Filters

Filters allow code to run before or after specific stages in the request processing pipeline.

Filter types

Authorization Filters

• Run first

• Determine whether the user is authorized for the request. If not, Short-circuit the pipeline.

Resource Filter

• Runs after authorization.

• OnResourceExecuting runs code before the rest of the filter pipeline. For example, before model binding.

• OnResourceExecuted runs code after the rest of the pipeline has completed.

Action/Endpoint Filters

• Runs immediately before and after an action method is called.

• Can change the arguments passed/result returned into/from an action.

• Not supported in Razor pages.

• Can be invoked on both actions and route handler-based endpoints.

Interaction between filters in the pipeline

• Synchronous Action Filters

public class SampleActionFilter : IActionFilter
{
    public void OnActionExecuting(ActionExecutingContext context)
    {
        // Do something before the action executes.
    }

    public void OnActionExecuted(ActionExecutedContext context)
    {
        // Do something after the action executes.
    }
}

• Asynchronous Action Filteres

public class SampleAsyncActionFilter : IAsyncActionFilter
{
    public async Task OnActionExecutionAsync(
        ActionExecutingContext context, ActionExecutionDelegate next)
    {
        // Do something before the action executes.
        await next();
        // Do something after the action executes.
    }
}

Built-in Filter Attributes

• ActionFilterAttribute

• ExceptionFilterAttribute

• ResultFilterAttribute

• FormatFilterAttribute

• ServiceFilterAttribute

public class ResponseHeaderAttribute : ActionFilterAttribute
{
    private readonly string _name;
    private readonly string _value;

    public ResponseHeaderAttribute(string name, string value) =>
        (_name, _value) = (name, value);

    public override void OnResultExecuting(ResultExecutingContext context)
    {
        context.HttpContext.Response.Headers.Add(_name, _value);

        base.OnResultExecuting(context);
    }
}

[ResponseHeader("Filter-Header", "Filter Value")]
public class ResponseHeaderController : ControllerBase
{
    public IActionResult Index() =>
        Content("Examine the response headers using the F12 developer tools.");

    // ...