Question

我以不同的方式找到了一个关于实现枚举的好例子。我认为这被称为类型安全的枚举模式。我开始使用它，但我意识到我不能在switch语句中使用它。
我的实现如下：

public sealed class MyState
{
    private readonly string m_Name;
    private readonly int m_Value;

    public static readonly MyState PASSED= new MyState(1, "OK");
    public static readonly MyState FAILED= new MyState(2, "ERROR");

    private MyState(int value, string name)
    {
        m_Name = name;
        m_Value = value;
    }

    public override string ToString()
    {
        return m_Name;
    }

    public int GetIntValue()
    {
        return m_Value;
    }
}

为了能够在C＃中的开关语句中使用此模式，我可以添加哪些内容？感谢。

Answer 1

您可以尝试这样的事情：

class Program
{
    static void Main(string[] args)
    {
        Gender gender = Gender.Unknown;

        switch (gender)
        {
            case Gender.Enum.Male:
                break;
            case Gender.Enum.Female:
                break;
            case Gender.Enum.Unknown:
                break;
        }
    }
}

public class Gender : NameValue
{
    private Gender(int value, string name)
        : base(value, name)
    {
    }

    public static readonly Gender Unknown = new Gender(Enum.Unknown, "Unknown");
    public static readonly Gender Male = new Gender(Enum.Male, "Male");
    public static readonly Gender Female = new Gender(Enum.Female, "Female");
    public class Enum
    {
        public const int Unknown = -1;
        public const int Male = 1;
        public const int Female = 2;
    }

}

public abstract class NameValue
{
    private readonly int _value;
    private readonly string _name;

    protected NameValue(int value, string name)
    {
        _value = value;
        _name = name;
    }

    public int Value
    {
        get { return _value; }
    }

    public string Name
    {
        get { return _name; }
    }

    public override string ToString()
    {
        return Name;
    }
    public override int GetHashCode()
    {
        return Value.GetHashCode();
    }

    public override bool Equals(object obj)
    {
        NameValue other = obj as NameValue;
        if (ReferenceEquals(other, null)) return false;
        return this.Value == other.Value;
    }

    public static implicit operator int(NameValue nameValue)
    {
        return nameValue.Value;
    }
}

Answer 2

类型安全的枚举模式很有趣，因为您可以将行为添加到单个枚举成员（即实例）中。因此，如果您要切换的行为可能是该类的一部分，那么只需使用多态。请注意，您可能需要为覆盖行为的每个成员创建子类：

public class MyState {

  public static readonly MyState Passed = new MyStatePassed();
  public static readonly MyState Failed = new MyStateFailed();

  public virtual void SomeLogic() {
    // default logic, or make it abstract
  }

  class MyStatePassed : MyState {
    public MyStatePassed() : base(1, "OK") { }
  }
  class MyStateFailed : MyState {
    public MyStateFailed() : base(2, "Error") { }
    public override void SomeLogic() { 
      // Error specific logic!
    }
  }

  ...
}

用法：

MyState state = ...
state.someLogic();

现在，如果逻辑显然不属于您并且您真的想要切换，我的建议是创建一个兄弟节点：

public enum MyStateValue { 
  Passed = 1, Failed = 2
}
public sealed class MyState {
  public static readonly MyState Passed = new MyState(MyStateValue.Passed, "OK");
  public static readonly MyState Failed = new MyState(MyStateValue.Failed, "Error");

  public MyStateValue Value { get; private set; }

  private MyState(MyStateValue value, string name) {
    ...
  }
}

然后开启：

switch (state.Value) {
  case MyStateValue.Passed: ...
  case MyStateValue.Failed: ...
}

在这种情况下，如果类型安全的枚举类没有任何行为，那么它没有太多理由代替枚举本身存在。但是，当然，你可以同时拥有逻辑和兄弟节点。

Answer 3

Jordão有正确的想法，但有更好的方法来实现多态，使用委托。

委托的使用比switch语句快。（事实上，我坚信，面向对象开发中switch语句的唯一位置是工厂方法。我总是寻找某种多态来替换我处理的任何代码中的任何switch语句。）

例如，如果您想要基于type-safe-enum的特定行为，我使用以下模式：

public sealed class EnumExample
{
    #region Delegate definitions
    /// <summary>
    /// This is an example of adding a method to the enum. 
    /// This delegate provides the signature of the method.
    /// </summary>
    /// <param name="input">A parameter for the delegate</param>
    /// <returns>Specifies the return value, in this case a (possibly 
    /// different) EnumExample</returns>
    private delegate EnumExample DoAction(string input);
    #endregion

    #region Enum instances
    /// <summary>
    /// Description of the element
    /// The static readonly makes sure that there is only one immutable 
    /// instance of each.
    /// </summary>
    public static readonly EnumExample FIRST = new EnumExample(1,
        "Name of first value",    
        delegate(string input)
           {
               // do something with input to figure out what state comes next
               return result;
           }
    );
    ...
    #endregion

    #region Private members
    /// <summary>
    /// The string name of the enum
    /// </summary>
    private readonly string name;
    /// <summary>
    /// The integer ID of the enum
    /// </summary>
    private readonly int value;
    /// <summary>
    /// The method that is used to execute Act for this instance
    /// </summary>
    private readonly DoAction action;
    #endregion

    #region Constructors
    /// <summary>
    /// This constructor uses the default value for the action method
    /// 
    /// Note all constructors are private to prevent creation of instances 
    /// by any other code
    /// </summary>
    /// <param name="value">integer id for the enum</param>
    /// <param name="name">string value for the enum</param>
    private EnumExample(int value, string name) 
            : this (value, name, defaultAction)
    {
    }

    /// <summary>
    /// This constructor sets all the values for a single instance.
    /// All constructors should end up calling this one.
    /// </summary>
    /// <param name="value">the integer ID for the enum</param>
    /// <param name="name">the string value of the enum</param>
    /// <param name="action">the method used to Act</param>
    private EnumExample(int value, string name, DoAction action)
    {
        this.name = name;
        this.value = value;
        this.action = action;
    }
    #endregion

    #region Default actions
    /// <summary>
    /// This is the default action for the DoAction delegate
    /// </summary>
    /// <param name="input">The inpute for the action</param>
    /// <returns>The next Enum after the action</returns>
    static private EnumExample defaultAction(string input)
    {
        return FIRST;
    }
    #endregion

    ...
}

如何在类型安全的枚举模式上使用switch语句

3 个答案: