c# - 我们可以在 c# 中定义枚举的隐式转换吗？

Question

是否可以在 c# 中定义枚举的隐式转换？

可以做到这一点的东西？

public enum MyEnum
{
    one = 1, two = 2
}

MyEnum number = MyEnum.one;
long i = number;

如果不是，为什么不呢？

Answer 1

有一个解决方案。考虑以下：

public sealed class AccountStatus
{
    public static readonly AccountStatus Open = new AccountStatus(1);
    public static readonly AccountStatus Closed = new AccountStatus(2);

    public static readonly SortedList<byte, AccountStatus> Values = new SortedList<byte, AccountStatus>();
    private readonly byte Value;

    private AccountStatus(byte value)
    {
        this.Value = value;
        Values.Add(value, this);
    }


    public static implicit operator AccountStatus(byte value)
    {
        return Values[value];
    }

    public static implicit operator byte(AccountStatus value)
    {
        return value.Value;
    }
}

以上提供了隐式转换：

        AccountStatus openedAccount = 1;            // Works
        byte openedValue = AccountStatus.Open;      // Works

这比声明一个普通的枚举要多得多（尽管你可以将上面的一些重构为一个通用的通用基类）。通过让基类实现 IComparable 和 IEquatable，以及添加方法来返回 DescriptionAttributes 的值、声明的名称等，您可以走得更远。

我编写了一个基类 (RichEnum<>) 来处理大部分繁琐的工作，这将上面的枚举声明简化为：

public sealed class AccountStatus : RichEnum<byte, AccountStatus>
{
    public static readonly AccountStatus Open = new AccountStatus(1);
    public static readonly AccountStatus Closed = new AccountStatus(2);

    private AccountStatus(byte value) : base (value)
    {
    }

    public static implicit operator AccountStatus(byte value)
    {
        return Convert(value);
    }
}

下面列出了基类 (RichEnum)。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq;
using System.Reflection;
using System.Resources;

namespace Ethica
{
    using Reflection;
    using Text;

    [DebuggerDisplay("{Value} ({Name})")]
    public abstract class RichEnum<TValue, TDerived>
                : IEquatable<TDerived>,
                  IComparable<TDerived>,
                  IComparable, IComparer<TDerived>
        where TValue : struct , IComparable<TValue>, IEquatable<TValue>
        where TDerived : RichEnum<TValue, TDerived>
    {
        #region Backing Fields

        /// <summary>
        /// The value of the enum item
        /// </summary>
        public readonly TValue Value;

        /// <summary>
        /// The public field name, determined from reflection
        /// </summary>
        private string _name;

        /// <summary>
        /// The DescriptionAttribute, if any, linked to the declaring field
        /// </summary>
        private DescriptionAttribute _descriptionAttribute;

        /// <summary>
        /// Reverse lookup to convert values back to local instances
        /// </summary>
        private static SortedList<TValue, TDerived> _values;

        private static bool _isInitialized;


        #endregion

        #region Constructors

        protected RichEnum(TValue value)
        {
            if (_values == null)
                _values = new SortedList<TValue, TDerived>();
            this.Value = value;
            _values.Add(value, (TDerived)this);
        }

        #endregion

        #region Properties

        public string Name
        {
            get
            {
                CheckInitialized();
                return _name;
            }
        }

        public string Description
        {
            get
            {
                CheckInitialized();

                if (_descriptionAttribute != null)
                    return _descriptionAttribute.Description;

                return _name;
            }
        }

        #endregion

        #region Initialization

        private static void CheckInitialized()
        {
            if (!_isInitialized)
            {
                ResourceManager _resources = new ResourceManager(typeof(TDerived).Name, typeof(TDerived).Assembly);

                var fields = typeof(TDerived)
                                .GetFields(BindingFlags.Static | BindingFlags.GetField | BindingFlags.Public)
                                .Where(t => t.FieldType == typeof(TDerived));

                foreach (var field in fields)
                {

                    TDerived instance = (TDerived)field.GetValue(null);
                    instance._name = field.Name;
                    instance._descriptionAttribute = field.GetAttribute<DescriptionAttribute>();

                    var displayName = field.Name.ToPhrase();
                }
                _isInitialized = true;
            }
        }

        #endregion

        #region Conversion and Equality

        public static TDerived Convert(TValue value)
        {
            return _values[value];
        }

        public static bool TryConvert(TValue value, out TDerived result)
        {
            return _values.TryGetValue(value, out result);
        }

        public static implicit operator TValue(RichEnum<TValue, TDerived> value)
        {
            return value.Value;
        }

        public static implicit operator RichEnum<TValue, TDerived>(TValue value)
        {
            return _values[value];
        }

        public static implicit operator TDerived(RichEnum<TValue, TDerived> value)
        {
            return value;
        }

        public override string ToString()
        {
            return _name;
        }

        #endregion

        #region IEquatable<TDerived> Members

        public override bool Equals(object obj)
        {
            if (obj != null)
            {
                if (obj is TValue)
                    return Value.Equals((TValue)obj);

                if (obj is TDerived)
                    return Value.Equals(((TDerived)obj).Value);
            }
            return false;
        }

        bool IEquatable<TDerived>.Equals(TDerived other)
        {
            return Value.Equals(other.Value);
        }


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

        #endregion

        #region IComparable Members

        int IComparable<TDerived>.CompareTo(TDerived other)
        {
            return Value.CompareTo(other.Value);
        }

        int IComparable.CompareTo(object obj)
        {
            if (obj != null)
            {
                if (obj is TValue)
                    return Value.CompareTo((TValue)obj);

                if (obj is TDerived)
                    return Value.CompareTo(((TDerived)obj).Value);
            }
            return -1;
        }

        int IComparer<TDerived>.Compare(TDerived x, TDerived y)
        {
            return (x == null) ? -1 :
                   (y == null) ? 1 :
                    x.Value.CompareTo(y.Value);
        }

        #endregion

        public static IEnumerable<TDerived> Values
        {
            get
            {
                return _values.Values;
            }
        }

        public static TDerived Parse(string name)
        {
            foreach (TDerived value in _values.Values)
                if (0 == string.Compare(value.Name, name, true) || 0 == string.Compare(value.DisplayName, name, true))
                    return value;

            return null;
        }
    }
}

Answer 2

您不能进行隐式转换（零除外），也不能编写自己的实例方法 - 但是，您可能可以编写自己的扩展方法：

public enum MyEnum { A, B, C }
public static class MyEnumExt
{
    public static int Value(this MyEnum foo) { return (int)foo; }
    static void Main()
    {
        MyEnum val = MyEnum.A;
        int i = val.Value();
    }
}

不过，这并没有给你很多东西（与仅仅做一个明确的演员相比）。

我看到人们想要这样做的主要时间之一是[Flags]通过泛型进行操作 - 即一种bool IsFlagSet<T>(T value, T flag);方法。不幸的是，C# 3.0 不支持泛型上的运算符，但您可以使用类似这样的东西来解决这个问题，这使得运算符完全可用于泛型。

Answer 3

struct PseudoEnum
{
    public const int 
              INPT = 0,
              CTXT = 1,
              OUTP = 2;
};

// ...

var arr = new String[3];

arr[PseudoEnum.CTXT] = "can";
arr[PseudoEnum.INPT] = "use";
arr[PseudoEnum.CTXT] = "as";
arr[PseudoEnum.CTXT] = "array";
arr[PseudoEnum.OUTP] = "index";

Answer 4

我改编了 Mark 出色的 RichEnum 通用基类。

定影

1. 由于他的库中缺少位而导致的一些编译问题（特别是：资源相关的显示名称没有完全删除；它们现在是）
2. 初始化并不完美：如果你做的第一件事是从基类访问静态 .Values 属性，你会得到一个 NPE。通过强制基类以奇怪的方式递归（CRTP）来解决这个问题，在 CheckInitialized 期间及时强制 TDerived 的静态构造
3. 最后将 CheckInitialized 逻辑移动到静态构造函数中（以避免每次检查的惩罚，多线程初始化的竞争条件；也许这是我的项目符号 1 解决的不可能。？）

感谢 Mark 出色的想法 + 实施，这是给大家的：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq;
using System.Reflection;
using System.Resources;

namespace NMatrix
{

    [DebuggerDisplay("{Value} ({Name})")]
    public abstract class RichEnum<TValue, TDerived>
                : IEquatable<TDerived>,
                  IComparable<TDerived>,
                  IComparable, IComparer<TDerived>
        where TValue : struct, IComparable<TValue>, IEquatable<TValue>
        where TDerived : RichEnum<TValue, TDerived>
    {
        #region Backing Fields

        /// <summary>
        /// The value of the enum item
        /// </summary>
        public readonly TValue Value;

        /// <summary>
        /// The public field name, determined from reflection
        /// </summary>
        private string _name;

        /// <summary>
        /// The DescriptionAttribute, if any, linked to the declaring field
        /// </summary>
        private DescriptionAttribute _descriptionAttribute;

        /// <summary>
        /// Reverse lookup to convert values back to local instances
        /// </summary>
        private static readonly SortedList<TValue, TDerived> _values = new SortedList<TValue, TDerived>();

        #endregion

        #region Constructors

        protected RichEnum(TValue value)
        {
            this.Value = value;
            _values.Add(value, (TDerived)this);
        }

        #endregion

        #region Properties

        public string Name
        {
            get
            {
                return _name;
            }
        }

        public string Description
        {
            get
            {
                if (_descriptionAttribute != null)
                    return _descriptionAttribute.Description;

                return _name;
            }
        }

        #endregion

        #region Initialization

        static RichEnum()
        {
            var fields = typeof(TDerived)
                .GetFields(BindingFlags.Static | BindingFlags.GetField | BindingFlags.Public)
                .Where(t => t.FieldType == typeof(TDerived));

            foreach (var field in fields)
            {
                /*var dummy =*/ field.GetValue(null); // forces static initializer to run for TDerived

                TDerived instance = (TDerived)field.GetValue(null);
                instance._name = field.Name;
                                    instance._descriptionAttribute = field.GetCustomAttributes(true).OfType<DescriptionAttribute>().FirstOrDefault();
            }
        }

        #endregion

        #region Conversion and Equality

        public static TDerived Convert(TValue value)
        {
            return _values[value];
        }

        public static bool TryConvert(TValue value, out TDerived result)
        {
            return _values.TryGetValue(value, out result);
        }

        public static implicit operator TValue(RichEnum<TValue, TDerived> value)
        {
            return value.Value;
        }

        public static implicit operator RichEnum<TValue, TDerived>(TValue value)
        {
            return _values[value];
        }

        public static implicit operator TDerived(RichEnum<TValue, TDerived> value)
        {
            return value;
        }

        public override string ToString()
        {
            return _name;
        }

        #endregion

        #region IEquatable<TDerived> Members

        public override bool Equals(object obj)
        {
            if (obj != null)
            {
                if (obj is TValue)
                    return Value.Equals((TValue)obj);

                if (obj is TDerived)
                    return Value.Equals(((TDerived)obj).Value);
            }
            return false;
        }

        bool IEquatable<TDerived>.Equals(TDerived other)
        {
            return Value.Equals(other.Value);
        }


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

        #endregion

        #region IComparable Members

        int IComparable<TDerived>.CompareTo(TDerived other)
        {
            return Value.CompareTo(other.Value);
        }

        int IComparable.CompareTo(object obj)
        {
            if (obj != null)
            {
                if (obj is TValue)
                    return Value.CompareTo((TValue)obj);

                if (obj is TDerived)
                    return Value.CompareTo(((TDerived)obj).Value);
            }
            return -1;
        }

        int IComparer<TDerived>.Compare(TDerived x, TDerived y)
        {
            return (x == null) ? -1 :
                   (y == null) ? 1 :
                    x.Value.CompareTo(y.Value);
        }

        #endregion

        public static IEnumerable<TDerived> Values
        {
            get
            {
                return _values.Values;
            }
        }

        public static TDerived Parse(string name)
        {
            foreach (TDerived value in Values)
                if (0 == string.Compare(value.Name, name, true))
                    return value;

            return null;
        }
    }
}

我在单声道上运行的使用示例：

using System.ComponentModel;
using System;

namespace NMatrix
{    
    public sealed class MyEnum : RichEnum<int, MyEnum>
    {
        [Description("aap")]  public static readonly MyEnum my_aap   = new MyEnum(63000);
        [Description("noot")] public static readonly MyEnum my_noot  = new MyEnum(63001);
        [Description("mies")] public static readonly MyEnum my_mies  = new MyEnum(63002);

        private MyEnum(int value) : base (value) { } 
        public static implicit operator MyEnum(int value) { return Convert(value); }
    }

    public static class Program
    {
        public static void Main(string[] args)
        {
            foreach (var enumvalue in MyEnum.Values)
                Console.WriteLine("MyEnum {0}: {1} ({2})", (int) enumvalue, enumvalue, enumvalue.Description);
        }
    }
}

产生输出

[mono] ~/custom/demo @ gmcs test.cs richenum.cs && ./test.exe 
MyEnum 63000: my_aap (aap)
MyEnum 63001: my_noot (noot)
MyEnum 63002: my_mies (mies)

注意：单声道 2.6.7 需要额外的显式转换，而使用单声道 2.8.2 时不需要......

Answer 5

您不能在枚举类型上声明隐式转换，因为它们不能定义方法。C#隐式关键字编译为以“op_”开头的方法，在这种情况下不起作用。

Answer 6

你可能可以，但不是枚举（你不能向它添加方法）。您可以向自己的类添加隐式转换，以允许将枚举转换为它，

public class MyClass {

    public static implicit operator MyClass ( MyEnum input ) {
        //...
    }
}

MyClass m = MyEnum.One;

问题是为什么？

一般来说，.Net 避免（你也应该）避免任何可能丢失数据的隐式转换。

Answer 7

如果将枚举的基数定义为 long，则可以执行显式转换。我不知道您是否可以使用隐式转换，因为枚举不能在其上定义方法。

public enum MyEnum : long
{
    one = 1,
    two = 2,
}

MyEnum number = MyEnum.one;
long i = (long)number;

另外，请注意，未初始化的枚举将默认为 0 值或第一项 - 因此在上述情况下，最好也定义zero = 0。

Answer 8

因此，枚举对我来说基本上没用，OP。

我最终一直在做与图片相关的事情：

简单的解决方案

经典的示例问题是用于检测按键的 VirtualKey 集。

enum VKeys : ushort
{
a = 1,
b = 2,
c = 3
}
// the goal is to index the array using predefined constants
int[] array = new int[500];
var x = array[VKeys.VK_LSHIFT]; 

这里的问题是你不能用枚举索引数组，因为它不能隐式地将枚举转换为 ushort（即使我们甚至将枚举基于 ushort）

在这个特定的上下文中，枚举被以下数据结构淘汰了。. . .

public static class VKeys
{
public const ushort
a = 1,
b = 2, 
c = 3;
}

Answer 9

我从这里https://codereview.stackexchange.com/questions/7566/enum-vs-int-wrapper-struct找到了更简单的解决方案，我从该链接粘贴了下面的代码，以防它在将来不起作用。

struct Day
{
    readonly int day;

    public static readonly Day Monday = 0;
    public static readonly Day Tuesday = 1;
    public static readonly Day Wednesday = 2;
    public static readonly Day Thursday = 3;
    public static readonly Day Friday = 4;
    public static readonly Day Saturday = 5;
    public static readonly Day Sunday = 6;

    private Day(int day)
    {
        this.day = day;
    }

    public static implicit operator int(Day value)
    {
        return value.day;
    }

    public static implicit operator Day(int value)
    {
        return new Day(value);
    }
}

Answer 10

我创建了这个实用程序来帮助我将Enum转换为PrimitiveEnum并将PrimitiveEnum转换为byte, sbyte, short, ushort, int, uint, long, or ulong.

