Filters in ASP.NET Core

2023-08-09 12:55:28

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

Built-in filters handle tasks such as:

Authorization (preventing access to resources a user isn't authorized for).
Response caching (short-circuiting the request pipeline to return a cached response).

Custom filters can be created to handle cross-cutting concerns. Examples of cross-cutting concerns include error handling, caching, configuration, authorization, and logging. Filters avoid duplicating code. For example, an error handling exception filter could consolidate error handling.

This document applies to Razor Pages, API controllers, and controllers with views.

View or download sample (how to download).

How filters work

Filters run within the ASP.NET Core action invocation pipeline, sometimes referred to as the filter pipeline. The filter pipeline runs after ASP.NET Core selects the action to execute.

Filter types

Each filter type is executed at a different stage in the filter pipeline:

Authorization filters run first and are used to determine whether the user is authorized for the request. Authorization filters short-circuit the pipeline if the request is unauthorized.
Resource filters:
- Run after authorization.
- OnResourceExecuting can run code before the rest of the filter pipeline. For example, OnResourceExecuting can run code before model binding.
- OnResourceExecuted can run code after the rest of the pipeline has completed.
Action filters can run code immediately before and after an individual action method is called. They can be used to manipulate the arguments passed into an action and the result returned from the action. Action filters are not supported in Razor Pages.
Exception filters are used to apply global policies to unhandled exceptions that occur before anything has been written to the response body.
Result filters can run code immediately before and after the execution of individual action results. They run only when the action method has executed successfully. They are useful for logic that must surround view or formatter execution.

The following diagram shows how filter types interact in the filter pipeline.

Implementation

Filters support both synchronous and asynchronous implementations through different interface definitions.

Synchronous filters can run code before (On-Stage-Executing) and after (On-Stage-Executed) their pipeline stage. For example, OnActionExecuting is called before the action method is called. OnActionExecuted is called after the action method returns.

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

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

Asynchronous filters define an On-Stage-ExecutionAsync method:

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

        // next() calls the action method.
        var resultContext = await next();
        // resultContext.Result is set.
        // Do something after the action executes.
    }
}

In the preceding code, the SampleAsyncActionFilter has an ActionExecutionDelegate (next) that executes the action method. Each of the On-Stage-ExecutionAsync methods take a FilterType-ExecutionDelegate that executes the filter's pipeline stage.

Multiple filter stages

Interfaces for multiple filter stages can be implemented in a single class. For example, the ActionFilterAttribute class implements IActionFilter, IResultFilter, and their async equivalents.

Implement either the synchronous or the async version of a filter interface, not both. The runtime checks first to see if the filter implements the async interface, and if so, it calls that. If not, it calls the synchronous interface's method(s). If both asynchronous and synchronous interfaces are implemented in one class, only the async method is called. When using abstract classes like ActionFilterAttribute override only the synchronous methods or the async method for each filter type.

Built-in filter attributes

ASP.NET Core includes built-in attribute-based filters that can be subclassed and customized. For example, the following result filter adds a header to the response:

public class AddHeaderAttribute : ResultFilterAttribute
{
    private readonly string _name;
    private readonly string _value;

    public AddHeaderAttribute(string name, string value)
    {
        _name = name;
        _value = value;
    }

    public override void OnResultExecuting(ResultExecutingContext context)
    {
        context.HttpContext.Response.Headers.Add( _name, new string[] { _value });
        base.OnResultExecuting(context);
    }
}

Attributes allow filters to accept arguments, as shown in the preceding example. Apply the AddHeaderAttribute to a controller or action method and specify the name and value of the HTTP header:

[AddHeader("Author", "Joe Smith")]
public class SampleController : Controller
{
    public IActionResult Index()
    {
        return Content("Examine the headers using the F12 developer tools.");
    }

    [ShortCircuitingResourceFilter]
    public IActionResult SomeResource()
    {
        return Content("Successful access to resource - header is set.");
    }

Several of the filter interfaces have corresponding attributes that can be used as base classes for custom implementations.

