对象传递把属性构造函数
问题说明
我正在尝试将对象传递给属性构造函数，如下所示:
I am attempting to pass objects into an Attributes constructor as follows:
[PropertyValidation(new NullOrEmptyValidatorScheme())]
public string Name { get; private set; }
使用此属性构造函数:
public PropertyValidationAttribute(IValidatorScheme validator) {
this._ValidatorScheme = validator;
}
代码无法编译.如上所述，如何将对象传递给属性?
The code won't compile. How can I pass an object into an attribute as above?
是的 NullOrEmptyValidatorScheme 实现了 IValidatorScheme.
Yes NullOrEmptyValidatorScheme implements IValidatorScheme.
错误:错误 CS0182:属性参数必须是属性参数类型的常量表达式、typeof 表达式或数组创建表达式.
The error: error CS0182: An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type.
正确答案
属性中的值仅限于简单类型；例如，基本常量(包括字符串)和 typeof...你不能使用 new 或其他更复杂的代码.简而言之;你不能这样做.你可以给它类型:
The values into attributes are limited to simple types; for example, basic constants (including strings) and typeof... you can't use new or other more complex code. In short; you can't do this. You can give it the type though:
[PropertyValidation(typeof(NullOrEmptyValidatorScheme)]
即PropertyValidation 构造函数采用 Type，并在代码中使用 Activator.CreateInstance 来创建对象.请注意，理想情况下，您应该只在内部存储字符串 (AssemblyQualifiedName).
i.e. the PropertyValidation ctor takes a Type, and use Activator.CreateInstance inside the code to create the object. Note that you should ideally just store the string internally (AssemblyQualifiedName).
来自 ECMA 334v4:
From ECMA 334v4:
§24.1.3 属性参数类型
§24.1.3 Attribute parameter types
位置和命名的类型属性类的参数是仅限于 属性参数类型，它们是:
The types of positional and named parameters for an attribute class are limited to the attribute parameter types, which are:
- 以下类型之一:
bool、byte、char、double、float、int、long、short、string. - 类型
object. - 类型
System.Type. - 枚举类型，前提是它具有公共可访问性并且嵌套的类型(如果有)还具有公共可访问性.
- 上述的一维数组类型.
- One of the following types:
bool,byte,char,double,float,int,long,short,string. - The type
object. - The type
System.Type. - An enum type, provided it has public accessibility and the types in which it is nested (if any) also have public accessibility.
- Single-dimensional arrays of the above types.
和
§24.2 属性规范
...
一个表达式 E 是一个属性参数表达式如果全部下列说法正确的是:
An expression E is an attribute-argument-expression if all of the following statements are true:
E的类型是一个属性参数类型(第 24.1.3 节).- 在编译时，E 的值可以是解决了以下问题之一:
- 一个常数值.
- 指定非泛型的类型表达式(第 14.5.11 节)类型，封闭构造类型(第 25.5.2 节)，或未绑定的泛型类型(第 25.5 节).
- 一维数组属性参数表达式.
- The type of
Eis an attribute parameter type (§24.1.3). - At compile-time, the value of E can be resolved to one of the following:
- A constant value.
- A typeof-expression (§14.5.11) specifying a non-generic type, a closed constructed type (§25.5.2), or an unbound generic type (§25.5).
- A one-dimensional array of attribute-argument-expressions.
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