Keep enum-to-object mapping with enum class?

问题:

I frequently need a global hard-coded mapping between an enum and another object (a string in this example). I want to co-locate the enum and mapping definitions to clarify maintenance.

As you can see, in this example, an annoying class with one static field is created.

public enum EmailTemplates
{
    // Remember to edit the corresponding mapping singleton!
    WelcomeEmail,
    ConfirmEmail
}

public class KnownTemplates
{
    public static Dictionary<EmailTemplates, string> KnownTemplates;
    static KnownTemplates() {
        KnownTemplates.Add(EmailTemplates.WelcomeEmail, "File1.htm");
        KnownTemplates.Add(EmailTemplates.ConfirmEmail, "File2.htm");
    }
}

Sometimes the mapping class can have more function and a meaningful name, and the mapping activity can even be private. But that only pollutes the maintenance/correlation problem.

Anyone have a good pattern for this?

回答1:

You can use attributes to annotate the enumeration and then use reflection to build the dictionary.

[AttributeUsage(AttributeTargets.Field)]
sealed class TemplateAttribute : Attribute {

  public TemplateAttribute(String fileName) {
    FileName = fileName;
  }

  public String FileName { get; set; }

}

enum EmailTemplate {

  [Template("File1.htm")]
  WelcomeEmail,

  [Template("File2.htm")]
  ConfirmEmail

}

class KnownTemplates {

  static Dictionary<EmailTemplate, String> knownTemplates;

  static KnownTemplates() {
    knownTemplates = typeof(EmailTemplates)
      .GetFields(BindingFlags.Static | BindingFlags.Public)
      .Where(fieldInfo => Attribute.IsDefined(fieldInfo, typeof(TemplateAttribute)))
      .Select(
        fieldInfo => new {
          Value = (EmailTemplate) fieldInfo.GetValue(null),
          Template = (TemplateAttribute) Attribute
            .GetCustomAttribute(fieldInfo, typeof(TemplateAttribute))
        }
      )
      .ToDictionary(x => x.Value, x => x.Template.FileName);
  }

}

If you do this a lot you can create a more general generic function that combines enumeration values with an attribute associated with that enumeration value:

static IEnumerable<Tuple<TEnum, TAttribute>> GetEnumAttributes<TEnum, TAttribute>()
  where TEnum : struct
  where TAttribute : Attribute {
  return typeof(TEnum)
    .GetFields(BindingFlags.Static | BindingFlags.Public)
    .Where(fieldInfo => Attribute.IsDefined(fieldInfo, typeof(TAttribute)))
    .Select(
      fieldInfo => Tuple.Create(
        (TEnum) fieldInfo.GetValue(null),
        (TAttribute) Attribute.GetCustomAttribute(fieldInfo, typeof(TAttribute))
      )
    );
}

And use it like this:

knownTemplates = GetEnumAttributes<EmailTemplate, TemplateAttribute>()
  .ToDictionary(tuple => tuple.Item1, tuple => tuple.Item2.FileName);

For even more fun you can create an extension method:

static class EmailTemplateExtensions {

  static Dictionary<EmailTemplate, String> templates;

  static EmailTemplateExtensions() {
    templates = GetEnumAttributes<EmailTemplate, TemplateAttribute>()
      .ToDictionary(tuple => tuple.Item1, tuple => tuple.Item2.FileName);
  }

  public static String FileName(this EmailTemplate emailTemplate) {
    String fileName;
    if (templates.TryGetValue(emailTemplate, out fileName))
      return fileName;
    throw new ArgumentException(
      String.Format("No template defined for EmailTemplate.{0}.", emailTemplate)
    );
  }

}

Then calling EmailTemplate.ConfirmEmail.FileName() will return File2.htm.

回答2:

Normally, when you want to add extra info or behaviors to your enum elements, that means you need a full blown class instead. You can borrow from (old-)Java the type-safe enum pattern and create something like this:

sealed class EmailTemplate {
  public static readonly EmailTemplate Welcome = new EmailTemplate("File1.html");
  public static readonly EmailTemplate Confirm = new EmailTemplate("File2.html");

  private EmailTemplate(string location) {
    Location = location;
  }
  public string Location { get; private set; }

  public string Render(Model data) { ... }
}

Now you can associate any properties or methods to your elements, like Location and Render above.

回答3:

Here is an approach which worked pretty well for me.

public class BaseErrWarn : Attribute
{
    public string Code { get; set; }
    public string Description { get; set; }

    public BaseErrWarn(string code, string description)
    {
        this.Code = code;
        this.Description = description;
    }
}

public enum ErrorCode
{
    [BaseErrWarn("ClientErrMissingOrEmptyField", "Field was missing or empty.")] ClientErrMissingOrEmptyField,
    [BaseErrWarn("ClientErrInvalidFieldValue", "Not a valid field value.")] ClientErrInvalidFieldValue,
    [BaseErrWarn("ClientErrMissingValue", "No value passed in.")] ClientErrMissingValue
}

Now you can use reflection to map the Enum to the BaseErrWarn class:

public static BaseErrWarn GetAttribute(Enum enumVal)
{
    return (BaseErrWarn)Attribute.GetCustomAttribute(ForValue(enumVal), typeof(BaseErrWarn));
}

private static MemberInfo ForValue(Enum errorEnum)
{
    return typeof(BaseErrWarn).GetField(Enum.GetName(typeof(BaseErrWarn), errorEnum));
}

Here is an example which uses this mapping to map an Enum to an object and then pull fields off of it:

    public BaseError(Enum errorCode)
    {
        BaseErrWarn baseError = GetAttribute(errorCode);
        this.Code = baseError.Code;
        this.Description = baseError.Description;
    }

    public BaseError(Enum errorCode, string fieldName)
    {
        BaseErrWarn baseError = GetAttribute(errorCode);
        this.Code = baseError.Code;
        this.Description = baseError.Description;
        this.FieldName = fieldName;
    }