Filter attributes:

Filter scopes and order of execution

A filter can be added to the pipeline at one of three scopes:

Using an attribute on an action.
Using an attribute on a controller.
Globally for all controllers and actions as shown in the following code:

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(MySampleActionFilter));         // By type
        options.Filters.Add(new SampleGlobalActionFilter());       // An instance
    }).SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
}

The preceding code adds three filters globally using the MvcOptions.Filters collection.

Default order of execution

When there are multiple filters for a particular stage of the pipeline, scope determines the default order of filter execution. Global filters surround class filters, which in turn surround method filters.

As a result of filter nesting, the after code of filters runs in the reverse order of the before code. The filter sequence:

The before code of global filters.
- The before code of controller filters.
  - The before code of action method filters.
  - The after code of action method filters.
- The after code of controller filters.
The after code of global filters.

The following example that illustrates the order in which filter methods are called for synchronous action filters.

Sequence	Filter scope	Filter method
1	Global	`OnActionExecuting`
2	Controller	`OnActionExecuting`
3	Method	`OnActionExecuting`
4	Method	`OnActionExecuted`
5	Controller	`OnActionExecuted`
6	Global	`OnActionExecuted`

This sequence shows:

The method filter is nested within the controller filter.
The controller filter is nested within the global filter.

Controller and Razor Page level filters

Every controller that inherits from the Controller base class includes Controller.OnActionExecuting, Controller.OnActionExecutionAsync, and Controller.OnActionExecuted OnActionExecuted methods. These methods:

Wrap the filters that run for a given action.
OnActionExecuting is called before any of the action's filters.
OnActionExecuted is called after all of the action filters.
OnActionExecutionAsync is called before any of the action's filters. Code in the filter after next runs after the action method.

For example, in the download sample, MySampleActionFilter is applied globally in startup.

The TestController:

Applies the SampleActionFilterAttribute ([SampleActionFilter]) to the FilterTest2 action.
Overrides OnActionExecuting and OnActionExecuted.

public class TestController : Controller
{
    [SampleActionFilter]
    public IActionResult FilterTest2()
    {
        return Content($"From FilterTest2");
    }

    public override void OnActionExecuting(ActionExecutingContext context)
    {
        // Do something before the action executes.
        base.OnActionExecuting(context);
    }

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

Navigating to https://localhost:5001/Test/FilterTest2 runs the following code:

TestController.OnActionExecuting
- MySampleActionFilter.OnActionExecuting
  - SampleActionFilterAttribute.OnActionExecuting
    - TestController.FilterTest2
  - SampleActionFilterAttribute.OnActionExecuted
- MySampleActionFilter.OnActionExecuted
TestController.OnActionExecuted

For Razor Pages, see Implement Razor Page filters by overriding filter methods.

Overriding the default order

The default sequence of execution can be overridden by implementing IOrderedFilter. IOrderedFilter exposes the Order property that takes precedence over scope to determine the order of execution. A filter with a lower Order value:

Runs the before code before that of a filter with a higher value of Order.
Runs the after code after that of a filter with a higher Order value.

The Order property can be set with a constructor parameter:

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

Consider the same 3 action filters shown in the preceding example. If the Order property of the controller and global filters is set to 1 and 2 respectively, the order of execution is reversed.

Sequence	Filter scope	`Order` property	Filter method
1	Method	0	`OnActionExecuting`
2	Controller	1	`OnActionExecuting`
3	Global	2	`OnActionExecuting`
4	Global	2	`OnActionExecuted`
5	Controller	1	`OnActionExecuted`
6	Method	0	`OnActionExecuted`

The Order property overrides scope when determining the order in which filters run. Filters are sorted first by order, then scope is used to break ties. All of the built-in filters implement IOrderedFilter and set the default Order value to 0. For built-in filters, scope determines order unless Order is set to a non-zero value.

Cancellation and short-circuiting

The filter pipeline can be short-circuited by setting the Result property on the ResourceExecutingContext parameter provided to the filter method. For instance, the following Resource filter prevents the rest of the pipeline from executing:

public class ShortCircuitingResourceFilterAttribute : Attribute, IResourceFilter
{
    public void OnResourceExecuting(ResourceExecutingContext context)
    {
        context.Result = new ContentResult()
        {
            Content = "Resource unavailable - header not set."
        };
    }

    public void OnResourceExecuted(ResourceExecutedContext context)
    {
    }
}

In the following code, both the ShortCircuitingResourceFilter and the AddHeader filter target the SomeResource action method. The ShortCircuitingResourceFilter:

Runs first, because it's a Resource Filter and AddHeader is an Action Filter.
Short-circuits the rest of the pipeline.

