如何在控制器中使用过滤器？

在控制器中使用过滤器时，若遇到多个操作共享的逻辑（如身份验证、日志记录等），应优先使用过滤器来保持代码整洁和可重用。1. 过滤器是在动作执行前后运行的逻辑块，用于处理跨多个操作的任务；2. 应用过滤器通常通过将属性添加到控制器或动作方法上实现，如[Authorize]；3. 创建自定义过滤器需实现特定接口，如IActionFilter，并可在动作执行前进行检查；4. 全局过滤器可通过注册方式应用于所有请求，适用于防伪保护、全站HTTPS强制等场景。通过合理使用过滤器，可以有效减少重复代码并提高应用程序的可维护性。

When you're working with filters in a controller—especially in frameworks like ASP.NET MVC or similar—you’re essentially applying logic before or after an action runs. The main idea is to keep your code clean and reusable by separating concerns like authentication, logging, or validation from the actual action methods.

Here’s how to actually use them without getting lost in abstraction.

What Are Filters and Why Use Them?

Filters are chunks of logic that run before or after a controller action. They help you handle tasks that apply across multiple actions, like checking user permissions, logging requests, or handling exceptions.

Instead of repeating code inside every action method, you attach a filter once and it runs automatically. Think of it like setting up a checkpoint: certain rules get enforced no matter which action is called.

Common uses:

• Authentication and authorization checks
• Logging or timing requests
• Handling errors consistently
• Caching responses

How to Apply Filters in Your Controller

Applying a filter is usually just a matter of adding an attribute to your controller class or a specific action method.

For example, in ASP.NET MVC:

[Authorize]
public class AccountController : Controller
{
    public IActionResult Login()
    {
        return View();
    }
}

In this case, the [Authorize] filter ensures only authenticated users can access any action in this controller.

You can also apply it to a single method:

public class BlogController : Controller
{
    [Authorize]
    public IActionResult Edit(int id)
    {
        return View();
    }

    public IActionResult Index()
    {
        return View();
    }
}

Now only the Edit action requires authorization, while Index remains public.

Creating Custom Filters (When Built-ins Aren’t Enough)

Sometimes the built-in filters don’t cut it. For example, maybe you want to check if a user has completed their profile before allowing them to post content.

To create a custom filter, you typically implement one of the base interfaces like IActionFilter, IAuthorizationFilter, or IResultFilter.

Here’s a basic example of a custom action filter:

public class RequireProfileCompleteAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext context)
    {
        var httpContext = context.HttpContext;
        var user = httpContext.User;

        // Check if user's profile is complete
        if (!IsProfileComplete(user))
        {
            context.Result = new RedirectResult("/Profile/Complete");
        }

        base.OnActionExecuting(context);
    }

    private bool IsProfileComplete(ClaimsPrincipal user)
    {
        // Logic to check profile status
        return false; // Just as an example
    }
}

Then apply it like any other filter:

[RequireProfileComplete]
public IActionResult PostArticle()
{
    return View();
}

This keeps your controller lean and makes the behavior reusable across different actions or controllers.

Registering Global Filters (Apply to All Actions)

If you want a filter to run on every request, you can register it globally instead of attaching it manually to each controller or action.

In ASP.NET Core, you do this in Startup.cs or Program.cs depending on the version:

services.AddControllersWithViews(options =>
{
    options.Filters.Add(new AutoValidateAntiforgeryTokenAttribute());
});

Or for a custom global filter:

services.AddControllersWithViews(options =>
{
    options.Filters.Add(typeof(MyGlobalFilterAttribute));
});

This is useful for things like anti-forgery protection, logging all requests, or enforcing HTTPS site-wide.

Using filters effectively helps you write cleaner, more maintainable code. You don’t need to sprinkle the same checks in every action—just define the rule once and let the framework enforce it.

And honestly, once you start using them regularly, you’ll wonder how you ever managed without them.

基本上就这些。

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