Filters in ASP.Net Core API

Introduction

Filters in ASP.NET Core API serve as powerful tools to address cross-cutting concerns, enhancing the modularity and maintainability of applications. This guide will explore the various types of filters through real-world examples, demonstrating their practical applications.

Authorization Filters

Authorization filters ensure controlled access to specific API endpoints. In the example, the [Authorize] filter is applied to an API endpoint within the AdminController. This ensures that only users with the specified role, in this case, “Admin,” can access the GetAdminData action method. This exemplifies how authorization filters enforce security policies, limiting access based on user roles.

[Authorize(Roles = "Admin")]
[ApiController]
[Route("api/[controller]")]
public class AdminController : ControllerBase
{
    [HttpGet]
    public IActionResult GetAdminData()
    {
       return Ok("Admin data retrieved successfully.");
    }
}

Action Filters

Action filters allow interaction with action methods by implementing the IActionFilter interface. The LogActionFilter example demonstrates the use of OnActionExecuting and OnActionExecuted methods, enabling tasks like logging or input validation before and after the execution of an action method.

public class LogActionFilter : IActionFilter
{
    public void OnActionExecuting(ActionExecutingContext context)
    {
      _loggerService.Log(LogLevel.Warning, "OnActionExecuting method called");
    }

    public void OnActionExecuted(ActionExecutedContext context)
    {
      _loggerService.Log(LogLevel.Warning, "OnActionExecuted method called");
    }
}

Exception Filters

Exception filters handle errors gracefully by implementing the IExceptionFilter interface. The CustomExceptionFilter example illustrates how to customize the handling of exceptions, providing a consistent error response. This enhances the application’s robustness and user experience during unexpected errors.

public class CustomExceptionFilter : IExceptionFilter
{
    public void OnException(ExceptionContext context)
    {
        context.Result = new ObjectResult("An error occurred.")
        {
            StatusCode = 500
        };
        context.ExceptionHandled = true;
    }
}

Result Filters

Result filters transform action results before and after their execution. By implementing the IResultFilter interface, developers can customize the behavior of the result. The CustomResultFilter example demonstrates how to modify the result, providing additional logging or transformations.

public class CustomResultFilter : IResultFilter
{
    public void OnResultExecuting(ResultExecutingContext context)
    {
      _loggerService.Log(LogLevel.Warning, "OnResultExecuting method called");
    }

    public void OnResultExecuted(ResultExecutedContext context)
    {
      _loggerService.Log(LogLevel.Warning, "OnResultExecuted method called");
    }
}

Conclusion

In conclusion, filters in ASP.NET Core API offer a modular and customizable approach to handling cross-cutting concerns. The provided examples showcase the practical applications of various filter types, highlighting their significance in enhancing security, logging, error handling, and result transformations. By incorporating filters into your ASP.NET Core API development workflow, you can achieve cleaner, more maintainable code and improve the overall performance and reliability of your applications.

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