How do I override ToString in C# enums?

Question

In the post Enum ToString, a method is described to use the custom attribute DescriptionAttribute like this:

Enum HowNice {
  [Description("Really Nice")]
  ReallyNice,
  [Description("Kinda Nice")]
  SortOfNice,
  [Description("Not Nice At All")]
  NotNice
}

And then, you call a function GetDescription, using syntax like:

GetDescription<HowNice>(NotNice); // Returns "Not Nice At All"

But that doesn't really help me when I want to simply populate a ComboBox with the values of an enum, since I cannot force the ComboBox to call GetDescription.

What I want has the following requirements:

• Reading (HowNice)myComboBox.selectedItem will return the selected value as the enum value.
• The user should see the user-friendly display strings, and not just the name of the enumeration values. So instead of seeing "NotNice", the user would see "Not Nice At All".
• Hopefully, the solution will require minimal code changes to existing enumerations.

Obviously, I could implement a new class for each enum that I create, and override its ToString(), but that's a lot of work for each enum, and I'd rather avoid that.

Any ideas?

Heck, I'll even throw in a hug as a bounty :-)

Answer 1

The best way to do this is to make a class.

class EnumWithToString {
    private String description;
    internal EnumWithToString(String desc){
        description = desc;
    }
    public override ToString(){
        return description;
    }
}

class HowNice : EnumWithToString {

    private HowNice(String desc) : base(desc){}

    public static readonly ReallyNice = new HowNice("Really Nice");
    public static readonly KindaNice = new HowNice("Kinda Nice");
    public static readonly NotVeryNice = new HowNice("Really Mean!");
}

I believe that is the best way to do it.

When stuffed in comboboxes the pretty ToString will be shown, and the fact that no one can make any more instances of your class essentially makes it an enum.

p.s. there may need to be some slight syntax fixes, I'm not super good with C#. (Java guy)

Answer 2

Not possible to override the ToString() of enums in C#. However, you can use extension methods;

public static string ToString(this HowNice self, int neverUsed)
{
    switch (self)
    {
        case HowNice.ReallyNice:
            return "Rilly, rilly nice";
            break;
    ...

Of course you will have to make an explicit call to the method, i.e;

HowNice.ReallyNice.ToString(0)

This is not a nice solution, with a switch statement and all - but it should work and hopefully whitout to many rewrites...

Answer 3

Given that you'd rather not create a class for each enum, I'd recommend creating a dictionary of the enum value/display text and binding that instead.

Note that this has a dependency on the GetDescription method methods in the original post.

public static IDictionary<T, string> GetDescriptions<T>()
 where T : struct
{
 IDictionary<T, string> values = new Dictionary<T, string>();

 Type type = enumerationValue.GetType();
 if (!type.IsEnum)
 {
  throw new ArgumentException("T must be of Enum type", "enumerationValue");
 }

 //Tries to find a DescriptionAttribute for a potential friendly name
 //for the enum
 foreach (T value in Enum.GetValues(typeof(T)))
 {
  string text = value.GetDescription();

  values.Add(value, text);
 }

 return values;
}

Answer 4

Create a collection that contains what you need (like simple objects containing a Value property containing the HowNice enum value and a Description property containing GetDescription<HowNice>(Value) and databind the combo to that collection.

Bit like this:

Combo.DataSource = new EnumeratedValueCollection<HowNice>();
Combo.ValueMember = "Value";
Combo.DisplayMember = "Description";

when you have a collection class like this:

using System;
using System.Linq;
using System.Collections.Generic;
using System.Collections.ObjectModel;

namespace Whatever.Tickles.Your.Fancy
{
    public class EnumeratedValueCollection<T> : ReadOnlyCollection<EnumeratedValue<T>>
    {
        public EnumeratedValueCollection()
            : base(ListConstructor()) { }
        public EnumeratedValueCollection(Func<T, bool> selection)
            : base(ListConstructor(selection)) { }
        public EnumeratedValueCollection(Func<T, string> format)
            : base(ListConstructor(format)) { }
        public EnumeratedValueCollection(Func<T, bool> selection, Func<T, string> format)
            : base(ListConstructor(selection, format)) { }
        internal EnumeratedValueCollection(IList<EnumeratedValue<T>> data)
            : base(data) { }

        internal static List<EnumeratedValue<T>> ListConstructor()
        {
            return ListConstructor(null, null);
        }

        internal static List<EnumeratedValue<T>> ListConstructor(Func<T, string> format)
        {
            return ListConstructor(null, format);
        }

        internal static List<EnumeratedValue<T>> ListConstructor(Func<T, bool> selection)
        {
            return ListConstructor(selection, null);
        }

        internal static List<EnumeratedValue<T>> ListConstructor(Func<T, bool> selection, Func<T, string> format)
        {
            if (null == selection) selection = (x => true);
            if (null == format) format = (x => GetDescription<T>(x));
            var result = new List<EnumeratedValue<T>>();
            foreach (T value in System.Enum.GetValues(typeof(T)))
            {
                if (selection(value))
                {
                    string description = format(value);
                    result.Add(new EnumeratedValue<T>(value, description));
                }
            }
            return result;
        }

        public bool Contains(T value)
        {
            return (Items.FirstOrDefault(item => item.Value.Equals(value)) != null);
        }

        public EnumeratedValue<T> this[T value]
        {
            get
            {
                return Items.First(item => item.Value.Equals(value));
            }
        }

        public string Describe(T value)
        {
            return this[value].Description;
        }
    }