因此，这在技术上将任何枚举转换为其任何原始值。

public enum MyEnum
{
    one = 1, two = 2
}

PrimitiveEnum number = MyEnum.one;
long i = number;

请参阅https://github.com/McKabue/McKabue.Extentions.Utility/blob/master/src/McKabue.Extentions.Utility/Enums/PrimitiveEnum.cs的提交

using System;

namespace McKabue.Extentions.Utility.Enums
{
    /// <summary>
    /// <see href="https://stackoverflow.com/q/261663/3563013">
    /// Can we define implicit conversions of enums in c#?
    /// </see>
    /// </summary>
    public struct PrimitiveEnum
    {
        private Enum _enum;

        public PrimitiveEnum(Enum _enum)
        {
            this._enum = _enum;
        }

        public Enum Enum => _enum;


        public static implicit operator PrimitiveEnum(Enum _enum)
        {
            return new PrimitiveEnum(_enum);
        }

        public static implicit operator Enum(PrimitiveEnum primitiveEnum)
        {
            return primitiveEnum.Enum;
        }

        public static implicit operator byte(PrimitiveEnum primitiveEnum)
        {
            return Convert.ToByte(primitiveEnum.Enum);
        }

        public static implicit operator sbyte(PrimitiveEnum primitiveEnum)
        {
            return Convert.ToSByte(primitiveEnum.Enum);
        }

        public static implicit operator short(PrimitiveEnum primitiveEnum)
        {
            return Convert.ToInt16(primitiveEnum.Enum);
        }

        public static implicit operator ushort(PrimitiveEnum primitiveEnum)
        {
            return Convert.ToUInt16(primitiveEnum.Enum);
        }

        public static implicit operator int(PrimitiveEnum primitiveEnum)
        {
            return Convert.ToInt32(primitiveEnum.Enum);
        }

        public static implicit operator uint(PrimitiveEnum primitiveEnum)
        {
            return Convert.ToUInt32(primitiveEnum.Enum);
        }

        public static implicit operator long(PrimitiveEnum primitiveEnum)
        {
            return Convert.ToInt64(primitiveEnum.Enum);
        }

        public static implicit operator ulong(PrimitiveEnum primitiveEnum)
        {
            return Convert.ToUInt64(primitiveEnum.Enum);
        }
    }
}

Answer 11

在 MS .net（非 Mono）上运行代码时，我已经解决了sehe 的答案问题。对我来说，这个问题特别发生在 .net 4.5.1 上，但其他版本似乎也受到了影响。

问题

通过反射访问 a public static TDervied MyEnumValue（通过FieldInfo.GetValue(null)不初始化所述字段。

解决方法

TDerived而不是在静态初始化器上为实例分配名称，而是RichEnum<TValue, TDerived>在第一次访问时懒惰地完成TDerived.Name. 编码：

public abstract class RichEnum<TValue, TDerived> : EquatableBase<TDerived>
    where TValue : struct, IComparable<TValue>, IEquatable<TValue>
    where TDerived : RichEnum<TValue, TDerived>
{
    // Enforcing that the field Name (´SomeEnum.SomeEnumValue´) is the same as its 
    // instances ´SomeEnum.Name´ is done by the static initializer of this class.
    // Explanation of initialization sequence:
    // 1. the static initializer of ´RichEnum<TValue, TDerived>´ reflects TDervied and 
    //    creates a list of all ´public static TDervied´ fields:
    //   ´EnumInstanceToNameMapping´
    // 2. the static initializer of ´TDerive´d assigns values to these fields
    // 3. The user is now able to access the values of a field.
    //    Upon first access of ´TDervied.Name´ we search the list 
    //    ´EnumInstanceToNameMapping´ (created at step 1) for the field that holds
    //    ´this´ instance of ´TDerived´.
    //    We then get the Name for ´this´ from the FieldInfo
    private static readonly IReadOnlyCollection<EnumInstanceReflectionInfo> 
                            EnumInstanceToNameMapping = 
        typeof(TDerived)
            .GetFields(BindingFlags.Static | BindingFlags.GetField | BindingFlags.Public)
            .Where(t => t.FieldType == typeof(TDerived))
            .Select(fieldInfo => new EnumInstanceReflectionInfo(fieldInfo))
            .ToList();

    private static readonly SortedList<TValue, TDerived> Values =
        new SortedList<TValue, TDerived>();

    public readonly TValue Value;

    private readonly Lazy<string> _name;

    protected RichEnum(TValue value)
    {
        Value = value;

