A shell extension is a guest in someone else's house; don't go changing the code page

A customer reported a problem with their shell extension:

    We want to format a floating point number according to the user's default locale. We do this by calling snprintf to convert the value from floating point to text with a period (U+002E) as the decimal separator, then using Get­Number­Format to apply the user's preferred grouping character, decimal separator, etc. We found, however, that if the user is running in (say) German, we find that sometimes (but not always) the snprintf function follows the German locale and uses a comma (U+002C) as the decimal separator with no thousands separator. This format prevents the Get­Number­Format function from working, since it requires the decimal separator to be U+002E. What is the recommended way of formatting a floating point number according to the user's locale?

The recommended way of formatting a floating point number according to the user's locale is indeed to use a function like snprintf to convert it to text with U+002E as the decimal separator (and other criteria), then use Get­Number­Format to apply the user's locale preferences.

The snprintf function follows the C/C++ runtime locale to determine how the floating point number should be converted, and the default C runtime locale is the so-called "C" locale which indeed uses U+002E as the decimal separator. Since you're getting U+002C as the decimal separator, somebody must have called set­locale to change the locale from "C" to a German locale, most likely by passing "" as the locale, which means "follow the locale of the environment."

    Our shell extension is running in Explorer. Under what conditions will Explorer call set­locale(LC_NUMERIC, "")? What should we do if the locale is not "C"?

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