCountChanged(EventArgs.Empty);

// if the page being previewed changed, refresh to show it
if (StartPage < 0) StartPage = 0;
if (i == StartPage || i == StartPage + 1)
{
Refresh();
}

// keep application responsive
Application.DoEvents();
}
}
This is the core of the preview code. The rest is concerned with housekeeping tasks such as
scaling the preview images, updating the scrollbars, handling navigation buttons, mouse,
keyboard, and so on. Please refer to the source code for the implementation details.
Updating the Page Layout
The preview dialog allows users to update the print layout. This is very easy to implement,
thanks to the .NET PageSetupDialog class. Here is the code that gets called when users
click the "Page Layout" button:
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void _btnPageSetup_Click(object sender, EventArgs e)
{

using (var dlg = new PageSetupDialog())
{
dlg.Document = Document;
if (dlg.ShowDialog(this) == DialogResult.OK)
{
// user changed the page layout, refresh preview images
_preview.RefreshPreview();
}
}
}
The code shows a PageSetupDialog that allows the user to change the paper size,
orientation, and margins. Changes made by the user are reflected in the document's
DefaultPageSettings property.
If the user clicks OK, then we assume that the page layout has been modified, and call the
RefreshPreview method on the preview control. This method regenerates all preview
images using the new settings, so the user can see the changes applied to margins, page
orientation, and so on.
Printing the Document
When the user clicks the "Print" button, the dialog shows a PrintDialog so the user can
select the printer, page range, or change his mind and cancel the printing.
Unfortunately, page range selections are not honored if you simply call the Print method
directly on the document. To remedy this, the dialog calls the Print method on the
enhanced preview control instead. That implementation uses the page images already
stored in the control, and honors page ranges defined in the document's PrinterSettings
properties.
This is the code that gets called when the user clicks the "Print" button:
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void _btnPrint_Click(object sender, EventArgs e)
{
using (var dlg = new PrintDialog())

{
// configure dialog
dlg.AllowSomePages = true;
dlg.AllowSelection = true;
dlg.Document = Document;

// show allowed page range
var ps = dlg.PrinterSettings;
ps.MinimumPage = ps.FromPage = 1;
ps.MaximumPage = ps.ToPage = _preview.PageCount;

// show dialog
if (dlg.ShowDialog(this) == DialogResult.OK)
{
// print selected page range
_preview.Print();
}
}
}
The Print method in the preview control starts by determining the range of pages that
should be rendered. This may be the full document, a specific range, or the current
selection (page being previewed). Once the page range has been determined, the code
creates a DocumentPrinter helper class to perform the actual printing:
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public void Print()
{
// select pages to print
var ps = _doc.PrinterSettings;
int first = ps.MinimumPage - 1;
int last = ps.MaximumPage - 1;

switch (ps.PrintRange)
{
case PrintRange.CurrentPage:
first = last = StartPage;
break;
case PrintRange.Selection:
first = last = StartPage;
if (ZoomMode == ZoomMode.TwoPages)
last = Math.Min(first + 1, PageCount - 1);
break;
case PrintRange.SomePages:
first = ps.FromPage - 1;
last = ps.ToPage - 1;
break;
}

// print using helper class
var dp = new DocumentPrinter(this, first, last);
dp.Print();
}
}
The DocumentPrinter class is simple. It inherits from PrintDocument and overrides the
OnPrintPage method to print only the pages selected by the user:
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internal class DocumentPrinter : PrintDocument
{
int _first, _last, _index;
List<Image> _imgList;

public DocumentPrinter(CoolPrintPreviewControl preview, int first, int

last)
{
// save page range and image list
_first = first;
_last = last;
_imgList = preview.PageImages;

// copy page and printer settings from original document
DefaultPageSettings = preview.Document.DefaultPageSettings;
PrinterSettings = preview.Document.PrinterSettings;
}

protected override void OnBeginPrint(PrintEventArgs e)
{
// start from the first page
_index = _first;
}
protected override void OnPrintPage(PrintPageEventArgs e)
{
// render the current page and increment the index
e.Graphics.PageUnit = GraphicsUnit.Display;
e.Graphics.DrawImage(_imgList[_index++], e.PageBounds);

// stop when we reach the last page in the range
e.HasMorePages = _index <= _last;
}
}
This implementation renders the page images assuming all pages have the same size and
orientation, which is the case for most documents. If the document contains pages of
different sizes, or with different orientation, this simple implementation will not work

correctly. To fix this, we would have to check that the current paper size and orientation
match the preview image size before printing each page and adjust the printer settings if
necessary. That is left as an exercise for the reader.
Previewing Really Long Documents
After I posted the first version of this project, I got some great feedback from other
CodeProject users. One of them mentioned a problem I also had a while ago. If the
document contains several thousand pages, caching all those images may cause problems.
Windows has a limit of 10,000 GDI objects, and each page image represents at least one. If
you use too many GDI objects, your application may crash, or cause other apps to crash.
Not nice...
One easy way to solve this problem is to convert the page images into streams. You can
then store the streams and create images on demand, only when they are needed for
previewing or printing.
The code below shows a PageImageList class that does the job. You can use it much like
a regular List, except when you get or set an image, it is automatically converted to and
from a byte array. This way, the images stored in the list are not GDI objects and don't use
up the system resources.
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// This version of the PageImageList stores images as byte arrays. It is
a little
// more complex and slower than a simple list, but doesn't consume GDI
resources.
// This is important when the list contains lots of images (Windows only
supports
// 10,000 simultaneous GDI objects!)
class PageImageList
{
// ** fields
var _list = new List<byte[]>();

// ** object model
public void Clear()
{
_list.Clear();
}
public int Count
{
get { return _list.Count; }
}