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Posted 6 Jun 2003

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Mapping Text to Enum entries

Reto Ravasio

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4.83/5 (64 votes)

6 Jun 2003

Attaching a description to each entry in an enum.

Introduction

While porting old C code to C#, I stumbled across lots of huge arrays which mapped constants to strings. While looking for the C# way of doing this, I discovered custom attributes and reflection.

How is it done?

Append a Description attribute to each enum entry:

C#

private enum MyColors
{
   [Description("yuk!")]       LightGreen    = 0x012020,
   [Description("nice :-)")]   VeryDeepPink  = 0x123456,
   [Description("so what")]    InvisibleGray = 0x456730,
   [Description("no comment")] DeepestRed    = 0xfafafa,
   [Description("I give up")]  PitchBlack    = 0xffffff,
}

To access the description from code, the following function is called:

C#

public static string GetDescription(Enum value)
{
   FieldInfo fi= value.GetType().GetField(value.ToString()); 
   DescriptionAttribute[] attributes = 
         (DescriptionAttribute[])fi.GetCustomAttributes(
         typeof(DescriptionAttribute), false);
   return (attributes.Length>0)?attributes[0].Description:value.ToString();
}

Without the following using declaration, the code will not compile.

C#

using System.ComponentModel;        
using System.Reflection;

Benefits

Instead of having constants and huge arrays all over the code, the above solution keeps declarations and descriptions nicely together.

History

I've only found out by accident that the framework already provides a DescriptionAttribute class.

I built the first version with my own custom attribute and got conflicts when I started using the ComponentModel namespace.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

Written By

Reto Ravasio

Switzerland

This member has not yet provided a Biography. Assume it's interesting and varied, and probably something to do with programming.

Comments and Discussions

Adaptation to work with extension methods

Joel Gonzalez22-Mar-21 9:58

22-Mar-21 9:58

First of all, Thank you !! it was very useful!!
I did this adaptation... hope it can be useful for someone else!
((I'm sorry about my "spanglish" ))

using's:

C#

using System.ComponentModel; //Tag Description
using System.Reflection; // Reflections en Metodos de extensión

add extension methods in some static class, if you want to use it in any Enum, you need to create it with "this Enum value" as parameter, you can also create your extension method for your own enumeration, writing "this [[MyEnumName]] value" as parameter in the extension method.

C#

public static class myClaseEstatica
    {
        public static string GetNameString(this Enum filtro)
        {
            return Enum.GetName(typeof(Enum), filtro);
        }
        public static string GetDescription(this Enum value)
        {
            FieldInfo fi = value.GetType().GetField(value.ToString());
            DescriptionAttribute[] attributes =
                  (DescriptionAttribute[])fi.GetCustomAttributes(
                  typeof(DescriptionAttribute), false);
            return (attributes.Length > 0) ? attributes[0].Description : value.ToString();
        }
    }

In your Enum, you shuld only use the Description Tag from ComponentModel with your string correlative message, '[Description("Enum_Element_Text_Value")]

C#

public enum myEnum
    {
        [Description("Enum_Element_Text_Value")]
        Enum_Element,
    }

Finaly, when you need it, you'll only need to call this method from your Enum Value object!!

C#

string valorEnTag = MyEnum.Enum_Element.GetDescription();

Thank's for your help... it makes my day!!

Lance A. Endres3-Apr-12 7:14

Lance A. Endres

3-Apr-12 7:14

I've used this solution for some time now and I find it is simple, flexible, and overall a very nice technique! Well done and thanks!

Dorofejev19-Dec-11 23:44

19-Dec-11 23:44

Nice solution!

How to use with dataset / tableadapter

Saelen Kenny8-Nov-09 0:01

8-Nov-09 0:01

I have a datagrid which is bound through dataset / tableadapter.
How can I get the enum value there because I do not add columns manually and I cannot call methods there...

Right now the table adapter loads the integer field from the database and puts in the integer value in the grid.
There I would need to tell it is the enum TaskStatus

Greetings,

Query regarding description Tag in Enum

NITINKADAM24-Nov-08 5:14

24-Nov-08 5:14

How to do internationalization for [Description] tag in enum

Awesome - thanks!

93522-Jun-07 5:48

22-Jun-07 5:48

Very helpful, .Net rocks!

Different versions for speed improvements

clementgatin12-Mar-07 11:37

12-Mar-07 11:37

I have created a small prototype using this attribute and trying to improve its performance using different caching methods. This code is available on my blog (in french, but you can download and understand the code in english). The cached results are up to 85 times faster than uncached results, depending on the caching mechanism.

