Is a deep nested Dictionary an antipattern?

Question

I have a structure that can be very easily represented using a three-deep nested dictionary, like so

private static Dictionary<string, Dictionary<string, Dictionary<string,string>>> PrerenderedTemplates;

Where the structure might be used something like this

PrerenderedTemplates[instanceID][templategroup][templatepart]

Now, I realise that this code is hard to read, because from looking at the definition statement, you can't tell what it's being used for. The only advantage I can really see in changing it to Dictionary<string, PrerenderedTemplate> is readability. Converting each nesting into its own class (e.g class PrerenderedTemplate{} class TemplateGroup{} class TemplatePart{}) would add many more lines of code for little (if any) computational advantage. As far as I can see.

• So, is my approach "ok" or should I go the extra mile and create seperate classes?
• Is it okay to cover how the nested Dictionary works in the documentation/comments
• Is there a best practice for handling this sort of nesting?
• Bear in mind, this is a private member, it doesn't need to be straightforward for people using the class.

Update

So, inspired by Reza, but unable to use Tuples, I decided to create my own key generator and implement his pattern like this:

private Dictionary<string, string> PrerenderedTemplates;
private string GetPrerenderedTemplateKey(string InstanceId, string FeatureId, string OptionId)
{
    return new StringBuilder(instanceId)
    .Append(FormatTools.LIST_ENTRY_DELIMITER)
    .Append(templategroup)
    .Append(FormatTools.LIST_ENTRY_DELIMITER)
    .Append(templatepart).ToString();
}

Where FormatTools.LIST_ENTRY_DELIMITER is the Unicode Private Use Character 0xe04d.

Answer 1

I offer another choice:

Dictionary<Tuple<string, string, string>, string> pt;

Access to dictionary:

pt[Tuple.Create("id","group","part")]

UPDATE:

Value Tuples introduced in C# 7 is most eye-catching:

Dictionary<(string id, string group, string part), string> pt;

Access to dictionary:

pt[("id", "group", "part")]

Answer 2

I would create a custom dictionary. Something like this

public class TrippleKeyDict
{
    private const string Separator = "<|>";
    private Dictionary<string, string> _dict = new Dictionary<string, string>();

    public string this[string key1, string key2, string key3]
    {
        get { return _dict[GetKey(key1, key2, key3)]; }
        set { _dict[GetKey(key1, key2, key3)] = value; }
    }

    public void Add(string key1, string key2, string key3, string value)
    {
        _dict.Add(GetKey(key1, key2, key3), value);
    }

    public bool TryGetValue(string key1, string key2, string key3, out string result)
    {
        return _dict.TryGetValue(GetKey(key1, key2, key3), out result);
    }

    private static string GetKey(string key1, string key2, string key3)
    {
        return String.Concat(key1, Separator, key2, Separator, key3);
    }
}

If you think, concatenating the strings is not safe enough, because the keys could contain the separators, then use your own key type or a Touple<string,string,string> as key. Since this implementation detail is hidden inside your custom dictionary, you can change it at any time.

You can use the dictionary like this

var dict = new TrippleKeyDict();

// Using the Add method
dict.Add(instanceID, templategroup, templatepart, "some value");

// Using the indexer
dict[instanceID, templategroup, templatepart] = "xy";
string result = dict[instanceID, templategroup, templatepart];

// Using the TryGetValue method
if (dict.TryGetValue(instanceID, templategroup, templatepart, out result)) {
    // Do something with result
}

Answer 3

I would like to offer an alternative approach, using a SortedDictionary and a custom comparer:

    public class PrerenderedTemplate
    {
        public string instanceID;
        public string templategroup;
        public string templatepart;

        public PrerenderedTemplate(string id, string tempGroup, string tempPart)
        {
            instanceID = id;
            templategroup = tempGroup;
            templatepart = tempPart;
        }

        // custom comparer instance used as argument 
        // to SortedDictionary constructor
        public class Comparer : IComparer<PrerenderedTemplate>
        {
            public int Compare(PrerenderedTemplate x, PrerenderedTemplate y)
            {
                int compare = 0;
                if (compare == 0) compare = x.instanceID.CompareTo(y.instanceID);
                if (compare == 0) compare = x.templategroup.CompareTo(y.templategroup);
                if (compare == 0) compare = x.templatepart.CompareTo(y.templatepart);
                return compare;
            }
        }
    }

Is used like so:

    var dictionary = new SortedDictionary<PrerenderedTemplate, string>(new PrerenderedTemplate.Comparer());

    dictionary.Add(new PrerenderedTemplate("1", "2", "3"), "123");
    dictionary.Add(new PrerenderedTemplate("4", "5", "6"), "456");
    dictionary.Add(new PrerenderedTemplate("7", "8", "9"), "789");

    Assert.AreEqual<string>(dictionary[new PrerenderedTemplate("7", "8", "9")], "789");

RezaArab's answer is fit for purpose but personally I dislike Tuples on the basis of their ambiguous properties and verbose syntax.

A custom class with comparer offers more clarity and also flexibility, should any requirements change.