    [System.Diagnostics.DebuggerDisplay("{Value} ({Description})")]
    public class EnumeratedValue<T>
    {
        private T value;
        private string description;
        internal EnumeratedValue(T value, string description) {
            this.value = value;
            this.description = description;
        }
        public T Value { get { return this.value; } }
        public string Description { get { return this.description; } }
    }

}

As you can see, this collection is easily customizable with lambda's to select a subset of your enumerator and/or implement a custom formatting to string instead of using the GetDescription<T>(x) function you mention.

Answer 5

Don't! Enums are primitives and not UI objects - making them serve the UI in .ToString() would be quite bad from a design standpoint. You are trying to solve the wrong problem here: the real issue is that you do not want Enum.ToString() to show up in the combo box!

Now this is a very solveable problem indeed! You create a UI object to represent your combo box items:

sealed class NicenessComboBoxItem
{
    public string Description { get { return ...; } }
    public HowNice Value { get; private set; }

    public NicenessComboBoxItem(HowNice howNice) { Value = howNice; }
}

And then just add instances of this class to your combo box's Items collection and set these properties:

comboBox.ValueMember = "Value";
comboBox.DisplayMember = "Description";

Answer 6

ComboBox has everything you need: the FormattingEnabled property, which you should set to true, and Format event, where you'll need to place desired formatting logic. Thus,

myComboBox.FormattingEnabled = true;
myComboBox.Format += delegate(object sender, ListControlConvertEventArgs e)
    {
        e.Value = GetDescription<HowNice>((HowNice)e.Value);
    }

Answer 7

I don't think you can do it without simply binding to a different type - at least, not conveniently. Normally, even if you can't control ToString(), you can use a TypeConverter to do custom formatting - but IIRC the System.ComponentModel stuff doesn't respect this for enums.

You could bind to a string[] of the descriptions, or a something essentially like a key/value pair? (desription/value) - something like:

class EnumWrapper<T> where T : struct
{
    private readonly T value;
    public T Value { get { return value; } }
    public EnumWrapper(T value) { this.value = value; }
    public string Description { get { return GetDescription<T>(value); } }
    public override string ToString() { return Description; }

    public static EnumWrapper<T>[] GetValues()
    {
        T[] vals = (T[])Enum.GetValues(typeof(T));
        return Array.ConvertAll(vals, v => new EnumWrapper<T>(v));
    }
}

And then bind to EnumWrapper<HowNice>.GetValues()

Answer 8

You could write an TypeConverter that reads specified attributes to look them up in your resources. Thus you would get multi-language support for display names without much hastle.

Look into the TypeConverter's ConvertFrom/ConvertTo methods, and use reflection to read attributes on your enum fields.

Answer 9

I would write a generic class for use with any type. I've used something like this in the past:

public class ComboBoxItem<T>
{
 /// The text to display.
 private string text = "";
 /// The associated tag.
 private T tag = default(T);

 public string Text
 {
  get
  {
   return text;
  }
 }

 public T Tag
 {
  get
  {
   return tag;
  }
 }

 public override string ToString()
 {
  return text;
 }

 // Add various constructors here to fit your needs
}

On top of this, you could add a static "factory method" to create a list of combobox items given an enum type (pretty much the same as the GetDescriptions method you have there). This would save you of having to implement one entity per each enum type, and also provide a nice/logical place for the "GetDescriptions" helper method (personally I would call it FromEnum(T obj) ...

Answer 10

You could make a generic struct that you could use for all of your enums that has descriptions. With implicit conversions to and from the class, your variables still works like the enum except for the ToString method:

public struct Described<T> where T : struct {

 private T _value;

 public Described(T value) {
  _value = value;
 }

 public override string ToString() {
  string text = _value.ToString();
  object[] attr =
   typeof(T).GetField(text)
   .GetCustomAttributes(typeof(DescriptionAttribute), false);
  if (attr.Length == 1) {
   text = ((DescriptionAttribute)attr[0]).Description;
  }
  return text;
 }

 public static implicit operator Described<T>(T value) {
  return new Described<T>(value);
 }

 public static implicit operator T(Described<T> value) {
  return value._value;
 }

}

Usage example:

Described<HowNice> nice = HowNice.ReallyNice;

Console.WriteLine(nice == HowNice.ReallyNice); // writes "True"
Console.WriteLine(nice); // writes "Really Nice"

Answer 11

TypeConverter. I think this is what I was looking for. All hail Simon Svensson!

