HowTo: Remove invalid filename characters in .NET

In ClipFlair Studio I use DotNetZip (Ionic.Zip) library for storing components (like the activity and its nested child components) to ZIP archives (.clipflair or .clipflair.zip files). Inside the ZIP archive its child components have their own .clipflair.zip file and so on (so that you could even nest activities at any depth) which construct their filename based on the component’s Title and ID (a GUID)

However, when the component Title used characters like " (double-quote) which are not allowed in filenames, then although Ionic.Zip created the archive with the double-quotes in the nested .clipflair.zip filenames, when trying to load those ZipEntries into a memory stream it failed. Obviously I had to filter those invalid filename characters (I opted to remove them to make those ZipEntry filenames a bit more readable/smaller).

So I added one more extension method for string type at StringExtensions static class (Utils.Silverlight project), based on info gathered from the links from related stackoverflow question. To calculated version of a string s without invalid file name characters, one can do s.ReplaceInvalidFileNameChars() or optionally pass a replacement token parameter (a string) to insert at the position of each char removed.

public static string ReplaceInvalidFileNameChars(this string s,
  string replacement = "")
{
  return Regex.Replace(s,
    "[" + Regex.Escape(new String(System.IO.Path.GetInvalidPathChars())) + "]",
    replacement, //can even use a replacement string of any length
    RegexOptions.IgnoreCase);
    //not using System.IO.Path.InvalidPathChars (deprecated insecure API)
}

For more info on Regular Expressions see http://www.regular-expressions.info/ and http://msdn.microsoft.com/en-us/library/hs600312.aspx

BTW, note that to convert the char[] returned by System.IO.Path.GetInvalidPathChars() to string we use new String(System.IO.Path.GetInvalidPathChars()).

It’s unfortunate that one can’t use ToString() method of char[] (using Visual Studio to go to definition of char[].ToString() takes us to Object.ToString() which means the array types don’t overload the virtual ToString() method of Object class to return something useful).

Another thing to note is that we don’t use System.IO.Path.InvalidPathChars field which is deprecated for security reasons, but use System.IO.Path.GetInvalidPathChars() method instead. MSDN explains the security issue, so better avoid that insecure API to be safe:

Do not use InvalidPathChars if you think your code might execute in the same application domain as untrusted code. InvalidPathChars is an array, so its elements can be overwritten. If untrusted code overwrites elements of InvalidPathChars, it might cause your code to malfunction in ways that could be exploited.

.NET String extension methods to check for array of prefixes or suffixes

Seems StartsWith and EndsWith methods of String class in .NET are missing a version that accepts multiple (as an array) prefixes or suffixes respectively when testing the string. To achieve this I just added the following extension methods to StringExtensions class (of Utils.Extensions namespace) under Utils.Silverlight project at the ClipFlair source code.

public static bool StartsWith(
  this string s, 
  string[] suffixes, 
  StringComparison comparisonType = StringComparison.CurrentCulture)
{
  foreach (string suffix in suffixes)
    if (s.StartsWith(suffix, comparisonType))
      return true;
  return false;
}

public static bool EndsWith(
  this string s, 
  string[] suffixes, 
  StringComparison comparisonType = StringComparison.CurrentCulture)
{
  foreach (string suffix in suffixes)
    if (s.EndsWith(suffix, comparisonType)) 
      return true;
  return false;
}

 

To use them, you add a reference to Utils.Silverlight project to your own one and then add a using clause for the namespace that hosts a static class with these extension methods (e.g. “using Utils.Extensions;”) and then you can use them on any String at the respective source file. Can even use them on literal strings, since most .NET compilers support Boxing of literals into respective types.

I’m using a default value for the comparisonType method argument to make it optional. I use StringComparison.CurrentCulture as the default value for it (performing a word case-sensitive and culture-sensitive comparison using the current culture), as Microsoft is doing at “String.StartsWith(String)” method. However, do note the following text from that method’s documentation:

Notes to Callers

As explained in Best Practices for Using Strings in the .NET Framework, we recommend that you avoid calling string comparison methods that substitute default values and instead call methods that require parameters to be explicitly specified. To determine whether a string begins with a particular substring by using the string comparison rules of the current culture, call the StartsWith(String, StringComparison) method overload with a value of StringComparison.CurrentCulture for its comparisonType parameter.