从枚举属性中获取枚举

Question

我有

public enum Als 
{
    [StringValue("Beantwoord")] Beantwoord = 0,
    [StringValue("Niet beantwoord")] NietBeantwoord = 1,
    [StringValue("Geselecteerd")] Geselecteerd = 2,
    [StringValue("Niet geselecteerd")] NietGeselecteerd = 3,
}

与

public class StringValueAttribute : Attribute
{
    private string _value;

    public StringValueAttribute(string value)
    {
        _value = value;
    }

    public string Value
    {
        get { return _value; }
    }
}

我想将我选择的组合框项目中的值放入一个 int 中：

int i = (int)(Als)Enum.Parse(typeof(Als), (string)cboAls.SelectedValue); //<- WRONG

这可能吗？如果可以，怎么做？ （StringValue 匹配从组合框中选择的值）。

Answer 1

这是一个帮助方法，应该可以为您指明正确的方向。

protected Als GetEnumByStringValueAttribute(string value)
{
    Type enumType = typeof(Als);
    foreach (Enum val in Enum.GetValues(enumType))
    {
        FieldInfo fi = enumType.GetField(val.ToString());
        StringValueAttribute[] attributes = (StringValueAttribute[])fi.GetCustomAttributes(
            typeof(StringValueAttribute), false);
        StringValueAttribute attr = attributes[0];
        if (attr.Value == value)
        {
            return (Als)val;
        }
    }
    throw new ArgumentException("The value '" + value + "' is not supported.");
}

要调用它，只需执行以下操作：

Als result = this.GetEnumByStringValueAttribute<Als>(ComboBox.SelectedValue);

这可能不是最好的解决方案，因为它与Als 相关联，您可能希望使此代码可重用。您可能想从我的解决方案中删除代码以返回属性值，然后像您在问题中所做的那样使用Enum.Parse。

Answer 2

我正在使用 Microsoft 的 DescriptionAttribute 和以下扩展方法：

public static string GetDescription(this Enum value)
{
    if (value == null)
    {
        throw new ArgumentNullException("value");
    }

    string description = value.ToString();
    FieldInfo fieldInfo = value.GetType().GetField(description);
    DescriptionAttribute[] attributes =
       (DescriptionAttribute[])
     fieldInfo.GetCustomAttributes(typeof(DescriptionAttribute), false);

    if (attributes != null && attributes.Length > 0)
    {
        description = attributes[0].Description;
    }
    return description;
}

Answer 3

这里有几个扩展方法，我用于这个确切的目的，我已经重写了这些以使用你的StringValueAttribute，但是像Oliver 我在我的代码中使用DescriptionAttribute。

    public static T FromEnumStringValue<T>(this string description) where T : struct {
        Debug.Assert(typeof(T).IsEnum);

        return (T)typeof(T)
            .GetFields()
            .First(f => f.GetCustomAttributes(typeof(StringValueAttribute), false)
                         .Cast<StringValueAttribute>()
                         .Any(a => a.Value.Equals(description, StringComparison.OrdinalIgnoreCase))
            )
            .GetValue(null);
    }

这在 .NET 4.5 中可以稍微简单一些：

    public static T FromEnumStringValue<T>(this string description) where T : struct {
        Debug.Assert(typeof(T).IsEnum);

        return (T)typeof(T)
            .GetFields()
            .First(f => f.GetCustomAttributes<StringValueAttribute>()
                         .Any(a => a.Value.Equals(description, StringComparison.OrdinalIgnoreCase))
            )
            .GetValue(null);
    }

要调用它，只需执行以下操作：

Als result = ComboBox.SelectedValue.FromEnumStringValue<Als>();

反之，这是一个从枚举值中获取字符串的函数：

    public static string StringValue(this Enum enumItem) {
        return enumItem
            .GetType()
            .GetField(enumItem.ToString())
            .GetCustomAttributes<StringValueAttribute>()
            .Select(a => a.Value)
            .FirstOrDefault() ?? enumItem.ToString();
    }

并称它为：

string description = Als.NietBeantwoord.StringValue()

Answer 4

来自this highly upvoted question and answer 的重复链接，这是一种适用于 C# 7.3 的新 Enum 类型约束的方法。请注意，您还需要指定它也是一个struct，以便您可以将其设为可为空的TEnum?，否则您将收到错误消息。

public static TEnum? GetEnumFromDescription<TEnum>(string description)
    where TEnum : struct, Enum 
{
    var comparison = StringComparison.OrdinalIgnoreCase;
    foreach (var field in typeof(TEnum).GetFields())
    {
        var attribute = Attribute.GetCustomAttribute(field, typeof(DescriptionAttribute)) as DescriptionAttribute;
        if (attribute != null)
        {
            if (string.Compare(attribute.Description, description, comparison) == 0)
                return (TEnum)field.GetValue(null);
        }
        if (string.Compare(field.Name, description, comparison) == 0)
            return (TEnum)field.GetValue(null);
    }
    return null;
}

Answer 5

不确定我是否在这里遗漏了什么，你能不能不要这样做，

Als temp = (Als)combo1.SelectedItem;
int t = (int)temp;

Answer 6

要根据应用于枚举成员的属性值解析字符串值，我建议您使用我的Enums.NET 开源库。

对于像 StringValueAttribute 这样的自定义属性，您可以这样做。

为StringValueAttribute.Value注册并存储自定义EnumFormat。

Format = Enums.RegisterCustomEnumFormat(m => m.Attributes.Get<StringValueAttribute>()?.Value);

然后使用自定义的EnumFormat。

Als result = Enums.Parse<Als>("Niet beantwoord", Format); // result == Als.NietBeantwoord

