仅在特定类型上允许自定义属性

Question

有没有办法强制编译器限制自定义属性的使用仅用于特定属性类型,如int,short,string(所有基本类型)？
类似于AttributeUsageAttribute的ValidOn- AttributeTargets枚举.

Answer 1

不,基本上你不能.你可以将它限制为structvs classvs interface,即它.另外:您无法向代码外部的类型添加属性(除了via TypeDescriptor,这是不一样的).

Answer 2

您可以运行此单元测试来检查它.

首先,声明验证属性PropertyType:

  [AttributeUsage(AttributeTargets.Class)]
    // [JetBrains.Annotations.BaseTypeRequired(typeof(Attribute))] uncomment if you use JetBrains.Annotations
    public class PropertyTypeAttribute : Attribute
    {
        public Type[] Types { get; private set; }

        public PropertyTypeAttribute(params Type[] types)
        {
            Types = types;
        }
    }

创建单元测试:

 [TestClass]
    public class TestPropertyType 
    {
        public static Type GetNullableUnderlying(Type nullableType)
        {
            return Nullable.GetUnderlyingType(nullableType) ?? nullableType;
        }

        [TestMethod]
        public void Test_PropertyType()
        {
            var allTypes = AppDomain.CurrentDomain.GetAssemblies().SelectMany(a => a.GetTypes());
            var allPropertyInfos = allTypes.SelectMany(a => a.GetProperties()).ToArray();

            foreach (var propertyInfo in allPropertyInfos)
            {
                var propertyType = GetNullableUnderlying(propertyInfo.PropertyType);
                foreach (var attribute in propertyInfo.GetCustomAttributes(true))
                {
                    var attributes = attribute.GetType().GetCustomAttributes(true).OfType<PropertyTypeAttribute>();
                    foreach (var propertyTypeAttr in attributes)
                        if (!propertyTypeAttr.Types.Contains(propertyType))
                            throw new Exception(string.Format(
                                "Property '{0}.{1}' has invalid type: '{2}'. Allowed types for attribute '{3}': {4}",
                                propertyInfo.DeclaringType,
                                propertyInfo.Name,
                                propertyInfo.PropertyType,
                                attribute.GetType(),
                                string.Join(",", propertyTypeAttr.Types.Select(x => "'" + x.ToString() + "'"))));
                }
            }
        }
    }

例如,您的属性仅允许小数属性类型:

 [AttributeUsage(AttributeTargets.Property)]
    [PropertyType(typeof(decimal))]
    public class PriceAttribute : Attribute
    {

    }

示例型号:

public class TestModel  
{
    [Price]
    public decimal Price1 { get; set; } // ok

    [Price]
    public double Price2 { get; set; } // error
}

Answer 3

如果属性放置在不是字符串列表的属性/字段上，下面的代码将返回错误。

该行if (!(value is List<string> list))可能是 C#6 或 7 功能。

[AttributeUsage(AttributeTargets.Property |
                AttributeTargets.Field, AllowMultiple = false)]
public sealed class RequiredStringListAttribute : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext context)
    {
        if (!(value is List<string> list))
            return new ValidationResult($"The required attrribute must be of type List<string>");

        bool valid = false;
        foreach (var item in list)
        {
            if (!string.IsNullOrWhiteSpace(item))
                valid = true;
        }

        return valid
            ? ValidationResult.Success
            : new ValidationResult($"This field is required"); ;
    }

}