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Action filter

As explained in Pro ASP.NET Core 7, Tenth Edition

An action filter is an ASP.NET Core MVC filter that runs around an action method after model binding and can modify action execution or its result.

An action filter runs around an ASP.NET Core MVC action after model binding. It can change bound arguments before invocation, replace the result afterward, or stop the action from running. section 30.4.3

Use action filters when request processing depends on model-bound action data:

  • Run logic in OnActionExecuting before the action method and OnActionExecuted afterward. section 30.4.3
  • Inspect, insert, remove, or change entries in ActionArguments. section 30.4.3
  • Assign Result to short-circuit the pipeline and return an action result without invoking the action. section 30.3 section 30.4.3
  • Inspect or replace the action result and examine cancellation or exceptions through ActionExecutedContext. section 30.4.3
  • Apply filters to controllers and action methods, not Razor Pages; use page filters for the Razor Pages equivalent. section 30.4.3

Changing a bound action argument

A filter can rewrite an argument after model binding and before the action runs: `public class ChangeArgAttribute : ActionFilterAttribute { public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next) { if (context.ActionArguments.ContainsKey("message1")) context.ActionArguments["message1"] = "New message"; await next(); } }`. Applying `[ChangeArg]` to a `Messages(string message1, string message2 = "None")` action causes the action to receive the replacement value.

Table 30.2 The filter types (view table figure)

Name

Description

Authorization filters

This type of filter is used to apply the application’s authorization policy.

Resource filters

This type of filter is used to intercept requests, typically to implement features such as caching.

Action filters

This type of filter is used to modify the request before it is received by an action method or to modify the action result after it has been produced. This type of filter can be applied only to controllers and actions.

Page filters

This type of filter is used to modify the request before it is received by a Razor Page handler method or to modify the action result after it has been produced. This type of filter can be applied only to Razor Pages.

Result filters

This type of filter is used to alter the action result before it is executed or to modify the result after execution.

Exception filters

This type of filter is used to handle exceptions that occur during the execution of the action method or page handler.

Compare action filters with authorization, resource, page, result, and exception filters by purpose and endpoint scope. section 30.3

How Pro ASP.NET Core 7, Tenth Edition covers it

How deep it goes
Explains synchronous and asynchronous interfaces, lifecycle contexts, argument mutation, short-circuiting, and result handling without detailing pipeline internals.
Examples
Demonstrates a ChangeArgAttribute that replaces message1 on a Messages controller action.
What it assumes
Assumes ASP.NET Core MVC controllers, action methods, model binding, and action results; distinguishes Razor Pages through page filters.
What you can do after
Apply an action filter to alter bound arguments, short-circuit execution, or change an action result.
How it connects
Connects action filters to model binding, resource filters, page filters, result filters, authorization filters, exception filters, and middleware.
Cover of Pro ASP.NET Core 7, Tenth Edition
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Related concepts

This explainer was generated by Manning's knowledge graph from the text of Pro ASP.NET Core 7, Tenth Edition. The section links point to the parts of the book it draws on.