如果您改为使用内置属性，例如 DescriptionAttribute，您可以这样做。

Als result = Enums.Parse<Als>("Niet beantwoord", EnumFormat.Description);

如果您有兴趣，这就是您获取与枚举值关联的字符串值的方式。

string value = Als.NietBeantwoord.AsString(Format); // value == "Niet beantwoord"

Answer 7

我做了一个更通用的解决方案。
您可以将它与 any 属性一起使用，甚至可以与字符串以外的其他类型一起使用。

using System;

namespace EnumTest
{
    public static class EnumExtensions
    {
        #region Get enum from Attribute
        /// <summary>
        /// Searches the enum element which has a [attributeType] attribute with a attributePropertyName equivalent to searchValue.
        /// Throws exception, if there is no enum element found which has a [attributeType] attribute with a attributePropertyName equivalent to searchValue.
        /// </summary>
        /// <param name="attributeType">the type of the attribute. e.g. typeof(System.ComponentModel.DescriptionAttribute)</param>
        /// <param name="attributePropertyName">the property of the attribute to search. At DescriptionAttribute, this is "Description"</param>
        /// <param name="searchValue">the value to search</param>
        public static TEnum FromAttributeValueToEnum<TEnum>(Type attributeType, string attributePropertyName, object searchValue)
        where TEnum : struct, Enum
        {
            TEnum? result = FromAttributeValueToNullableEnum<TEnum>(attributeType, attributePropertyName, searchValue);

            if (result.HasValue)
            {
                return result.Value;
            }

            Type enumType = typeof(TEnum);
            throw new ArgumentException($"The enum type {enumType.FullName} has no element with a {attributeType.FullName} with {attributePropertyName} property equivalent to '{searchValue}'");
        }

        /// <summary>
        /// Searches the enum element which has a [attributeType] attribute with a attributePropertyName equivalent to searchValue.
        /// Returns fallBackValue, if there is no enum element found which has a [attributeType] attribute with a attributePropertyName equivalent to searchValue.
        /// </summary>
        /// <param name="attributeType">the type of the attribute. e.g. typeof(System.ComponentModel.DescriptionAttribute)</param>
        /// <param name="attributePropertyName">the property of the attribute to search. At DescriptionAttribute, this is "Description"</param>
        /// <param name="searchValue">the value to search</param> 
        public static TEnum FromAttributeValueToEnum<TEnum>(Type attributeType, string attributePropertyName, object searchValue, TEnum fallBackValue)
        where TEnum : struct, Enum
        {
            TEnum? result = FromAttributeValueToNullableEnum<TEnum>(attributeType, attributePropertyName, searchValue);

            if (result.HasValue)
            {
                return result.Value;
            }

            return fallBackValue;
        }

        /// <summary>
        /// Searches the enum element which has a [attributeType] attribute with a attributePropertyName equivalent to searchValue.
        /// Returns null, if there is no enum element found which has a [attributeType] attribute with a attributePropertyName equivalent to searchValue.
        /// </summary>
        /// <param name="attributeType">the type of the attribute. e.g. typeof(System.ComponentModel.DescriptionAttribute)</param>
        /// <param name="attributePropertyName">the property of the attribute to search. At DescriptionAttribute, this is "Description"</param>
        /// <param name="searchValue">the value to search</param> 
        public static TEnum? FromAttributeValueToNullableEnum<TEnum>(Type attributeType, string attributePropertyName, object searchValue)
        where TEnum : struct, Enum
        {
            Type enumType = typeof(TEnum);
            if (!enumType.IsEnum)
            {
                throw new ArgumentException($"The type {enumType.FullName} is no Enum!");
            }

            if (attributeType == null)
            {
                throw new ArgumentNullException(nameof(attributeType));
            }

            if (!typeof(Attribute).IsAssignableFrom(attributeType))
            {
                throw new ArgumentException($"The type {attributeType.FullName} is no Attribute!", nameof(attributeType));
            }

            var propertyInfoAttributePropertyName = attributeType.GetProperty(attributePropertyName);

            if (propertyInfoAttributePropertyName == null)
            {
                throw new ArgumentException($"The type {attributeType.FullName} has no (public) property with name {attributePropertyName}", nameof(attributeType));
            }

            foreach (var field in enumType.GetFields())
            {
                var attributes = Attribute.GetCustomAttributes(field, attributeType);
                if (attributes.Length == 0)
                {
                    continue;
                }

                foreach (var attribute in attributes)
                {
                    object attributePropertyValue = propertyInfoAttributePropertyName.GetValue(attribute);

                    if (attributePropertyValue == null)
                    {
                        continue;
                    }

                    if (attributePropertyValue.Equals(searchValue))
                    {
                        return (TEnum)field.GetValue(null);
                    }
                }
            }

            return null;
        }
        #endregion
    }
}

@RubenHerman 的初始帖子：

Als Beantwoord = EnumExtensions.FromAttributeValueToEnum<Als>(typeof(StringValueAttribute), nameof(StringValueAttribute.Value), "Beantwoord");

高级示例：

public class IntValueAttribute : Attribute
{
    public IntValueAttribute(int value)
    {
        Value = value;
    }

    public int Value { get; private set; }
}

public enum EnumSample
{
    [Description("Eins")]
    [IntValue(1)]
    One,

    [Description("Zwei")]
    [IntValue(2)]
    Two,

    [Description("Drei")]
    [IntValue(3)]
    Three
}

EnumSample one = EnumExtensions.FromAttributeValueToEnum<EnumSample>(typeof(DescriptionAttribute), nameof(DescriptionAttribute.Description), "Eins");
EnumSample two = EnumExtensions.FromAttributeValueToEnum<EnumSample>(typeof(IntValueAttribute), nameof(IntValueAttribute.Value), 2);