Therefore the AddHeader filter never runs for the SomeResource action. This behavior would be the same if both filters were applied at the action method level, provided the ShortCircuitingResourceFilter ran first. The ShortCircuitingResourceFilter runs first because of its filter type, or by explicit use of Order property.

[AddHeader("Author", "Joe Smith")]
public class SampleController : Controller
{
    public IActionResult Index()
    {
        return Content("Examine the headers using the F12 developer tools.");
    }

    [ShortCircuitingResourceFilter]
    public IActionResult SomeResource()
    {
        return Content("Successful access to resource - header is set.");
    }

Dependency injection

Filters can be added by type or by instance. If an instance is added, that instance is used for every request. If a type is added, it's type-activated. A type-activated filter means:

An instance is created for each request.
Any constructor dependencies are populated by dependency injection (DI).

Filters that are implemented as attributes and added directly to controller classes or action methods cannot have constructor dependencies provided by dependency injection (DI). Constructor dependencies cannot be provided by DI because:

Attributes must have their constructor parameters supplied where they're applied.
This is a limitation of how attributes work.

The following filters support constructor dependencies provided from DI:

ServiceFilterAttribute
TypeFilterAttribute
IFilterFactory implemented on the attribute.

The preceding filters can be applied to a controller or action method:

Loggers are available from DI. However, avoid creating and using filters purely for logging purposes. The built-in framework logging typically provides what's needed for logging. Logging added to filters:

Should focus on business domain concerns or behavior specific to the filter.
Should not log actions or other framework events. The built in filters log actions and framework events.

ServiceFilterAttribute

Service filter implementation types are registered in ConfigureServices. A ServiceFilterAttribute retrieves an instance of the filter from DI.

The following code shows the AddHeaderResultServiceFilter:

public class AddHeaderResultServiceFilter : IResultFilter
{
    private ILogger _logger;
    public AddHeaderResultServiceFilter(ILoggerFactory loggerFactory)
    {
        _logger = loggerFactory.CreateLogger<AddHeaderResultServiceFilter>();
    }

    public void OnResultExecuting(ResultExecutingContext context)
    {
        var headerName = "OnResultExecuting";
        context.HttpContext.Response.Headers.Add(
            headerName, new string[] { "ResultExecutingSuccessfully" });
        _logger.LogInformation("Header added: {HeaderName}", headerName);
    }

    public void OnResultExecuted(ResultExecutedContext context)
    {
        // Can't add to headers here because response has started.
    }
}

In the following code, AddHeaderResultServiceFilter is added to the DI container:

public void ConfigureServices(IServiceCollection services)
{
    // Add service filters.
    services.AddScoped<AddHeaderResultServiceFilter>();
    services.AddScoped<SampleActionFilterAttribute>();

    services.AddMvc(options =>
    {
        options.Filters.Add(new AddHeaderAttribute("GlobalAddHeader",
            "Result filter added to MvcOptions.Filters"));         // An instance
        options.Filters.Add(typeof(MySampleActionFilter));         // By type
        options.Filters.Add(new SampleGlobalActionFilter());       // An instance
    }).SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
}

In the following code, the ServiceFilter attribute retrieves an instance of the AddHeaderResultServiceFilter filter from DI:

[ServiceFilter(typeof(AddHeaderResultServiceFilter))]
public IActionResult Index()
{
    return View();
}

When using ServiceFilterAttribute, setting ServiceFilterAttribute.IsReusable:

Provides a hint that the filter instance may be reused outside of the request scope it was created within. The ASP.NET Core runtime doesn't guarantee:
- That a single instance of the filter will be created.
- The filter will not be re-requested from the DI container at some later point.
Should not be used with a filter that depends on services with a lifetime other than singleton.

ServiceFilterAttribute implements IFilterFactory. IFilterFactory exposes the CreateInstance method for creating an IFilterMetadata instance. CreateInstance loads the specified type from DI.

