可覆盖的方法不能是静态的：我怎样才能做我想做的事情？

Question

I have a series of static classes that I use to get strings for enum values. They all look something like this:

public static class MyEnumToString
{
  private static Dictionary<MyEnum, string> map
   = new Dictionary<MyEnum, string>();

  public static string Get(MyEnum type)
  {
    PopulateEmptyMap();
    return map[type];
  }

  static void PopulateEmptyMap()
  {
    if (!map.Any())
    {
      PopulateMap();
    }
  }

  private static void PopulateMap()
  {
    map[MyEnum.enum1] = "string for enum 1";
    map[MyEnum.enum2] = "string for enum 2";
  }
}

I have multiple classes like this, that differ in the Enum type they use, and the string values. Clearly, I should combine the classes to reduce duplicated code.

What I tried doing was create generic base class so that it can handle any type, then implement the PopulateMap for the inherited classes. If it were possible, it would look something like this:

public static class TypeToString<TType>
{
  public static Dictionary<TType, string> map
   = new Dictionary<TType, string>();

  public static string Get(TType type)
  {
    PopulateEmptyMap();
    return map[type];
  }

  static void PopulateEmptyMap()
  {
    if (!map.Any())
    {
      PopulateMap();
    }
  }

  public abstract static void PopulateMap();
}

public static class MyEnumToString: TypeToString<MyEnum>
{
  public static void PopulateMap()
  {
    map[MyEnum.enum1] = "string for enum 1";
    map[MyEnum.enum2] = "string for enum 2";
  }
}

I had to make the Dictionary and the method PopulateMap public, because apparently generic classes cannot have protected or protected-internal members. Having to make that public isn't ideal, but not a deal-breaker.

What I am getting hung up on is the fact that "overridable methods cannot be static", so my PopulateMap method cannot be both abstract and static. And if it's not static, it can't be called from other static methods. And if it's not abstract, then the inheriting classes' PopulateMap doesn't get called.

This version doesn't even build.

Is there any way to do what I'm trying to do and still keep my class static? I'd really like to avoid having to have an instantiated TypeToString object every time I want to call TypeToString.Get().

Answer 1

Here's a handy extension method, as I'm guessing you're trying to map some description text to an enum value:

public static class EnumExtensions
{
    public static string GetDescription(this Enum value)
    {
        var field = value.GetType().GetField(value.ToString());
        if (field == null)
            return value.ToString();

        var attribute = field.GetCustomAttributes(typeof(DescriptionAttribute), false)
                             .OfType<DescriptionAttribute>()
                             .SingleOrDefault();

        return attribute != null
            ? attribute.Description
            : value.ToString();
    }
}

Use it like this:

public enum Foo
{
    [Description("Hello")]
    Bar,

    [Description("World")]
    Baz
}

var value = Foo.Bar;
var description = value.GetDescription(); // Hello

Depending on your needs, you could cache the descriptions if reflection proves to be too slow for you, just modify the GetDescription method.

---

EDIT: to account for the additional info in the comment.

As it looks like you need something more extensible, you could use a custom attribute:

[AttributeUsage(AttributeTargets.Field, AllowMultiple = true, Inherited = false)]
public sealed class DescriptionEntryAttribute : Attribute
{
    public string Key { get; private set; }
    public string Value { get; private set; }

    public DescriptionEntryAttribute(string key, string value)
    {
        Key = key;
        Value = value;
    }
}

Which would let you to do this:

public enum Foo
{
    [DescriptionEntry("Name", "Hello")]
    [DescriptionEntry("Title", "Some title")]
    Bar,

    [DescriptionEntry("Name", "World")]
    [DescriptionEntry("Title", "Some title")]
    Baz
}

Now, to read this thing, I'd advise you to store it in a cache like that:

public static class EnumExtensions
{
    private static readonly ConcurrentDictionary<Type, DescriptionCache> Caches = new ConcurrentDictionary<Type, DescriptionCache>();

    public static string GetDescription(this Enum value, string key)
    {
        var enumType = value.GetType();
        var cache = Caches.GetOrAdd(enumType, type => new DescriptionCache(type));
        return cache.GetDescription(value, key);
    }

    public static IEnumerable<TEnum> GetValuesFromDescription<TEnum>(string key, string description)
        where TEnum : struct
    {
        var cache = Caches.GetOrAdd(typeof(TEnum), type => new DescriptionCache(type));
        return cache.GetValues(key, description).Select(value => (TEnum)(object)value);
    }

    private class DescriptionCache
    {
        private readonly ILookup<Enum, Tuple<string, string>> _items;
        private readonly ILookup<Tuple<string, string>, Enum> _reverse;

        public DescriptionCache(Type enumType)
        {
            if (!enumType.IsEnum)
                throw new ArgumentException("Not an enum");

            _items = (from value in Enum.GetValues(enumType).Cast<Enum>()
                      let field = enumType.GetField(value.ToString())
                      where field != null
                      from attribute in field.GetCustomAttributes(typeof (DescriptionEntryAttribute), false).OfType<DescriptionEntryAttribute>()
                      select new {value, key = attribute.Key, description = attribute.Value})
                .ToLookup(i => i.value, i => Tuple.Create(i.key, i.description));

            _reverse = (from grp in _items
                        from description in grp
                        select new {value = grp.Key, description})
                .ToLookup(i => i.description, i => i.value);
        }

        public string GetDescription(Enum value, string key)
        {
            var tuple = _items[value].FirstOrDefault(i => i.Item1 == key);
            return tuple != null ? tuple.Item2 : null;
        }

        public IEnumerable<Enum> GetValues(string key, string description)
        {
            return _reverse[Tuple.Create(key, description)];
        }
    }
}

This way:

• Foo.Bar.GetDescription("Name") returns "Hello"
• EnumExtensions.GetValuesFromDescription<Foo>("Title", "Some title") returns a sequence containing Foo.Bar and Foo.Baz

That should be enough to get you started, now you should tweak it to your needs. For instance, you could use an enum instead of a string for the keys, it would help avoid typing mistakes, but I don't know if this would suit your needs.

Answer 2

Your problem is that static methods and variables are essentially not inherited. They are variables that don't act on instances of the class themselves, but provide some functionality to the class.

So you have a bunch of different enums, and you want to populate them based on different stuff. So let's look at what parts you have, and what is common:

• PopulateMap: Not common
• Enum type: Not common
• Storage variable: Common
• Populate Map if empty: Common

So all you really want is a way to populate the map once, when it's used. There is already a class for this, it's called Lazy . Using that, the code becomes:

public abstract class TypeToString<Type>
{
    protected TypeToString()
    {
        storage = new Lazy<Dictionary<Type, string>>(GetMap);
    }
    private Lazy<Dictionary<Type, string>> storage;
    protected abstract Dictionary<Type, string> GetMap();
    public string Get(Type t) {return storage.Value[t];}
}
public class MyEnumToString : TypeToString<MyEnum>
{
    protected override Dictionary<MyEnum, string> GetMap()
    {
        return null;
    }
    public static Get(MyEnum e) { return new MyEnumToString.Get(e); }
}

Alternatively, you can decorate your enums with a [DescriptionAttribute] and then create a method to get the description of a specific enum. This is what I did when I was faced with a similar problem. (Be sure to cache the results for the enum, as it used reflection which was slow.)