Question

I’m working on a migration project in which a database actually stores display sizes in twips.
Since I can’t use twips to assign sizes to WPF or Winforms controls, I was wondering if .NET has a conversion method usable at runtime?

Answer 1

It turns out that the migration tool has something, but it wouldn’t do any good at runtime. Here’s what I did (if the hard-coded value in the extension method were changed to the value for points per inch it would work as a point converter too):

1 Twip = 1/1440th of an inch.
The .NET Graphics object has a method DpiX and DpiY which can be used to determine how many pixels are in an inch.
Using those measurements I created the following extension methods for Graphics:

using System.Drawing;

static class Extensions
{
    /// <summary>
    /// Converts an integer value in twips to the corresponding integer value
    /// in pixels on the x-axis.
    /// </summary>
    /// <param name="source">The Graphics context to use</param>
    /// <param name="inTwips">The number of twips to be converted</param>
    /// <returns>The number of pixels in that many twips</returns>
    public static int ConvertTwipsToXPixels(this Graphics source, int twips)
    {
        return (int)(((double)twips) * (1.0 / 1440.0) * source.DpiX);
    }

    /// <summary>
    /// Converts an integer value in twips to the corresponding integer value
    /// in pixels on the y-axis.
    /// </summary>
    /// <param name="source">The Graphics context to use</param>
    /// <param name="inTwips">The number of twips to be converted</param>
    /// <returns>The number of pixels in that many twips</returns>
    public static int ConvertTwipsToYPixels(this Graphics source, int twips)
    {
        return (int)(((double)twips) * (1.0 / 1440.0) * source.DpiY);
    }
}

To use these methods one simply has to do the following (assuming you’re in a context where CreateGraphics returns a Drawing.Graphics object (here this is a Form, so CreateGraphics is inherited from Form‘s super-class Control):

using( Graphics g = CreateGraphics() )
{
    Width = g.ConvertTwipsToXPixels(sizeInTwips);
    Height = g.ConvertTwipsToYPixels(sizeInTwips);
}

See the “Remarks” section in the Graphics Class documentation for a list of ways to obtain a graphics object. More complete documentation is available in the tutorial How to: Create Graphics Objects.

Brief summary of easiest ways:

• Control.CreateGraphics
• a Paint event’s PaintEventArgs has a Graphics available in its Graphics property.
• Hand Graphics.FromImage an image and it will return a Graphics object that can draw on that image. (NOTE: It is unlikely that you’ll want to use twips for an actual image)