TypeFilterAttribute

TypeFilterAttribute is similar to ServiceFilterAttribute, but its type isn't resolved directly from the DI container. It instantiates the type by using Microsoft.Extensions.DependencyInjection.ObjectFactory.

Because TypeFilterAttribute types aren't resolved directly from the DI container:

Types that are referenced using the TypeFilterAttribute don't need to be registered with the DI container. They do have their dependencies fulfilled by the DI container.
TypeFilterAttribute can optionally accept constructor arguments for the type.

When using TypeFilterAttribute, setting TypeFilterAttribute.IsReusable:

Provides hint that the filter instance may be reused outside of the request scope it was created within. The ASP.NET Core runtime provides no guarantees that a single instance of the filter will be created.
Should not be used with a filter that depends on services with a lifetime other than singleton.

The following example shows how to pass arguments to a type using TypeFilterAttribute:

[TypeFilter(typeof(LogConstantFilter),
    Arguments = new object[] { "Method 'Hi' called" })]
public IActionResult Hi(string name)
{
    return Content($"Hi {name}");
}

public class LogConstantFilter : IActionFilter
{
    private readonly string _value;
    private readonly ILogger<LogConstantFilter> _logger;

    public LogConstantFilter(string value, ILogger<LogConstantFilter> logger)
    {
        _logger = logger;
        _value = value;
    }

    public void OnActionExecuting(ActionExecutingContext context)
    {
        _logger.LogInformation(_value);
    }

    public void OnActionExecuted(ActionExecutedContext context)
    { }
}

Authorization filters

Authorization filters:

Are the first filters run in the filter pipeline.
Control access to action methods.
Have a before method, but no after method.

Custom authorization filters require a custom authorization framework. Prefer configuring the authorization policies or writing a custom authorization policy over writing a custom filter. The built-in authorization filter:

Calls the authorization system.
Does not authorize requests.

Do not throw exceptions within authorization filters:

The exception will not be handled.
Exception filters will not handle the exception.

Consider issuing a challenge when an exception occurs in an authorization filter.

Learn more about Authorization.

Resource filters

Resource filters:

Implement either the IResourceFilter or IAsyncResourceFilter interface.
Execution wraps most of the filter pipeline.
Only Authorization filters run before resource filters.

Resource filters are useful to short-circuit most of the pipeline. For example, a caching filter can avoid the rest of the pipeline on a cache hit.

Resource filter examples:

The short-circuiting resource filter shown previously.
DisableFormValueModelBindingAttribute:
- Prevents model binding from accessing the form data.
- Used for large file uploads to prevent the form data from being read into memory.

Action filters

Important

Action filters do not apply to Razor Pages. Razor Pages supports IPageFilter and IAsyncPageFilter . For more information, see Filter methods for Razor Pages.

Action filters:

Implement either the IActionFilter or IAsyncActionFilter interface.
Their execution surrounds the execution of action methods.

The following code shows a sample action filter:

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

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

The ActionExecutingContext provides the following properties:

ActionArguments - enables the inputs to an action method be read.
Controller - enables manipulating the controller instance.
Result - setting Result short-circuits execution of the action method and subsequent action filters.

Throwing an exception in an action method:

Prevents running of subsequent filters.
Unlike setting Result, is treated as a failure instead of a successful result.

The ActionExecutedContext provides Controller and Result plus the following properties:

Canceled - True if the action execution was short-circuited by another filter.
Exception - Non-null if the action or a previously run action filter threw an exception. Setting this property to null:
- Effectively handles the exception.
- Result is executed as if it was returned from the action method.

For an IAsyncActionFilter, a call to the ActionExecutionDelegate:

Executes any subsequent action filters and the action method.
Returns ActionExecutedContext.

To short-circuit, assign Microsoft.AspNetCore.Mvc.Filters.ActionExecutingContext.Result to a result instance and don't call next (the ActionExecutionDelegate).

The framework provides an abstract ActionFilterAttribute that can be subclassed.

