取消装箱通用参数中的可空枚举会导致问题

Question

我在通过通用变量传递可为 null 的枚举时遇到问题。

我目前正在使用反射为通用 类 创建 属性 setter。 这是代码

public static class program
{
    static void Main(string[] args)
    {
        someClass testclass = new someClass();
        PropertyInfo enumProperty = typeof(someClass).GetProperties()[0];
        Action<someClass, int> enumDelegate = CreateSetterFromPropertyInfo<someClass, int>(enumProperty);
        enumDelegate(testclass, 2);
        Console.WriteLine(testclass.enumVal.ToString()); // writes B

        object nullableSetter = null;
        PropertyInfo enumPropertyNullable = typeof(someClass).GetProperties()[1];

        if (enumPropertyNullable.PropertyType.GetGenericTypeDefinition() == typeof(Nullable<>))
        {
            if (enumPropertyNullable.PropertyType.GetGenericArguments().First().IsEnum)
            {
                // the step below determines the method signature. I cannot deviate from what the property tells me.
                // if I use int? instead of the PropertyType I get an ArgumentException.
                MethodInfo method = typeof(program).GetMethod("CreateSetterFromPropertyInfo", BindingFlags.Static | BindingFlags.NonPublic).MakeGenericMethod(new Type[] { typeof(someClass), enumPropertyNullable.PropertyType });
                nullableSetter = method.Invoke(null, new object[] { enumPropertyNullable });
            }
        }
        if (nullableSetter != null)
        {
            Action<someClass, someEnum?> cast1 = (Action<someClass, someEnum?>) nullableSetter;
            Action<someClass, Nullable<someEnum>> cast2 = (Action<someClass, Nullable<someEnum>>)nullableSetter;
            // Action<someClass, Nullable<int>> cast3 = (Action<someClass, Nullable<int>>)nullableSetter;
            // Action<someClass, Enum> cast4 = (Action<someClass, Enum>)nullableSetter;

            // at this point I am trying to cast a string to a enum. That works. However the parameter should be a nullable enum.
            Type subtype = enumPropertyNullable.PropertyType.GetGenericArguments().First();
            Enum result = Enum.Parse(subtype, "B") as Enum;
            // here is where I am getting the error. I can't find a way to cast the string I have to a nullable enum. 
            //cast1(testclass, result);
        }

    }

    private static Action<C, P> CreateSetterFromPropertyInfo<C, P>(PropertyInfo property)
    {

        Action<C, P> setForAnyProperty = (Action<C, P>)Delegate.CreateDelegate(typeof(Action<C, P>), null, property.GetSetMethod());
        return setForAnyProperty;
    }

    public class someClass
    {
        public someEnum enumVal {get; set;}
        public someEnum? enumValNullable { get; set; }
    }

    public enum someEnum
    {
        A = 1,
        B = 2
    }

}

我只遇到可空枚举的问题，普通枚举没有。如果你 运行 这个例子你可以看到使用一个带有整数作为签名的委托，代码 运行 没问题。

问题 可空枚举迫使我在委托 setter 签名中使用枚举本身。当我实际上试图用一个值填充对象时，代码没有引用有问题的枚举，这是使用泛型的一点。我也不确定为什么我不能使用 int？对于可为空的枚举。欢迎任何见解！

此时我很想使用 PropertyInfo.SetValue 方法而不是委托。好像还行。

Answer 1

更新：下面的新 NullableEnumCopier class 不了解 someClass 或 someEnum。

public static class program
{
    static void Main(string[] args)
    {
        someClass testclass = new someClass();
        NullableEnumCopier<someClass, someEnum>.Copy(testclass);
    }

    public static class NullableEnumCopier<TClass, TEnum> where TEnum : struct
    {
        public static void Copy(TClass item)
        {
            PropertyInfo enumProperty = typeof(TClass).GetProperties()[0];
            Action<TClass, int> enumDelegate = CreateSetterFromPropertyInfo<TClass, int>(enumProperty);
            enumDelegate(item, 2);
            //Console.WriteLine(item.enumVal.ToString()); // writes B

            object nullableSetter = null;
            PropertyInfo enumPropertyNullable = typeof(TClass).GetProperties()[1];

            if (enumPropertyNullable.PropertyType.GetGenericTypeDefinition() == typeof(Nullable<>))
            {
                if (enumPropertyNullable.PropertyType.GetGenericArguments().First().IsEnum)
                {
                    // the step below determines the method signature. I cannot deviate from what the property tells me.
                    // if I use int? instead of the PropertyType I get an ArgumentException.
                    MethodInfo method = typeof(program).GetMethod("CreateSetterFromPropertyInfo", BindingFlags.Static | BindingFlags.NonPublic).MakeGenericMethod(new Type[] { typeof(TClass), enumPropertyNullable.PropertyType });
                    nullableSetter = method.Invoke(null, new object[] { enumPropertyNullable });
                }
            }
            if (nullableSetter != null)
            {
                Action<TClass, TEnum?> cast1 = (Action<TClass, TEnum?>) nullableSetter;
                Action<TClass, Nullable<TEnum>> cast2 = (Action<TClass, Nullable<TEnum>>)nullableSetter;
                // Action<TClass, Nullable<int>> cast3 = (Action<TClass, Nullable<int>>)nullableSetter;
                // Action<TClass, Enum> cast4 = (Action<TClass, Enum>)nullableSetter;

                // at this point I am trying to cast a string to a enum. That works. However the parameter should be a nullable enum.
                Type subtype = enumPropertyNullable.PropertyType.GetGenericArguments().First();
                Enum result = Enum.Parse(subtype, "B") as Enum;
                // here is where I am getting the error. I can't find a way to cast the string I have to a nullable enum. 
                cast1(item, (TEnum?)(object)result);
            }
        }
    }

    private static Action<C, P> CreateSetterFromPropertyInfo<C, P>(PropertyInfo property)
    {

        Action<C, P> setForAnyProperty = (Action<C, P>)Delegate.CreateDelegate(typeof(Action<C, P>), null, property.GetSetMethod());
        return setForAnyProperty;
    }

    public class someClass
    {
        public someEnum enumVal {get; set;}
        public someEnum? enumValNullable { get; set; }
    }

    public enum someEnum
    {
        A = 1,
        B = 2
    }

}