C#特性中this关键字使用疑问：未显式创建对象为何可用

Great question! Let me break this down clearly for you:

First, let's clarify how attributes work in C#:

• When you apply an attribute like [Help("this is a do-nothing class")] to a class (or any other target like methods, properties), you're not directly creating an instance of HelpAttribute in your code. However, the .NET runtime automatically creates an instance of your attribute class when you access it via reflection.

Now, about the this keyword in the constructor:

• The this keyword always refers to the current instance of the class whose constructor is being executed—regardless of whether you created that instance manually with new or the runtime created it for you.
• When the runtime processes your attribute (for example, when you call typeof(AnyClass).GetCustomAttributes(...)), it calls the HelpAttribute constructor with the string you provided. At that moment, an instance of HelpAttribute is already allocated in memory, and this points to that instance. The constructor uses this to assign the incoming parameter to the instance's description field.

Let me add a concrete example to make it tangible. If you run this code:

using System;
using System.Reflection;

public class HelpAttribute : Attribute {
    public HelpAttribute(String Description_in) {
        this.description = Description_in;
    }
    protected String description;
    public String Description {
        get { return this.description; }
    }
}
[Help("this is a do-nothing class")]
public class AnyClass { }

class Program {
    static void Main() {
        // This line triggers the runtime to create the HelpAttribute instance
        var helpAttribute = (HelpAttribute)typeof(AnyClass)
            .GetCustomAttributes(typeof(HelpAttribute), false)[0];
        
        Console.WriteLine(helpAttribute.Description); // Outputs: this is a do-nothing class
    }
}

When GetCustomAttributes is called, the runtime:

1. Allocates memory for a HelpAttribute instance
2. Calls its constructor with the string "this is a do-nothing class"
3. Inside the constructor, this refers to that newly allocated instance, so this.description = Description_in sets the instance's field correctly

To sum up: You don't see the new keyword in your code, but the runtime handles creating the attribute instance for you. Whenever a constructor runs (no matter who invokes it), this will always refer to the instance being initialized.

内容的提问来源于stack exchange，提问作者Breathing