How do you convert multistring to/from C# string collection?

Question

Multistrings (double null-terminated string of null-separated strings) are common in the Windows API. What's a good method for converting a multistring returned from an API to a C# string collection and vice versa?

I'm especially interested in proper handling of character encoding (Windows XP an later).

The following method seems to be okay for creating a multistring, but I don't have an example of decoding a multistring.

static string StringArrayToMultiString(
    ICollection<string> stringArray
    )
{
    StringBuilder multiString = new StringBuilder();


    if (stringArray != null)
    {
        foreach (string s in stringArray)
        {
            multiString.Append(s);
            multiString.Append('\0');
        }
    }

    return multiString.ToString();
}

Answer 1

This might be naïve, but how about:

static string[] MultiStringToArray(string multiString)
{
    return multiString.TrimEnd('\0').Split('\0');
}

Also - aren't you missing the final \0 (you state double-null-terminated) in StringArrayToMultiString? And it might be easier to call if the array was a params string[] array - something like:

    static string StringArrayToMultiString(params string[] values)
{
    if (values == null) throw new ArgumentNullException("values");
    StringBuilder multiString = new StringBuilder();

    foreach (string s in values)
    {
        multiString.Append(s);
        multiString.Append('\0');
    }
    return multiString.ToString();
}

[edited after clarification about final \0]

Answer 2

I've tested the StringArrayToMultiString method, using the ChangeServiceConfig() function to change the dependencies of a Windows service, and it works nicely for zero, one and many strings.

In the meantime, I've worked out a solution for decoding a multistring received from an API call. For example, the SCardListReaders() function returns a multistring of PC/SC reader names. I declared this as:

[DllImport("winscard.dll", CharSet = CharSet.Auto)]
static extern int SCardListReaders(
    IntPtr context,
    string groups,
    char[] readers,
    ref uint readersLen
    );

Note that the readers parameter, which returns the multistring, is declared as char[]. The return value is easily parsed and converted into a collection of strings:

static string[] MultiStringToArray(
    char[] multistring
    )
{
    List<string> stringList = new List<string>();
    int i = 0;
    while (i < multistring.Length)
    {
        int j = i;
        if (multistring[j++] == '\0') break;
        while (j < multistring.Length)
        {
            if (multistring[j++] == '\0')
            {
                stringList.Add(new string(multistring, i, j - i - 1));
                i = j;
                break;
            }
        }
    }

    return stringList.ToArray();
}

Answer 3

In the MSDN documentation for RegistryKey.SetValue() and RegistryKey.GetValue(), the examples simply pass in, or cast to, string[], respectively. Is there something wrong with that?