Four of your ten test cases were valid filenames which opened correctly. Five of the six invalid names correctly failed to open. For the sixth invalid name, windows took the strange corrective action you describe as a 'feature'. I do not have a clue what makes this name special. We have no idea how other invalid names may be handled. Code which makes any assumptions about invalid names is simply not 'bullet-proof'. Your simple test probably finds enough errors to be useful. You may even consider it an advantage that it allows some special cases that are not strictly valid.