        // SortedList doesn't allow duplicates so we don't need to do
        // duplicate checking ourselves
        Values.Add(value, (TDerived)this);

        _name = new Lazy<string>(
                    () => EnumInstanceToNameMapping
                         .First(x => ReferenceEquals(this, x.Instance))
                         .Name);
    }

    public string Name
    {
        get { return _name.Value; }
    }

    public static implicit operator TValue(RichEnum<TValue, TDerived> richEnum)
    {
        return richEnum.Value;
    }

    public static TDerived Convert(TValue value)
    {
        return Values[value];
    }

    protected override bool Equals(TDerived other)
    {
        return Value.Equals(other.Value);
    }

    protected override int ComputeHashCode()
    {
        return Value.GetHashCode();
    }

    private class EnumInstanceReflectionInfo
    {
        private readonly FieldInfo _field;
        private readonly Lazy<TDerived> _instance;

        public EnumInstanceReflectionInfo(FieldInfo field)
        {
            _field = field;
            _instance = new Lazy<TDerived>(() => (TDerived)field.GetValue(null));
        }

        public TDerived Instance
        {
            get { return _instance.Value; }
        }

        public string Name { get { return _field.Name; } }
    }
}

这 - 在我的情况下 - 基于EquatableBase<T>：

public abstract class EquatableBase<T>
    where T : class 
{
    public override bool Equals(object obj)
    {
        if (this == obj)
        {
            return true;
        }

        T other = obj as T;
        if (other == null)
        {
            return false;
        }

        return Equals(other);
    }

    protected abstract bool Equals(T other);

    public override int GetHashCode()
    {
        unchecked
        {
            return ComputeHashCode();
        }
    }

    protected abstract int ComputeHashCode();
}

笔记

上面的代码没有包含Mark原始答案的所有功能！

谢谢

感谢Mark提供了他的RichEnum实现，感谢sehe提供了一些改进！

Answer 12

@BatteryBackupUnit 嘿，这听起来像是一个很酷的解决方案，但你能在这里解释一下这部分吗？

由于我使用 .NET 4.7.2 得到了一个“InvalidCastException”，可悲的是：/

 _name = new Lazy<string>(
                () => EnumInstanceToNameMapping
                     .First(x => ReferenceEquals(this, x.Instance))
                     .Name);

我不知道为什么，我创建了 RichEnum 的派生类型并初始化为您在示例中所做的一切，但我得到了这个 annyoingg 异常..

会很高兴对此有所帮助，因为我非常喜欢这种方法。

Answer 13

这是基于adminSoftDK 的答案的不同风格。

/// <summary>
/// Based on https://datatracker.ietf.org/doc/html/rfc4346#appendix-A.1
/// </summary>
[DebuggerDisplay("{_value}")]
public struct HandshakeContentType
{
    #region Types
    public const byte ChangeCipher = 0x14;
    public const byte Alert = 0x15;
    public const byte Handshake = 0x16;
    public const byte ApplicationData = 0x17;
    #endregion

    byte _value;
    private HandshakeContentType(byte value)
    {
        _value = value;

        switch (_value)
        {
            case ChangeCipher:
            case Alert:
            case Handshake:
            case ApplicationData:
                break;

            default:
                throw new InvalidOperationException($"An invalid handshake content type (${value}) was provided.");
        }
    }

    #region Methods
    public static implicit operator byte(HandshakeContentType type) => type._value;
    public static implicit operator HandshakeContentType(byte b) => new HandshakeContentType(b);
    #endregion
}

这使您可以将其struct与switch我认为非常棒的语句一起使用。

Answer 14

为枚举类型引入隐式转换会破坏类型安全，所以我不建议这样做。你为什么想这么做？我见过的唯一用例是当您想将枚举值放入具有预定义布局的结构中时。但即便如此，您也可以在结构中使用 enum 类型，然后告诉 Marshaller 他应该如何处理它。