英文:
Getting generic attribute from a property using reflection without knowing the generic type
问题
C# 11引入了通用属性。是否可以使用GetCustomAttribute方法（或类似方法）获取附加到PropertyInfo的属性，而不知道其通用类型，特别是当属性继承自其他属性时？
以下是您的代码：
public class MyTypedAttribute<TValue> : Attribute
{
public virtual TValue ProcessValue(object value) { }
}
public class MyNumberAttribute : MyTypedAttribute<int>
{
public override int ProcessValue(object value) { }
}
public class MyStringAttribute : MyTypedAttribute<string>
{
public override string ProcessValue(object value) { }
}
我的类具有多个具有这些属性的属性，并且我想使用反射获取它们。通常我会使用通用的 PropertyInfo.GetCustomAttribute<> 函数，但是当属性本身也期望有通用参数时（除非通用类型在编译时已知，否则似乎无法工作，在我的情况下不知道通用类型是什么）
PropertyInfo myProp = ...;
var attr = myProp.GetCustomAttribute<MyTypedAttribute<>>();
编译错误：意外使用未绑定的通用名称
似乎我可以使用非通用的等效函数，但是返回的属性类型是Attribute?，并且似乎我无法在不知道通用类型的情况下进行转换。我们无法在转换时指定空尖括号，因此必须指定类型，但我不知道类型是什么。我唯一想到的是使用object作为通用类型，但这在运行时失败：
var attr = (MyTypedAttribute<object>?)myProp.GetCustomAttribute(typeof(MyTypedAttribute<>))
运行时错误：无法将类型为 'MyNumberAttribute' 的对象强制转换为类型为 'MyTypedAttribute`1[System.Object]' 的对象。
基本上，我试图获取属性并调用ProcessValue方法，捕获返回的值。有没有办法做到这一点？
英文:
C# 11 has introduced generic attributes. Is it possible to get the attributes that are attached to a PropertyInfo using the GetCustomAttribute method (or similar) without knowing it's generic type, particularly where the attribute inherits from another?
Here's what I have:
public class MyTypedAttribute<TValue> : Attribute
{
public virtual TValue ProcessValue(object value) { }
}
public class MyNumberAttribute : MyTypedAttribute<int>
{
public override int ProcessValue(object value) { }
}
public class MyStringAttribute : MyTypedAttribute<string>
{
public override string ProcessValue(object value) { }
}
My class has properties with a range of these attributes on them, and I want to get them using reflection. I would normally use the generic PropertyInfo.GetCustomAttribute<> function, but this doesnt seem to work when the attributes themselves also expect a generic parameter (unless the generic type is known at compile time, which in my case they it's not)
PropertyInfo myProp = ...;
var attr = myProp.GetCustomAttribute<MyTypedAttribute<>>();
>Compile error: Unexpected use of an unbound generic name
It seems I can use the non-generic equivalent function, but the returned attribute is of type Attribute?, and it seems I cannot cast this without knowing the generic type. We cannot specify empty angle brackets when casting, so a type must be specified, but I dont know what the type is. The only thing I can think of doing is using object as the generic type, but this fails at run time:
var attr = (MyTypedAttribute<object>?)myProp.GetCustomAttribute(typeof(MyTypedAttribute<>))
>Runtime error: Unable to cast object of type 'MyNumberAttribute' to type 'MyTypedAttribute`1[System.Object]'.'
Essentially what I'm trying to do is get the attributes and call the ProcessValue method, capturing the returned value. Is there any way to do this?
答案1
得分: 1
以下是翻译好的部分：
你可以创建一个非泛型的基类
public class MyTypedAttribute<TValue> : MyUntypedAttribute
{
public virtual TValue ProcessValue(object value)
{
// 不管什么操作
}
public override object ProcessValueObject(object value) =>
ProcessValue(value);
}
public class MyUntypedAttribute : Attribute
{
public abstract object ProcessValueObject(object value);
}
然后你可以这样做
var attr = myProp.GetCustomAttribute<MyUntypedAttribute>();
英文:
You can make a non-generic base class
public class MyTypedAttribute<TValue> : MyUntypedAttribute
{
public virtual TValue ProcessValue(object value)
{
// whatever
}
public override object ProcessValueObject(object value) =>
ProcessValue(value);
}
public class MyUntypedAttribute : Attribute
{
public abstract object ProcessValueObject(object value);
}
Then you can do
var attr = myProp.GetCustomAttribute<MyUntypedAttribute>();
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库，让每个人都能够通过互相帮助和分享经验来进步。
评论