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This article introduces the relevant knowledge of "how to implement the filter in ASP.NET Core MVC". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

How does the filter work?

Filters run in the MVC Action call pipeline, sometimes referred to as the filter pipeline. The filter pipe is not executed until MVC selects the Action method to execute:

Realize

The filter supports both synchronous and asynchronous interface definitions. Depending on the type of task you perform, you can choose a synchronous or asynchronous implementation.

Synchronization filters define OnStageExecuting and OnStageExecuted methods that run code before and after a specific phase of the pipeline. For example, the IActionFilter filter calls OnActionExecuting before calling the Action method and OnActionExecuted after the return of the Action method:

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}}

The asynchronous filter defines an OnStageExecutionAsync method. This method provides a delegate to FilterTypeExecutionDelegate that performs specific pipeline phase work when the delegate is called. For example, ActionExecutionDelegate is used to call the Action method, and you can execute code before and after calling it.

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}}

You can implement multiple filter interfaces in a single class. For example, the ActionFilterAttribute abstract class implements IActionFilter and IResultFilter and their corresponding asynchronous interfaces.

It is suggested that you do not need to implement both filter interfaces at the same time, either synchronous or asynchronous. The framework first checks whether the filter implements the asynchronous interface, and if so, executes the asynchronous method directly. If not, it executes the methods of the synchronous interface. If you implement both interfaces on the same class, only asynchronous methods are called. When using an abstract class like ActionFilterAttribute, you only need to override the synchronous or asynchronous methods of the filter.

Filter Typ

ASP.NET Core has the following five types of filters, each of which is executed at different stages in the filter pipeline:

Authorization Filter

The authorization filter is executed first in the filter pipeline, which is usually used to verify the validity of the current request, and the pipeline following the illegality is skipped directly. They have only one Before method, unlike most other filters that support pre-and post-phase methods. Note that you do not throw exceptions in authorization filters because there is no code to handle exceptions (exception filters do not handle them).

Resource Filter

The resource filter is the second run, executed after Authorization Filter and before Model Binding. In terms of performance, resource filters are particularly important in implementing caching or truncating filter pipes.

Action Filter

The most frequently used filter executes code before and after calling the Acioin method. Similar to Resource Filter, but Model Binding executes later.

Exception Filter

Used to execute exception handling policies for applications.

Result Filter

When the Action execution is complete, the filter is finally executed. Used to handle the ActionResult result output strategy.

Filter running sequence

Every request from ASP.NET Core passes through the registered `Middleware` before the filter is executed: all filters of the same type are executed on a first-in-first-out basis.

The color arrow is a normal procedure.

The gray arrow is the exception handling flow.

Scope and execution order of filters

Filters have three different levels of scope. You can register the filter with the specified controller or Action method through Attribute, or you can register the filter with the collection of MvcOptions.Filters as a global filter in the ConfigureServices method of the Startup class (valid for all controllers and Action methods):

Public class Startup {public void ConfigureServices (IServiceCollection services) {services.AddMvc (options = > {options.Filters.Add (new AddHeaderAttribute ("GlobalAddHeader", "Result filter added to MvcOptions.Filters")); / / an instance options.Filters.Add (typeof (SampleActionFilter)) / / by type options.Filters.Add (new SampleGlobalActionFilter ()); / / an instance}); services.AddScoped ();}}

Examples come from ASP.NET Core MVC English documentation

Default execution order

When there are multiple filters at some stage of the pipeline, the default order in which the filters are executed is determined by the scope: the global filter takes precedence over the controller filter, and the controller filter takes precedence over the Action method filter.

The following example is the order in which synchronous Action filter calls are called:

Ordinal filter scope filter method 1GlobalOnActionExecuting2ControllerOnActionExecuting3MethodOnActionExecuting4MethodOnActionExecuted5ControllerOnActionExecuted6GlobalOnActionExecuted

Prompt that the base class Controller for each controller contains OnActionExecuting and OnActionExecuted methods. Where OnActionExecuting is called before all filters and OnActionExecuted is called after all filters.

Override the default execution order

You can override the default execution order by implementing the IOrderedFilter interface. This interface exposes the Order attribute to indicate priority to determine the order of execution; a filter with a lower Order value will perform a pre-method before a filter with a higher Order value; and a filter with a lower Order value will perform a post method after a filter with a higher Order value.

You can set the Order property using the constructor parameter:

[MyFilter (Name = "Controller Level Attribute", Order=1)]

If you set the Order of the Action filter in the above example to 1, and the Order property of the controller and the global filter to 2 and 3, respectively, the order of execution will be the opposite of the default.

Ordinal filter scope Order attribute filter method 1Method1OnActionExecuting2Controller2OnActionExecuting3Global3OnActionExecuting4Global3OnActionExecuted5Controller2OnActionExecuted6Method1OnActionExecuted

When the filter is executed, the Order attribute takes precedence over scope. The filter is sorted first by the Order attribute and then by scope. All built-in filters implement the IOrderedFilter interface and set the Order value to 0 by default; therefore, unless the Order property is set to a non-zero value, it is executed according to the priority of the scope.

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