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Posted 2 Apr 2008

Tree Structured Enumerations

, 2 Apr 2008
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The way to maintain a tree structured enumeration while having all the advantages of the standard ones


In this article, I want to show you an approach of how a structured enumeration can be handled by C#.


While playing with my little home project, I stumbled upon a problem of having all those category enumerations displayed in a tree. I wanted to keep it simple - as the enumerations are - while avoiding the need to create structured object hierarchies for every one of them. So, after a bit of thinking, I came up with this solution. I hope you'll find it useful or at least interesting.

Structured Enumeration

First, we have to tickle our old plain list enumeration a bit and convert it to a structured one. I chose the animal categories.. well.. to confess, I'm always having slight difficulties to find a good example, but here it is anyway.

public enum AnimalKind
    Unknown         = 00000,

    DomesticAnimals = 00001,
      Dog           = 00100,
        Dalmatin    = 10000,
        Greyhound   = 10001,
        Malamute    = 10002,
        Terrier     = 10003,
      Cat           = 00101,
    WildAnimals     = 00002,
      Ape           = 00200,
        Chimpanzee  = 20000,
        Gorrila     = 20001,
        Orangutan   = 20002,
      Deer          = 00201

As you might notice, some unknown attribute is used there. Let me introduce it to you.

public class StructuredAttribute : Attribute
    public int span;

    public int Span
        get { return span; }
        set { span = value; }

    public StructuredAttribute(int span)
        this.span = span;

This simple attribute is composed of only one property (an automatic property may be used in C# 3.0, God I love those). The attribute is responsible for determining the span (multiplicator) of the tree levels thus allowing us to distinguish the hierarchy later.

Well.. later is now because this ought to be a short article. I took the liberty to create the utility class to help us deal with the structured enumerations. It consists of two static methods. Let's take a closer look at them.


The IsChild method is used to determine if one enumeration value is placed under another one. I guess the utility can be easily extended by a method determining the whole chain of the parents from a particular enum value.

public static bool IsChild<ttype>(TType child, TType parent)
    Type enumType = typeof(TType);
    Object[] attributeList = enumType.GetCustomAttributes
        (typeof(StructuredAttribute), true);

    if (attributeList.Length > 0)
        StructuredAttribute attribute = (StructuredAttribute)attributeList[0];
        int span = attribute.Span;
        int parentIndex = (int)Convert.ChangeType(parent, typeof(int));
        int childIndex = (int)Convert.ChangeType(child, typeof(int));
        int index = childIndex / span;
        return (index == parentIndex && childIndex != parentIndex);

    return true;

Another useful method that may be of interest to us is the CreateList method. It obviously creates a list of the child enumeration values under a particular parental value. It will also allow us to use the output list for the display or various cycle purposes. I can imagine an iterator here.

public static List<ttype> CreateList<ttype>(TType parent)
    List<ttype> result = new List<ttype>();
    Type enumType = typeof (TType);
    TType[] enumValues = (TType[]) Enum.GetValues(enumType);
    foreach (TType enumValue in enumValues)
        if (IsChild(enumValue, parent))

    return result;

Some Examples of Use

This example took all I mentioned above and put it to use. It will dump the tree to a console output while making the levels indented.

private static void DumpTree(AnimalKind parent, Int32 level)
    foreach (AnimalKind animalKind in EnumUtility.CreateList(parent))
        string caption = animalKind.ToString();
        int width = caption.Length;
        string output = caption.PadLeft(width + level, ' ');
        DumpTree(animalKind, level + 1);

static void Main()
    DumpTree(AnimalKind.Unknown, 0);

Possible Enhancements

  1. As I said earlier in this article, I can imagine some kind of iterator (or perhaps an indexer) instead of the CreateList method.
  2. The default indexing capabilities of enumerations can be widened by "inheriting" the enumeration from ulong type instead of default uint.
  3. The utility can also be extended with any kind of structuring routine which suits your need such as retrieving the chain of parents for a particular value.


  • The complex trees with many levels may find their limit because the indexing will reach the limit of enumeration (ulong). This limitation can be reduced by lowering the span value on the attribute, thus allowing to scale for the count of branches against the count of levels.
  • It is recommended to use some default (zero) value which will then be used to retrieve the level one branches.

Personal Note

I found these structured enumerations quite useful myself dealing with countless - now waiting to be structured - category enumerations. They cut the time needed to create the editable trees where the categories are distinguished from the instance items. Moreover, the enumeration is still one type in the end.

