How to understand and use the C# attribute attributeUsage feature?

AttributeUsage is an attribute used to specify the scope and usage of custom attributes. It can be applied to the declaration of a custom attribute class to specify which target objects the attribute can be applied to.

The constructor of AttributeUsage has three parameters: validOn, AllowMultiple, and Inherited.

1. The validOn parameter specifies the target objects to which the attribute can be applied, via an enumeration of AttributeTargets values. Common target objects include classes, methods, fields, and properties.
2. The AllowMultiple parameter indicates whether this feature can be applied to the same target object multiple times, with a default value of false, meaning it does not allow multiple applications.
3. The inherited parameter indicates if the feature can be inherited, with a default value of true, meaning it can be inherited.

By utilizing AttributeUsage, restrictions can be added to custom attribute classes on their applicable scope and usage, making the use of attributes more precise and flexible.

Here is an example using AttributeUsage:

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
public class MyAttribute : Attribute
{
    // 自定义特性的实现
}

[My]
public class MyClass
{
    [My]
    public void MyMethod()
    {
        // 方法的实现
    }
}

In the example above, the validOn parameter of the MyAttribute attribute specifies that the attribute can be applied to classes and methods, the AllowMultiple parameter specifies that it cannot be applied multiple times, and the Inherited parameter specifies that it can be inherited.

The use of MyAttribute in the declaration of the MyClass class and the MyMethod method aligns with the specified scope and usage defined in AttributeUsage.

It is important to note that AttributeUsage can only be used in the declaration of custom attribute classes and cannot be directly used in other logic in the code. The specific usage of attributes also needs to be determined based on the requirements of the custom attribute.