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PO.NET invalid cast exception on double division
 

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    <p>I am working on a .NET application to reduce image file sizes using <a href="http://en.wikipedia.org/wiki/ImageMagick" rel="nofollow noreferrer">ImageMagick</a>. My application stores some "settings" in a <code>Hashtable</code> for use in various areas of the application. I am now adding a feature to reduce image size based on desired <a href="http://en.wikipedia.org/wiki/Dots_per_inch" rel="nofollow noreferrer">DPI</a> in the transformed image. Basically, I am looking up the DPI in the unaltered image, calculating the percentage of the desired DPI in relation to the current DPI, and then I will resize the image dimensions by this proportion.</p> <p>ImageMagick reports image DPI as floating point values. So 200x200 DPI is really 199.975x199.975. Thus I use <code>Math.Ceiling()</code> to get the values into my application. When I try to use the desired DPI from my settings <code>Hashtable</code> to do the percentage calculation I get an invalid cast exception. I don't know why this is happening.</p> <p>Here is a test case that fails in the same manner as my actual code:</p> <pre><code> using System; using System.Collections; namespace typetest { class Program { private struct DPI { public double x; public double y; } public static void Main(string[] args) { Hashtable vars = new Hashtable(); DPI dpi; string dpiString = "199.547:199.547"; string[] ret; vars["newDpiX"] = 150; vars["newDpiY"] = 150; ret = dpiString.Split(':'); dpi.x = ( (double)vars["newDpiX"] / Math.Ceiling(double.Parse(ret[0])) ) * 100; dpi.y = ( (double)vars["newDpiY"] / Math.Ceiling(double.Parse(ret[1])) ) * 100; Console.WriteLine("New DPI percentage = " + dpi.x + "%x" + dpi.y +"%"); } } } </code></pre> <p>If I change the division to</p> <pre><code> double newDpiX = 150.0; dpi.x = ( newDpiX / Math.Ceiling(double.Parse(ret[0])) ) * 100; </code></pre> <p>it will work as expected. What is going on here?</p>
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