The Old New Thing

Why does my hard drive light flash every few second?

Back in Windows 95, people would notice that their hard drive light would blink every few seconds. What's that all about? Actually, it wasn't the hard drive light after all. Windows 95 was polling your CD-ROM drive to see whether you had inserted a new CD. Some computers wired up the "hard drive light" not to the hard drive ...

What is __purecall?

Both C++ and C# have the concept of virtual functions. These are functions which always invoke the most heavily derived implementation, even if called from a pointer to the base class. However, the two languages differ on the semantics of virtual functions during object construction and destruction. C# objects exist as their final type ...

How to retrieve text under the cursor (mouse pointer)

Microsoft Active Accessibilty is the technology that exposes information about objects on the screen to accessibility aids such as screen readers. But that doesn't mean that only screen readers can use it. Here's a program that illustrates the use of Active Accessibility at the most rudimentary level: Reading text. There's much more to ...

Cleaner, more elegant, and wrong

Just because you can't see the error path doesn't mean it doesn't exist. Here's a snippet from a book on C# programming, taken from the chapter on how great exceptions are. Notice how much cleaner and more elegant [this] solution is. Cleaner, more elegant, and wrong. Suppose an exception is thrown during CreateIndexes(). The ...

Why the compiler can’t autoconvert foreach to for

People have discovered that the "natural" C# loop construct is fractionally slower than the corresponding manual loop: The first thing that needs to be said here is that The performance difference is almost certainly insignificant. Don't go running around changing all your foreach loops into corresponding for loops thinking that your ...

WM_KILLFOCUS is the wrong time to do field validation

"I'll do my field validation when I get a WM_KILLFOCUS message." This is wrong for multiple reasons. First, you may not get your focus loss message until it's too late. Consider a dialog box with an edit control and an OK button. The edit control validates its contents on receipt of the WM_KILLFOCUS message. Suppose the user fills in ...

Mapping all those "strange" digits to "0" through "9"

In an earlier article, I discussed how the Char.IsDigit() method and its Win32 counterpart, GetStringTypeEx report things to be digits that aren't just "0" through "9". If you really care just about "0" through "9", then you can test for them explicitly. For example, as a regular expression, use [0-9] instead of \d. Alternatively, for a ...

Not all short filenames contain a tilde

I'm sure everybody has seen the autogenerated short names for long file names. For the long name "Long name for file.txt", you might get "LONGNA~1.TXT" or possibly "LO18C9~1.TXT" if there are a lot of collisions. What you may not know is that sometimes there is no tilde at all! Each filesystem decides how it wants to implement short ...

Unicode collation is hard

The principle of "garbage in, garbage out" applies to Unicode collation. If you hand it a meaningless string and ask to compare it to another meaningless string, you get meaningless results. I am not a Unicode expert; I just play one on the web. A real Unicode expert is Michael Kaplan, whose explanation of how comparing invalid ...

The random number seed can be the weakest link

Random number generation is hard. That's why you should leave it to the experts. But even if you choose a good random number generator, you still have to seed it properly. The best random number generator in the world isn't very useful if people can guess the seed. That's why seeding the random number generator with the current ...