Good luck and I will be pleased to hear your comments.


  • 2008-04-02: Missing example was added (Shall I ever get it right the first time?)
  • 2008-04-02: Initial article posted


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Smart K8
Software Developer
Czech Republic Czech Republic
Contacts: EMAIL -

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Comments and Discussions

GeneralSuggestions [modified] Pin
PIEBALDconsult6-Apr-08 18:20
memberPIEBALDconsult6-Apr-08 18:20 
GeneralSuggestions Part 2 Pin
PIEBALDconsult6-Apr-08 19:11
memberPIEBALDconsult6-Apr-08 19:11 
GeneralRe: Suggestions Part 2 Pin
Smart K86-Apr-08 20:43
memberSmart K86-Apr-08 20:43 
GeneralRe: Suggestions Part 2 Pin
PIEBALDconsult7-Apr-08 14:42
memberPIEBALDconsult7-Apr-08 14:42 
Smart K8 wrote:
I've just stripped away all the code non-concerned in the article

And didn't test it?

I meant, in the line:

public static bool IsChild<ttype>(TType child, TType parent)

where is ttype used? Is it supposed to be TType and you didn't capitalize it properly?

Smart K8 wrote:
There's no constraint for Enum

Yeah, and there should be. Stupid Microsoft Mad | :mad:

Anyway, I spent another whole day today tweaking my implementation of this.
GeneralRe: Suggestions Part 2 Pin
Smart K87-Apr-08 21:43
memberSmart K87-Apr-08 21:43 
GeneralToo complex Pin
PIEBALDconsult2-Apr-08 11:54
memberPIEBALDconsult2-Apr-08 11:54 
GeneralP.S. Too complex Pin
PIEBALDconsult2-Apr-08 12:09
memberPIEBALDconsult2-Apr-08 12:09 
GeneralRe: P.S. Too complex Pin
Smart K82-Apr-08 12:31
memberSmart K82-Apr-08 12:31 
GeneralRe: P.S. Too complex Pin
PIEBALDconsult2-Apr-08 12:35
memberPIEBALDconsult2-Apr-08 12:35 
GeneralRe: P.S. Too complex Pin
Smart K82-Apr-08 12:44
memberSmart K82-Apr-08 12:44 
GeneralRe: P.S. Too complex Pin
PIEBALDconsult2-Apr-08 13:42
memberPIEBALDconsult2-Apr-08 13:42 
AnswerRe: P.S. Too complex [modified] Pin
Smart K82-Apr-08 20:29
memberSmart K82-Apr-08 20:29 
GeneralRe: P.S. Too complex Pin
PIEBALDconsult2-Apr-08 15:26
memberPIEBALDconsult2-Apr-08 15:26 
GeneralReinventing the wheel Pin
leppie2-Apr-08 11:17
member leppie 2-Apr-08 11:17 
AnswerRe: Reinventing the wheel Pin
Smart K82-Apr-08 11:35
memberSmart K82-Apr-08 11:35 
GeneralRe: Reinventing the wheel Pin
leppie2-Apr-08 12:10
member leppie 2-Apr-08 12:10 
GeneralRe: Reinventing the wheel Pin
Smart K82-Apr-08 12:35
memberSmart K82-Apr-08 12:35 
GeneralRe: Reinventing the wheel Pin
peterchen2-Apr-08 20:30
memberpeterchen2-Apr-08 20:30 
GeneralRe: Reinventing the wheel Pin
Smart K82-Apr-08 20:53
memberSmart K82-Apr-08 20:53 
GeneralRe: Reinventing the wheel Pin
Smart K82-Apr-08 22:22
memberSmart K82-Apr-08 22:22 
GeneralRe: Reinventing the wheel Pin
PIEBALDconsult5-Apr-08 18:17
memberPIEBALDconsult5-Apr-08 18:17 
GeneralRe: Reinventing the wheel Pin
Smart K85-Apr-08 22:38
memberSmart K85-Apr-08 22:38 
GeneralRe: Reinventing the wheel Pin
PIEBALDconsult5-Apr-08 19:13
memberPIEBALDconsult5-Apr-08 19:13 
GeneralRe: Reinventing the wheel Pin
PIEBALDconsult6-Apr-08 18:22
memberPIEBALDconsult6-Apr-08 18:22 

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