Detailed explanation of attributeusage usage

attributeusage usage: 1. Create a csharp sample file; 2. Define an attribute named MyAttribute and use AttributeUsage to limit the usage specifications of the attribute; 3. Set the validOn parameter to "AttributeTargets.Class" , indicating that this feature can only be applied to classes; 4. The allowMultiple parameter is set to "false", indicating that this feature can only be applied once, and vice versa.

AttributeUsage is an attribute class in C# that is used to define the usage specifications of custom attributes. When writing a custom attribute, you can use the AttributeUsage attribute to clarify which target elements the attribute can be used on, such as classes, methods, properties, and so on. This article will introduce in detail the usage of AttributeUsage and how to use it.

1. The syntax and attributes of the AttributeUsage attribute

The syntax of the AttributeUsage attribute is as follows:

[AttributeUsage(validOn, AllowMultiple = allowMultiple, Inherited = inherited)]

Among them, the validOn parameter specifies the target type that the attribute can be used for, the allowMultiple parameter indicates whether the same attribute is allowed to be applied multiple times, and the inherited parameter indicates whether the derived class inherits the attribute. The options for these parameters are as follows:

- validOn: One or more values ​​of the AttributeTargets enumeration that represent the target elements to which the attribute can be applied. The AttributeTargets enumeration includes the following values:

- Assembly: assembly

- Module: module

- Class: class

- Struct: structure

- Enum: Enumeration

- Constructor: Constructor

- Method: Method

- Property: Property

- Field: Field

- Event: event

- Interface: interface

- Parameter: parameter

- Delegate: delegate

- ReturnValue: Return Value

- allowMultiple: A Boolean value indicating whether multiple applications of the same attribute are allowed. The default value is false, which means it can only be applied once.

- inherited: A Boolean value indicating whether derived classes are allowed to inherit this attribute. The default value is true, which means inheritance is allowed.

2. Example of using AttributeUsage

The following uses a code example to demonstrate the use of AttributeUsage.

csharp
[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
public class MyAttribute : Attribute
{
    // 属性、方法等
}
[My]
public class MyClass
{
    // 类的定义
}
[My]
public interface IMyInterface
{
    // 接口的定义
}
[My]
public abstract class MyBaseClass
{
    // 抽象类的定义
}
[My]
public enum MyEnum
{
    // 枚举的定义
}
[My]
public delegate void MyDelegate();
[My]
public struct MyStruct
{
    // 结构体的定义
}
[My]
public delegate void MyMethod([My] int myParam);
[My]
public event MyDelegate MyEvent;
[My]
public int MyProperty { get; set; }
[My]
public int MyField;
[My]
public void MyMethod([My] int myParam)
{
    // 方法的定义
}
[My]
public int MyProperty
{
    [My]
    get { return myField; }
    [My]
    set { myField = value; }
}

In the above example, we defined an attribute named MyAttribute and used AttributeUsage to restrict the usage specification of the attribute. When using AttributeUsage, we set the validOn parameter to AttributeTargets.Class, indicating that the attribute can only be applied to classes. The allowMultiple parameter is set to false, indicating that the feature can only be applied once. The inherited parameter is set to true to allow derived classes to inherit this feature.

Then, we use the MyAttribute attribute on each target element, including classes, interfaces, abstract classes, enumerations, delegates, structures, methods, events, properties, fields, etc.

It should be noted that according to the restrictions of AttributeUsage, in this example the MyAttribute attribute can only be applied to the class and not to other target elements. At the same time, this attribute can only be applied once to each target element, but derived classes can inherit this attribute.

3. Summary

Through the AttributeUsage attribute, we can limit the usage specifications of custom attributes, clarify which target elements the attributes can be applied to, and control whether the attributes are allowed to be applied multiple times and whether inheritance is allowed. . This approach allows our features to be applied more flexibly and precisely to the appropriate target elements.

The above is a detailed explanation of the usage of AttributeUsage, and an introduction to how to use the AttributeUsage attribute to define the usage specifications of custom attributes. I hope readers can better understand and apply the AttributeUsage attribute through this article.

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