The OnActionExecuting action filter can be used to:

Validate model state.
Return an error if the state is invalid.

public class ValidateModelAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext context)
    {
        if (!context.ModelState.IsValid)
        {
            context.Result = new BadRequestObjectResult(context.ModelState);
        }
    }

The OnActionExecuted method runs after the action method:

And can see and manipulate the results of the action through the Result property.
Canceled is set to true if the action execution was short-circuited by another filter.
Exception is set to a non-null value if the action or a subsequent action filter threw an exception. Setting Exception to null:
- Effectively handles an exception.
- ActionExecutedContext.Result is executed as if it were returned normally from the action method.

public class ValidateModelAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext context)
    {
        if (!context.ModelState.IsValid)
        {
            context.Result = new BadRequestObjectResult(context.ModelState);
        }
    }


    public override void OnActionExecuted(ActionExecutedContext context)
    {
        var result = context.Result;
        // Do something with Result.
        if (context.Canceled == true)
        {
            // Action execution was short-circuited by another filter.
        }

        if(context.Exception != null)
        {
            // Exception thrown by action or action filter.
            // Set to null to handle the exception.
            context.Exception = null;
        }
        base.OnActionExecuted(context);
    }
}

Exception filters

Exception filters:

Implement IExceptionFilter or IAsyncExceptionFilter.
Can be used to implement common error handling policies.

The following sample exception filter uses a custom error view to display details about exceptions that occur when the app is in development:

public class CustomExceptionFilter : IExceptionFilter
{
    private readonly IHostingEnvironment _hostingEnvironment;
    private readonly IModelMetadataProvider _modelMetadataProvider;

    public CustomExceptionFilter(
        IHostingEnvironment hostingEnvironment,
        IModelMetadataProvider modelMetadataProvider)
    {
        _hostingEnvironment = hostingEnvironment;
        _modelMetadataProvider = modelMetadataProvider;
    }

    public void OnException(ExceptionContext context)
    {
        if (!_hostingEnvironment.IsDevelopment())
        {
            return;
        }
        var result = new ViewResult {ViewName = "CustomError"};
        result.ViewData = new ViewDataDictionary(_modelMetadataProvider,
                                                    context.ModelState);
        result.ViewData.Add("Exception", context.Exception);
        // TODO: Pass additional detailed data via ViewData
        context.Result = result;
    }
}

Exception filters:

Don't have before and after events.
Implement OnException or OnExceptionAsync.
Handle unhandled exceptions that occur in Razor Page or controller creation, model binding, action filters, or action methods.
Do not catch exceptions that occur in resource filters, result filters, or MVC result execution.

To handle an exception, set the ExceptionHandled property to true or write a response. This stops propagation of the exception. An exception filter can't turn an exception into a "success". Only an action filter can do that.

Exception filters:

Are good for trapping exceptions that occur within actions.
Are not as flexible as error handling middleware.

Prefer middleware for exception handling. Use exception filters only where error handling differs based on which action method is called. For example, an app might have action methods for both API endpoints and for views/HTML. The API endpoints could return error information as JSON, while the view-based actions could return an error page as HTML.

Result filters

Result filters:

Implement an interface:
- IResultFilter or IAsyncResultFilter
- IAlwaysRunResultFilter or IAsyncAlwaysRunResultFilter
Their execution surrounds the execution of action results.

IResultFilter and IAsyncResultFilter

The following code shows a result filter that adds an HTTP header:

public class AddHeaderResultServiceFilter : IResultFilter
{
    private ILogger _logger;
    public AddHeaderResultServiceFilter(ILoggerFactory loggerFactory)
    {
        _logger = loggerFactory.CreateLogger<AddHeaderResultServiceFilter>();
    }

    public void OnResultExecuting(ResultExecutingContext context)
    {
        var headerName = "OnResultExecuting";
        context.HttpContext.Response.Headers.Add(
            headerName, new string[] { "ResultExecutingSuccessfully" });
        _logger.LogInformation("Header added: {HeaderName}", headerName);
    }

    public void OnResultExecuted(ResultExecutedContext context)
    {
        // Can't add to headers here because response has started.
    }
}

The kind of result being executed depends on the action. An action returning a view would include all razor processing as part of the ViewResult being executed. An API method might perform some serialization as part of the execution of the result. Learn more about action results.

