Learn About ASP.NET Core Filters

Introduction

ASP.NET Core Filters are a powerful toolset for implementing cross-cutting concerns in your applications, providing a modular and reusable way to handle tasks such as authorization, caching, logging, exception handling, and more. In this article, we'll delve deep into the world of ASP.NET Core Filters, exploring each type—Authorization, Resource, Action, Exception, Result, and Endpoint—providing clear examples to demonstrate their usage and benefits.

1. Authorization Filter

Authorization filters are crucial for enforcing security policies in your application. They execute before the action method and are used to determine whether the current user is authorized to access a particular resource.

Example

public class CustomAuthorizationFilter : IAuthorizationFilter
{
    public void OnAuthorization(AuthorizationFilterContext context)
    {
        // Check if the user is authenticated and has necessary permissions
        if (!context.HttpContext.User.Identity.IsAuthenticated)
        {
            context.Result = new UnauthorizedResult();
            return;
        }

        // Additional authorization logic...
    }
}

[Authorize]
public IActionResult SecureAction()
{
    return Ok("Access granted.");
}

2. Resource Filter

Resource filters run before and after the action method, encapsulating logic around the execution of the action method. They are often used for caching or setting up resources that the action method requires.

Example

public class CacheResourceFilter : IResourceFilter
{
    public void OnResourceExecuting(ResourceExecutingContext context)
    {
        // Check if the response is cached
        // If cached, return the cached response
    }

    public void OnResourceExecuted(ResourceExecutedContext context)
    {
        // Cache the response if it's not already cached
    }
}

3. Action Filter

Action filters execute before and after the action method, allowing you to perform tasks such as logging, validation, or modifying the action's parameters.

Example

public class LogActionFilter : IActionFilter
{
    public void OnActionExecuting(ActionExecutingContext context)
    {
        // Log action execution start
    }

    public void OnActionExecuted(ActionExecutedContext context)
    {
        // Log action execution end
    }
}

4. Exception Filter

Exception filters handle exceptions that occur during the execution of the action method, providing a centralized location for exception logging and handling.

Example

public class CustomExceptionFilter : IExceptionFilter
{
    public void OnException(ExceptionContext context)
    {
        // Log the exception
        // Handle the exception
        // Set appropriate HTTP status code
    }
}

5. Result Filter

Result filters execute before and after the execution of the action result, allowing you to modify the result of the action method.

Example

public class ModifyResultFilter : IResultFilter
{
    public void OnResultExecuting(ResultExecutingContext context)
    {
        // Modify the result before it's executed
    }

    public void OnResultExecuted(ResultExecutedContext context)
    {
        // Perform additional tasks after the result has been executed
    }
}

6. Endpoint Filter

Endpoint filters execute before the middleware pipeline, allowing you to perform actions such as endpoint metadata modification or endpoint selection based on conditions.

Example

public class CustomEndpointFilter : IEndpointFilter
{
    public Task OnEndpointExecutingAsync(EndpointExecutingContext context)
    {
        // Perform actions before the endpoint is executed
    }

    public Task OnEndpointExecutedAsync(EndpointExecutedContext context)
    {
        // Perform actions after the endpoint has been executed
    }
}

Conclusion

ASP.NET Core Filters provide a flexible and efficient mechanism for handling cross-cutting concerns in your application. By understanding and leveraging each type of filter—Authorization, Resource, Action, Exception, Result, and Endpoint—you can improve the security, performance, and maintainability of your ASP.NET Core applications. With the provided examples and guidelines, you're now equipped to harness the power of ASP.NET Core Filters effectively in your projects.