I'm writing a function that finds the full path of a directory based on a database table of entries. Each record contains a key, the directory's name, and the key of the parent directory (it's the Directory table in an MSI if you're familiar). I had an iterative solution, but it started looking a little nasty. I thought I could write an elegant tail recursive solution, but I'm not sure anymore.

I'll show you my code and then explain the issues I'm facing.

Dictionary<string, string> m_directoryKeyToFullPathDictionary = new Dictionary<string, string>();
...
private string ExpandDirectoryKey(Database database, string directoryKey)
{
    // check for terminating condition
    string fullPath;
    if (m_directoryKeyToFullPathDictionary.TryGetValue(directoryKey, out fullPath))
    {
        return fullPath;
    }

    // inductive step
    Record record = ExecuteQuery(database, "SELECT DefaultDir, Directory_Parent FROM Directory where Directory.Directory='{0}'", directoryKey);
    // null check
    string directoryName = record.GetString("DefaultDir");
    string parentDirectoryKey = record.GetString("Directory_Parent");

    return Path.Combine(ExpandDirectoryKey(database, parentDirectoryKey), directoryName);
}

This is how the code looked when I realized I had a problem (with some minor validation/massaging removed). I want to use memoization to short circuit whenever possible, but that requires me to make a function call to the dictionary to store the output of the recursive ExpandDirectoryKey call. I realize that I also have a Path.Combine call there, but I think that can be circumvented with a ... + Path.DirectorySeparatorChar + ....

I thought about using a helper method that would memoize the directory and return the value so that I could call it like this at the end of the function above:

return MemoizeHelper(
    m_directoryKeyToFullPathDictionary,
    Path.Combine(ExpandDirectoryKey(database, parentDirectoryKey)),
    directoryName);

But I feel like that's cheating and not going to be optimized as tail recursion.

Any ideas? Should I be using a completely different strategy? This doesn't need to be a super efficient algorithm at all, I'm just really curious. I'm using .NET 4.0, btw.

Thanks!