Result filters are only executed when an action or action filter produces an action result. Result filters are not executed when:

An authorization filter or resource filter short-circuits the pipeline.
An exception filter handles an exception by producing an action result.

The Microsoft.AspNetCore.Mvc.Filters.IResultFilter.OnResultExecuting method can short-circuit execution of the action result and subsequent result filters by setting Microsoft.AspNetCore.Mvc.Filters.ResultExecutingContext.Cancel to true. Write to the response object when short-circuiting to avoid generating an empty response. Throwing an exception in IResultFilter.OnResultExecuting will:

Prevent execution of the action result and subsequent filters.
Be treated as a failure instead of a successful result.

When the Microsoft.AspNetCore.Mvc.Filters.IResultFilter.OnResultExecuted method runs, the response has likely already been sent to the client. If the response has already been sent to the client, it cannot be changed further.

ResultExecutedContext.Canceled is set to true if the action result execution was short-circuited by another filter.

ResultExecutedContext.Exception is set to a non-null value if the action result or a subsequent result filter threw an exception. Setting Exception to null effectively handles an exception and prevents the exception from being rethrown by ASP.NET Core later in the pipeline. There is no reliable way to write data to a response when handling an exception in a result filter. If the headers have been flushed to the client when an action result throws an exception, there's no reliable mechanism to send a failure code.

For an IAsyncResultFilter, a call to await next on the ResultExecutionDelegate executes any subsequent result filters and the action result. To short-circuit, set ResultExecutingContext.Cancel to true and don't call the ResultExecutionDelegate:

public class MyAsyncResponseFilter : IAsyncResultFilter
{
    public async Task OnResultExecutionAsync(ResultExecutingContext context,
                                             ResultExecutionDelegate next)
    {
        if (!(context.Result is EmptyResult))
        {
            await next();
        }
        else
        {
            context.Cancel = true;
        }

    }
}

The framework provides an abstract ResultFilterAttribute that can be subclassed. The AddHeaderAttribute class shown previously is an example of a result filter attribute.

IAlwaysRunResultFilter and IAsyncAlwaysRunResultFilter

The IAlwaysRunResultFilter and IAsyncAlwaysRunResultFilter interfaces declare an IResultFilter implementation that runs for all action results. This includes action results produced by:

Authorization filters and resource filters that short-circuit.
Exception filters.

For example, the following filter always runs and sets an action result (ObjectResult) with a 422 Unprocessable Entity status code when content negotiation fails:

public class UnprocessableResultFilter : Attribute, IAlwaysRunResultFilter
{
    public void OnResultExecuting(ResultExecutingContext context)
    {
        if (context.Result is StatusCodeResult statusCodeResult &&
            statusCodeResult.StatusCode == 415)
        {
            context.Result = new ObjectResult("Can't process this!")
            {
                StatusCode = 422,
            };
        }
    }

    public void OnResultExecuted(ResultExecutedContext context)
    {
    }
}

IFilterFactory

IFilterFactory implements IFilterMetadata. Therefore, an IFilterFactory instance can be used as an IFilterMetadata instance anywhere in the filter pipeline. When the runtime prepares to invoke the filter, it attempts to cast it to an IFilterFactory. If that cast succeeds, the CreateInstance method is called to create the IFilterMetadata instance that is invoked. This provides a flexible design, since the precise filter pipeline doesn't need to be set explicitly when the app starts.

IFilterFactory can be implemented using custom attribute implementations as another approach to creating filters:

public class AddHeaderWithFactoryAttribute : Attribute, IFilterFactory
{
    // Implement IFilterFactory
    public IFilterMetadata CreateInstance(IServiceProvider serviceProvider)
    {
        return new InternalAddHeaderFilter();
    }

    private class InternalAddHeaderFilter : IResultFilter
    {
        public void OnResultExecuting(ResultExecutingContext context)
        {
            context.HttpContext.Response.Headers.Add(
                "Internal", new string[] { "My header" });
        }

        public void OnResultExecuted(ResultExecutedContext context)
        {
        }
    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }
}

The preceding code can be tested by running the download sample:

Invoke the F12 developer tools.
Navigate to https://localhost:5001/Sample/HeaderWithFactory.

