How to give a textbox a fixed width of 17,5 cm?

Question

I have an application with a textbox, and the width of the textbox on the screen must always be 17,5 centimeters on the screen of the user.

This is what I tried so far:

const double centimeter = 17.5; // the width I need
const double inches = centimeter * 0.393700787; // convert centimeter to inches

float dpi = GetDpiX(); // get the dpi. 96 in my case.

var pixels = dpi*inches; // this should give me the amount of pixels
textbox1.Width = Convert.ToInt32(pixels); // set it. Done.



private float GetDpiX()
{
    floar returnValue;
    Graphics graphics = CreateGraphics();
    returnValue = graphics.DpiX;
    graphics.Dispose(); // don’t forget to release the unnecessary resources
    return returnValue;
}

But this gives me different sizes with different resolutions.

It gives me 13 cm with 1680 x 1050 and 21,5 cm with 1024 x 768.

What am I doing wrong?

Answer 1

The method graphics.DpiX does not give the real dots per inch of the monitor. It returns the DPI set in Windows Display properties, mostly either 96 or 120 DPI.

It is not possible to read the real DPI of the monitor. Microsoft did research this for Windows Vista/7 but as long as manufactures of monitors do not provide a standard way to read the value from the monitor hardware it will not be possible.

Answer 2

Yes, unfortunately Xenan is right. To workaround the problem you could allow a sort of by hand calibration, done by the user.

e.g. draw a line of 400 pixel on the screen, ask the user to measure it on the screen and set the result. Now is really simple to calculate the PPI (pixels per inch) that is your calibration.