Marc Clifton15-Oct-06 3:39

15-Oct-06 3:39

for posting this simple and elegant solution. It just came in handy now!

Marc

Thyme In The Country

People are just notoriously impossible. --DavidCrow
There's NO excuse for not commenting your code. -- John Simmons / outlaw programmer
People who say that they will refactor their code later to make it "good" don't understand refactoring, nor the art and craft of programming. -- Josh Smith

Solution for C / C++

JimmyO5-May-05 9:56

5-May-05 9:56

Is this solution also for C / C++ available ?

Greetings
Jimmy Smile | :)

Smile | :)

Re: Solution for C / C++

Uwe Keim10-Apr-06 19:53

10-Apr-06 19:53

Yes [^]

Smile | :)

.

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koo918-Jan-05 10:53

18-Jan-05 10:53

MyEnum.tostring("G") will do the same thing.

Reto Ravasio19-Jan-05 8:36

19-Jan-05 8:36

no

What about Flags?

Spliffy7322-Mar-04 4:22

22-Mar-04 4:22

Nice article. But i get into trouble when i`m using [Sytstem.Flags] attribute and concat values ex:

MyColors cols = MyColors.LightGreen | MyColors.PitchBlack;

Anybody know how to solve this?;)

Re: What about Flags?

JSpens14-Aug-07 7:50

14-Aug-07 7:50

Here's an easy way to do it (if you're still interested), since Enum.ToString() will give you the enum names, you can use that to get the descriptions.

public static string[] GetFlagDescriptions(Enum value)
{
string[] names = value.ToString().Split(new string[] { ", " }, StringSplitOptions.None);
string[] descriptions = new string[names.Length];
for (int i = 0; i < names.Length; i++)
{
Enum enumValue = (Enum)Enum.Parse(value.GetType(), names[i]);
descriptions[i] = (GetDescription(enumValue));
}
return descriptions;
}

John

Re: What about Flags?

frinkfree11-Nov-08 12:55

11-Nov-08 12:55

This method attempts to break an enum down into its greatest common values. The return value indicates if the enum was completely covered by the returned enum value list. Perhaps it should be called TryGetEnumValues Wink | ;)

Wink | ;)

private bool GetEnumValues(Type enumType, object value, out IEnumerable<object> enumValues)
{
  bool enumCompletelyCovered = false;
  List<object> enumValueCollection = new List<object>();

  // perform a shortcut comparison for the common case
  if(Enum.IsDefined(enumType, value))
  {
    // the specified value completely matches an enumeration entry
    enumValueCollection.Add(value);
    enumCompletelyCovered = true;
  }
  else
  {
    // attempt to decompose the specified value into several enumeration entries
    // compare enumeration values as the largest comparable type (UInt64)
    ulong currentEnumValue = Convert.ToUInt64(value);

    // get an array of all the possible enumeration entries
    Array values = Enum.GetValues(enumType);

    // process the enumeration entries in reverse order to compare the largest values first
    int index = values.Length - 1;
    while(index >= 0)
    {
      // convert currently indexed enumeration entry value to a comparable type
      ulong currentIndexedValue = System.Convert.ToUInt64(values.GetValue(index));

      // special case the None enumeration
      if((index == 0) && (currentIndexedValue == 0L))
      {
        break;
      }

      // use a bit-field comparison technique
      if((currentEnumValue & currentIndexedValue) == currentIndexedValue)
      {
        currentEnumValue -= currentIndexedValue;
        enumValueCollection.Add(values.GetValue(index));
      }

      // move to the next up enumeration entry
      index--;
    }

    // special case the None enumeration
    // Add the assumed None enumeration if:
    //  the enumeration value collection is empty;
    //  and the current value is 0;
    //  and the current enumeration entry is the first entry;
    //  and the current enumeration entry matches the current value.
    if((enumValueCollection.Count == 0) && (currentEnumValue == 0L) && (index == 0) && (System.Convert.ToUInt64(values.GetValue(index)) == 0L))
    {
      enumValueCollection.Add(values.GetValue(index));
    }

    // reverse the enumeration value collection to go from least to greatest values
    enumValueCollection.Reverse();

    // the enumeration was completely covered if the current value is 0 and there is at lease one entry in the enumeration value collection
    enumCompletelyCovered = (currentEnumValue == 0L) && (enumValueCollection.Count > 0);
  }

  enumValues = enumValueCollection;
  return enumCompletelyCovered;
}

Hope this helps, -Parx

Dave Miller4-Feb-04 17:50

4-Feb-04 17:50

I've just written a nice console application that uses reflection to analyse any assembly for enumerated types for description attributes. It generates a minimal XML documentation set with the descriptions used as the summaries for each enumeration. This is really useful for keeping your documentation of error codes, descriptors in sync with your source code -> if you use NANT & NDoc.

