Split string containing command-line parameters into string[] in C#

It annoys me that there’s no function to split a string based on a function that examines each character. If there was, you could write it like this:

    public static IEnumerable<string> SplitCommandLine(string commandLine)
    {
        bool inQuotes = false;

        return commandLine.Split(c =>
                                 {
                                     if (c == '\"')
                                         inQuotes = !inQuotes;

                                     return !inQuotes && c == ' ';
                                 })
                          .Select(arg => arg.Trim().TrimMatchingQuotes('\"'))
                          .Where(arg => !string.IsNullOrEmpty(arg));
    }

Although having written that, why not write the necessary extension methods. Okay, you talked me into it…

Firstly, my own version of Split that takes a function that has to decide whether the specified character should split the string:

    public static IEnumerable<string> Split(this string str, 
                                            Func<char, bool> controller)
    {
        int nextPiece = 0;

        for (int c = 0; c < str.Length; c++)
        {
            if (controller(str[c]))
            {
                yield return str.Substring(nextPiece, c - nextPiece);
                nextPiece = c + 1;
            }
        }

        yield return str.Substring(nextPiece);
    }

It may yield some empty strings depending on the situation, but maybe that information will be useful in other cases, so I don’t remove the empty entries in this function.

Secondly (and more mundanely) a little helper that will trim a matching pair of quotes from the start and end of a string. It’s more fussy than the standard Trim method – it will only trim one character from each end, and it will not trim from just one end:

    public static string TrimMatchingQuotes(this string input, char quote)
    {
        if ((input.Length >= 2) && 
            (input[0] == quote) && (input[input.Length - 1] == quote))
            return input.Substring(1, input.Length - 2);

        return input;
    }

And I suppose you’ll want some tests as well. Well, alright then. But this must be absolutely the last thing! First a helper function that compares the result of the split with the expected array contents:

    public static void Test(string cmdLine, params string[] args)
    {
        string[] split = SplitCommandLine(cmdLine).ToArray();

        Debug.Assert(split.Length == args.Length);

        for (int n = 0; n < split.Length; n++)
            Debug.Assert(split[n] == args[n]);
    }

Then I can write tests like this:

        Test("");
        Test("a", "a");
        Test(" abc ", "abc");
        Test("a b ", "a", "b");
        Test("a b \"c d\"", "a", "b", "c d");

Here’s the test for your requirements:

        Test(@"/src:""C:\tmp\Some Folder\Sub Folder"" /users:""[email protected]"" tasks:""SomeTask,Some Other Task"" -someParam",
             @"/src:""C:\tmp\Some Folder\Sub Folder""", @"/users:""[email protected]""", @"tasks:""SomeTask,Some Other Task""", @"-someParam");

Note that the implementation has the extra feature that it will remove quotes around an argument if that makes sense (thanks to the TrimMatchingQuotes function). I believe that’s part of the normal command-line interpretation.

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