<p>A Java version of this question was just answered, and, well, I don't know how to do this in .net. </p>

<p>So how do you calculate the display width of a string in C# / .net?</p>

You've got the same problem in this question as was present in the Java question - not enough information! It will differ between WinForms and WPF.
For WinForms:
Graphics.MeasureString
For WPF I'm not sure, but I suspect it will depend on the exact way you're drawing the text...