[TypeConverter(typeof(EnumToStringUsingDescription))]
Enum HowNice {
  [Description("Really Nice")]
  ReallyNice,
  [Description("Kinda Nice")]
  SortOfNice,
  [Description("Not Nice At All")]
  NotNice
}

All I need to change in my current enum is add this line before their declaration.

[TypeConverter(typeof(EnumToStringUsingDescription))]

Once I do that, any enum will get displayed using the DescriptionAttribute of its fields.

Oh, and the TypeConverter would be defined like this:

public class EnumToStringUsingDescription : TypeConverter
{
    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
    {
        return (sourceType.Equals(typeof(Enum)));
    }

    public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
    {
        return (destinationType.Equals(typeof(String)));
    }

    public override object ConvertFrom(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value)
    {
        return base.ConvertFrom(context, culture, value);
    }

    public override object ConvertTo(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value, Type destinationType)
    {
        if (!destinationType.Equals(typeof(String)))
        {
            throw new ArgumentException("Can only convert to string.", "destinationType");
        }

        if (!value.GetType().BaseType.Equals(typeof(Enum)))
        {
            throw new ArgumentException("Can only convert an instance of enum.", "value");
        }

        string name = value.ToString();
        object[] attrs = 
            value.GetType().GetField(name).GetCustomAttributes(typeof(DescriptionAttribute), false);
        return (attrs.Length > 0) ? ((DescriptionAttribute)attrs[0]).Description : name;
    }
}

This helps me with my ComboBox case, but obviously doesn't actually override the ToString(). I guess I'll settle for this meanwhile...

Answer 12

You could use PostSharp to target Enum.ToString and add aditionall code you want. This doesn't require any code changes.

Answer 13

What you need is to turn an enum into a ReadonlyCollection and bind the collection to the combobox (or any Key-Value Pair enabled control for that matter)

First off you need a class to contain the items of the list. Since all you need is the int/string pair I suggest using an interface and a base class combo so that you can implement the functionality in any object you want:

public interface IValueDescritionItem
{
    int Value { get; set;}
    string Description { get; set;}
}

public class MyItem : IValueDescritionItem
{
    HowNice _howNice;
    string _description;

    public MyItem()
    {

    }

    public MyItem(HowNice howNice, string howNice_descr)
    {
        _howNice = howNice;
        _description = howNice_descr;
    }

    public HowNice Niceness { get { return _howNice; } }
    public String NicenessDescription { get { return _description; } }


    #region IValueDescritionItem Members

    int IValueDescritionItem.Value
    {
        get { return (int)_howNice; }
        set { _howNice = (HowNice)value; }
    }

    string IValueDescritionItem.Description
    {
        get { return _description; }
        set { _description = value; }
    }

    #endregion
}

Here is the interface and a sample class that implements it.Notice that the class' Key is strongly typed to the Enum, and that the IValueDescritionItem proprties are implemented explicitely (so the class can have whatever properties and you can CHOOSE the ones that implement the Key/Value pair.

Now the EnumToReadOnlyCollection class:

public class EnumToReadOnlyCollection<T,TEnum> : ReadOnlyCollection<T> where T: IValueDescritionItem,new() where TEnum : struct
{
    Type _type;

    public EnumToReadOnlyCollection() : base(new List<T>())
    {
        _type = typeof(TEnum);
        if (_type.IsEnum)
        {
            FieldInfo[] fields = _type.GetFields();

            foreach (FieldInfo enum_item in fields)
            {
                if (!enum_item.IsSpecialName)
                {
                    T item = new T();
                    item.Value = (int)enum_item.GetValue(null);
                    item.Description = ((ItemDescription)enum_item.GetCustomAttributes(false)[0]).Description;
                    //above line should be replaced with proper code that gets the description attribute
                    Items.Add(item);
                }
            }
        }
        else
            throw new Exception("Only enum types are supported.");
    }

    public T this[TEnum key]
    {
        get 
        {
            return Items[Convert.ToInt32(key)];
        }
    }

}

So all you need in your code is :

private EnumToReadOnlyCollection<MyItem, HowNice> enumcol;
enumcol = new EnumToReadOnlyCollection<MyItem, HowNice>();
comboBox1.ValueMember = "Niceness";
comboBox1.DisplayMember = "NicenessDescription";
comboBox1.DataSource = enumcol;

Remember that your collection is typed with MyItem so the combobox value should return an enum value if you bind to the appropriate proprtie.

I added the T this[Enum t] property to make the collection even more usefull than a simple combo consumable, for example textBox1.Text = enumcol[HowNice.ReallyNice].NicenessDescription;

You can of course chose to turn MyItem into a Key/Value class used only for this puprose effectively skipping MyItem in the type arguments of EnumToReadnlyCollection altogether, but then you'd be forced to go with int for the key (meaning getting combobox1.SelectedValue would return int and not the enum type). You work around that if you create a KeyValueItem class to replace MyItem and so on and so forth...

Answer 14

Could you post an example of using postsharp to target the enum.tostring() method?