The F12 developer tools display the following response headers added by the sample code:

author: Joe Smith
globaladdheader: Result filter added to MvcOptions.Filters
internal: My header

The preceding code creates the internal: My header response header.

IFilterFactory implemented on an attribute

Filters that implement IFilterFactory are useful for filters that:

Don't require passing parameters.
Have constructor dependencies that need to be filled by DI.

TypeFilterAttribute implements IFilterFactory. IFilterFactory exposes the CreateInstance method for creating an IFilterMetadata instance. CreateInstance loads the specified type from the services container (DI).

public class SampleActionFilterAttribute : TypeFilterAttribute
{
    public SampleActionFilterAttribute():base(typeof(SampleActionFilterImpl))
    {
    }

    private class SampleActionFilterImpl : IActionFilter
    {
        private readonly ILogger _logger;
        public SampleActionFilterImpl(ILoggerFactory loggerFactory)
        {
            _logger = loggerFactory.CreateLogger<SampleActionFilterAttribute>();
        }

        public void OnActionExecuting(ActionExecutingContext context)
        {
            _logger.LogInformation("Business action starting...");
            // perform some business logic work

        }

        public void OnActionExecuted(ActionExecutedContext context)
        {
            // perform some business logic work
            _logger.LogInformation("Business action completed.");
        }
    }
}

The following code shows three approaches to applying the [SampleActionFilter]:

[SampleActionFilter]
public IActionResult FilterTest()
{
    return Content($"From FilterTest");
}

[TypeFilter(typeof(SampleActionFilterAttribute))]
public IActionResult TypeFilterTest()
{
    return Content($"From ServiceFilterTest");
}

// ServiceFilter must be registered in ConfigureServices or
// System.InvalidOperationException: No service for type '<filter>' has been registered.
// Is thrown.
[ServiceFilter(typeof(SampleActionFilterAttribute))]
public IActionResult ServiceFilterTest()
{
    return Content($"From ServiceFilterTest");
}

In the preceding code, decorating the method with [SampleActionFilter] is the preferred approach to applying the SampleActionFilter.

Using middleware in the filter pipeline

Resource filters work like middleware in that they surround the execution of everything that comes later in the pipeline. But filters differ from middleware in that they're part of the ASP.NET Core runtime, which means that they have access to ASP.NET Core context and constructs.

To use middleware as a filter, create a type with a Configure method that specifies the middleware to inject into the filter pipeline. The following example uses the localization middleware to establish the current culture for a request:

public class LocalizationPipeline
{
    public void Configure(IApplicationBuilder applicationBuilder)
    {
        var supportedCultures = new[]
        {
            new CultureInfo("en-US"),
            new CultureInfo("fr")
        };

        var options = new RequestLocalizationOptions
        {

            DefaultRequestCulture = new RequestCulture(culture: "en-US", 
                                                     uiCulture: "en-US"),
            SupportedCultures = supportedCultures,
            SupportedUICultures = supportedCultures
        };
        options.RequestCultureProviders = new[] 
            { new RouteDataRequestCultureProvider() { Options = options } };

        applicationBuilder.UseRequestLocalization(options);
    }
}

Use the MiddlewareFilterAttribute to run the middleware:

[Route("{culture}/[controller]/[action]")]
[MiddlewareFilter(typeof(LocalizationPipeline))]
public IActionResult CultureFromRouteData()
{
    return Content($"CurrentCulture:{CultureInfo.CurrentCulture.Name},"
        + $"CurrentUICulture:{CultureInfo.CurrentUICulture.Name}");
}

Middleware filters run at the same stage of the filter pipeline as Resource filters, before model binding and after the rest of the pipeline.

码农公寓

How filters work

Filter types

Implementation

Multiple filter stages

Built-in filter attributes

Filter scopes and order of execution

Default order of execution

Controller and Razor Page level filters

Overriding the default order

Cancellation and short-circuiting

Dependency injection

ServiceFilterAttribute

TypeFilterAttribute

Authorization filters

Resource filters

Action filters

Exception filters

Result filters

IResultFilter and IAsyncResultFilter

IAlwaysRunResultFilter and IAsyncAlwaysRunResultFilter

IFilterFactory

IFilterFactory implemented on an attribute

Using middleware in the filter pipeline

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