我有一个包含扩展方法的类，可以让我读取这样的数据注释：
int maxRefLen = ReflectionAPI.GetProperty<Organisation, String>(x => x.Name)
.GetAttribute<StringLengthAttribute>()
.GetValueOrDefault(x => x.MaximumLength, 256);
所以如果你使用它，你应该能够得到这样的值EditableAttribute：
bool isEditable = ReflectionAPI.GetProperty<Foo, String>(x => x.NomInvertebrat)
.GetAttribute<EditableAttribute>()
.GetValueOrDefault(x => x.AllowEdit, true);
至于在运行时设置数据注释，我自己没有做过，但我读到这里有一个解决方案：在运行时设置数据注释
我认为获取特定类型的所有数据注释的列表需要阅读实体框架元数据。我再次没有尝试过这个。
如果你把它加在一起，我个人认为它感觉笨重而不是优雅，但你已经要求使用DataAnnotations 的解决方案，更优雅的东西可能意味着进入你的架构。
我倾向于这样做：
public bool StartDateIsReadOnly
{
//use this property client-side to disable the input
get{ return Project.IsStarted;}
}
//Implement IValidatable object to do server side validation
public IEnumerable<ValidationResult> Validate(ValidationContext validationContext
{
bool startdateIsChanged = // I'll leave you to work out this bit
var results = new List<ValidationResult>();
if(StartDateIsReadOnly && startdateIsChanged)
results.Add(new ValidationResult("Start Date cannot be changed after project is started");
}
这是 ReflectionAPI 类：
请注意，该课程包括@JonSkeet 发布并描述为“邪恶”的部分黑客攻击。我个人认为这一点还不错，但您应该阅读以下参考资料：
覆盖值类型和引用类型的通用方法。
邪恶代码 - 重载解决方法
public static class ReflectionAPI
{
public static int GetValueOrDefault<TInput>(this TInput a, Func<TInput, int> func, int defaultValue)
where TInput : Attribute
//Have to restrict to struct or you get the error:
//The type 'R' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable<T>'
{
if (a == null)
return defaultValue;
return func(a);
}
public static Nullable<TResult> GetValueOrDefault<TInput, TResult>(this TInput a, Func<TInput, TResult> func, Nullable<TResult> defaultValue)
where TInput : Attribute
where TResult : struct
//Have to restrict to struct or you get the error:
//The type 'R' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable<T>'
{
if (a == null)
return defaultValue;
return func(a);
}
//In order to constrain to a class without interfering with the overload that has a generic struct constraint
//we need to add a parameter to the signature that is a reference type restricted to a class
public class ClassConstraintHack<T> where T : class { }
//The hack means we have an unused parameter in the signature
//http://msmvps.com/blogs/jon_skeet/archive/2010/11/02/evil-code-overload-resolution-workaround.aspx
public static TResult GetValueOrDefault<TInput, TResult>(this TInput a, Func<TInput, TResult> func, TResult defaultValue, ClassConstraintHack<TResult> ignored = default(ClassConstraintHack<TResult>))
where TInput : Attribute
where TResult : class
{
if (a == null)
return defaultValue;
return func(a);
}
//I don't go so far as to use the inheritance trick decribed in the evil code overload resolution blog,
//just create some overloads that take nullable types - and I will just keep adding overloads for other nullable type
public static bool? GetValueOrDefault<TInput>(this TInput a, Func<TInput, bool?> func, bool? defaultValue)
where TInput : Attribute
{
if (a == null)
return defaultValue;
return func(a);
}
public static int? GetValueOrDefault<TInput>(this TInput a, Func<TInput, int?> func, int? defaultValue)
where TInput : Attribute
{
if (a == null)
return defaultValue;
return func(a);
}
public static T GetAttribute<T>(this PropertyInfo p) where T : Attribute
{
if (p == null)
return null;
return p.GetCustomAttributes(false).OfType<T>().LastOrDefault();
}
public static PropertyInfo GetProperty<T, R>(Expression<Func<T, R>> expression)
{
if (expression == null)
return null;
MemberExpression memberExpression = expression.Body as MemberExpression;
if (memberExpression == null)
return null;
return memberExpression.Member as PropertyInfo;
}
}
