How to check a string to see if it’s valid for use in creating a file name

earlgraham

In my old c++ days, I would use my old function below; BOOL IsValidFilename(const char *szFilename) { if ( (!szFilename) || (!strlen(szFilename)) ) return (FALSE); if ( (strchr(szFilename, '.')) || (strchr(szFilename, '\\')) || (strchr(szFilename, '/')) || (strchr(szFilename, ':')) || (strchr(szFilename, '<')) || (strchr(szFilename, '>')) || (strchr(szFilename, '*')) || (strchr(szFilename, '?')) || (strchr(szFilename, '|')) || (strchr(szFilename, '\"')) ) return (FALSE); return (TRUE); } I thought there might be a cleaner way to test a string before I used it for a file name.

Programmer Glenn Earl Graham Austin, TX

Brady Kelly

Maybe slightly cleaner. The

Path.GetInvalidFileNameChars

should give you an array of invalid chars, which you can iterate and check

IndexOf

in your candidate file name.

"Once in Africa I lost the corkscrew and we were forced to live off food and water for weeks." - Ernest Hemingway My New Blog

PIEBALDconsult

I'd just try it and catch the exception if any. Let the OS/framework determine validity; it knows best.

earlgraham

Thanks here is the final code it works perfectly...

bool IsValidFilename(String ^fileName)
{
  if ( String::IsNullOrEmpty( fileName ) == true )
    return false;
  
  array^ invalidFileChar = System::IO::Path::GetInvalidFileNameChars();

  for each ( Char ^i in invalidFileChar )
  {
    if ( fileName->Contains(String::Format("{0}", i)) )
    {
      String ^tmp = String::Format(L" '{0}'  is invalid in a file name.\n", i);
      System::Windows::Forms::MessageBox::Show(gcnew String(tmp),gcnew String(L"Warning"), 
        MessageBoxButtons::OK);
      return false;
    }
  }

  return true;
}

Programmer Glenn Earl Graham Austin, TX

pbraun

But if its speed you want, catching the exception may not be the best solution.

Phil

PIEBALDconsult

That doesn't look like C# to me. :~

PIEBALDconsult

Absolutely, why check what the OS/framework will check anyway? That's wasteful. The custom check may be faulty, causing a false valid or invalid. A false valid will likely result in an exception anyway, so you've wasted the time on the extra check, plus the time writing the faulty check. I know I couldn't write a filename validity checking routine as fast and accurate as the built-in one. You don't know to what OSs the framework may be ported in future (I know, it's unlikely) and what their validity rules are. I could certainly see adding some additional checking, like ensuring the name doesn't include a SPACE, or is 8.3 or something, but other than that you're just wasting your time. This is a wheel best not reinvented.

PIEBALDconsult

Or a Regular Expression, matching any of the invalid characters.

Luc Pattyn

String.IndexOfAny() seems appropriate here. :)

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Brady Kelly

I like that idea. A regex over a path size string should be blindingly fast as well.

"Once in Africa I lost the corkscrew and we were forced to live off food and water for weeks." - Ernest Hemingway My New Blog