Some .net/mono frameworks do not support the 3 argument GetDirectories method. The code below provides a simple replacement.

/// <summary>
/// Builds an array of folders & sub folders.
/// </summary>
/// <param name="path">The path to search.</param>
/// <param name="pattern">The search string to match against the names of files in path. The parameter cannot end in two periods 
/// ("..") or contain two periods ("..") followed by System.IO.Path.DirectorySeparatorChar or System.IO.Path.AltDirectorySeparatorChar,
/// nor can it contain any of the characters in System.IO.Path.InvalidPathChars.
///</param>
/// <param name="searchOptions">One of the System.IO.SearchOption values that specifies whether the search operation should include 
/// all subdirectories or only the current directory.
///</param>
/// <returns>A String array of directories that match the search pattern.</returns>
public static string[] GetDirectories(string path, string pattern, SearchOption searchOptions)
{
    // check if searching for all directories
    if (searchOptions == SearchOption.AllDirectories)
    {
        // add start paths to list
        var list = Directory.GetDirectories(path, pattern);
        var index = 0;
        var count = list.Length;

        // process list and add folders to end of list
        while (index < count)
        {
            var directories = Directory.GetDirectories(list[index++], pattern);
            if (directories.Length > 0)
            {
                // check if we need more space to store the directories
                if (count + directories.Length > list.Length - 1)
                {
                    Array.Resize(ref list, list.Length + directories.Length + 1000);
                }

                // add directories to end of the list
                foreach (var directory in directories)
                {
                    list[count++] = directory;
                }
            }

            // trim unused index from end of the array
            if (list.Length > count)
            {
                Array.Resize(ref list, count);
            }
        }

        return list;
    }

    // just return initial list of folder with no sub folders
    return Directory.GetDirectories(path, pattern);
}