It also can accept pre-generated XML doc files and insert the summaries into the correct member positions.

I will post the application on a public HTTP site soon.

Any ideas and suggestions would be

Palladino26-Aug-03 2:39

26-Aug-03 2:39

I liked this. Great idea!! Here it is going an idea on the message posted by Uwe Keim. The attribute could contain a property that would be destined to the name of resource (e.g. "AnyLib.Resources.Common.Yes"). In moment of solving string (GetDescription method) the mechanism could verify this property, if search be filled out the resource otherwise it uses the default description (hardcoded).

Marcelo Palladino
Brazil

What About using the System Enum

vbinfo25-Jun-03 22:10

25-Jun-03 22:10

Your way to get the description of an enum is very good but a bit too involved
what about this:
Public Enum ProfilePermission
Manager= 0
Supervisors= 1
Directors=2
End Enum

MsgBox(System.Enum.GetName(GetType(ProfilePermission), ProfilePermission.Supervisors)

Thanks A lot
Gabriel
vbnetuk@yahoo.co.uk

Re: What About using the System Enum

Reto Ravasio4-Jul-03 1:04

4-Jul-03 1:04

That's a possible solution as long as the description doesn't contain any characters that are valid for an enum. I can't use enum names like this is a very long description { ## ... }.

reto

Re: What About using the System Enum

vbinfo6-Jul-03 21:05

6-Jul-03 21:05

Hi
I didnt know that

Are you saying that using the system enum is not good if you are using long description or if the enum contains characters that are in the enum>?

Thanks

Thanks A lot
Gabriel
vbnetuk@yahoo.co.uk

Re: What About using the System Enum

Reto Ravasio7-Jul-03 4:47

7-Jul-03 4:47

sorry if I wasn't clear

enum entries can only contain characters which the compiler can compile.
this means that there cannot be spaces and any other funny characters...

reto

Re: What About using the System Enum

devaulr6-Nov-03 2:40

6-Nov-03 2:40

Reto,

I think you need to embellish us with a more detailed look at enumerations with spaces. I understand what you are saying about invalid characters, but the example isn't quite clear on it. Especially if you have an XML enumeration that you would be using in a web service environment.

In a web service environment, if the service uses a string enumeration VS. compacts the names for the enumeration and lays on the XMLAttribute tag with the actual name for the value which contained spaces.

example:

/// <remarks>
[System.Xml.Serialization.XmlEnumAttribute("Accounts Receivable")]
AccountsReceivable,

If you then want to present the correct strings in a listbox you must first iterate through the enum and create a new string[] containing the display strings.

When the user selects an element, the string must be compressed again back to the no spaces version which matches the enum names so that the correct enum entry can be located. whew!!

Russ DeVaul
Software Pathologist

Re: What About using the System Enum

datavalue19-Oct-05 3:06

19-Oct-05 3:06

I like the above proposal using Reflection.

D'Oh! | :doh:

D'Oh! | :doh:

Previously I used Enums with _ between each part of the name
Then my code did a replace of _ with space.
And the reverse for converting a description to an enum.

eg

public enum eFoods
{
Baked_Beans,
New_Potatoes
}

...

public static string ItemColumnTitle(eFoods pColumn)
{
string lItemName = Enum.Format(typeof(eFoods), pColumn, "G");
return(lItemName.Replace("_", " "));
}

Re: What About using the System Enum

ConradC1-Feb-05 8:22

1-Feb-05 8:22

They are two different things if I understand your message correctly. Just like property name and property description are two different attributes of a property.

Enum name is more useful for developers and compilers. It is not practically useful for display to end user since it has all the naming limits and is not localizable.

Just like the other comment from other thread, it would be nice to have this attribute to use resources instead of hard coded string. Also performance would be a concern since it uses reflection alot. How about caching the result from GetDescription static method?

Just my 2 cents
Conrad:->

Uwe Keim7-Jun-03 21:36

7-Jun-03 21:36

Is it also possible to provide multiple descriptions for an item - i.e. for different user languages? (e.g. through the mysterious ".resx" files?)

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