c# - 如何将对象传递给属性构造函数

Question

我正在尝试将对象传递给属性构造函数，如下所示:

[PropertyValidation(new NullOrEmptyValidatorScheme())]
public string Name { get; private set; }

使用这个属性构造函数:

 public PropertyValidationAttribute(IValidatorScheme validator) {
      this._ValidatorScheme = validator;
    }

代码无法编译。如何将对象传递给上述属性？

编辑:是的 NullOrEmptyValidatorScheme 实现了 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。

Answer 1

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...您不能使用new 或其他更复杂的代码。简而言之;你不能这样做。不过你可以给它类型:

[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)。

来自 ECMA 334v4:

§24.1.3 Attribute parameter types

The types of positional and named parameters for an attribute class are limited to the attribute parameter types, which are:

• 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 Attribute specification

...

An expression E is an attribute-argument-expression if all of the following statements are true:

• The type of E is 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.