Filters
By Tom Dykstra and Steve Smith
Filters in ASP.NET Core MVC allow you to run code before or after certain stages in the request processing pipeline.
Built-in filters handle tasks such as authorization (preventing access to resources a user isn’t authorized for), ensuring that all requests use HTTPS, and response caching (short-circuiting the request pipeline to return a cached response).
You can create custom filters to handle cross-cutting concerns for your application. Anytime you want to avoid duplicating code across actions, filters are the solution. For example, you can consolidate error handling code in a exception filter.
View or download sample from GitHub.
How do filters work?
Filters run within the MVC action invocation pipeline, sometimes referred to as the filter pipeline. The filter pipeline runs after MVC 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 current user is authorized for the current request. They can short-circuit the pipeline if a request is unauthorized.
Resource filters are the first to handle a request after authorization. They can run code before the rest of the filter pipeline, and after the rest of the pipeline has completed. They’re useful to implement caching or otherwise short-circuit the filter pipeline for performance reasons. Since they run before model binding, they’re useful for anything that needs to influence model binding.
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.
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 and are useful for logic that must surround view or formatter execution.
The following diagram shows how these filter types interact in the filter pipeline.
Implementation
Filters support both synchronous and asynchronous implementations through different interface definitions. Choose either the sync or async variant depending on the kind of task you need to perform.
Synchronous filters that can run code both before and after their pipeline stage define OnStageExecuting and OnStageExecuted methods. For example, OnActionExecuting is called before the action method is called, and OnActionExecuted is called after the action method returns.
[!code-csharpMain]
1: using FiltersSample.Helper;
2: using Microsoft.AspNetCore.Mvc.Filters;
3:
4: namespace FiltersSample.Filters
5: {
6: #region snippet_ActionFilter
7: public class SampleActionFilter : IActionFilter
8: {
9: public void OnActionExecuting(ActionExecutingContext context)
10: {
11: // do something before the action executes
12: }
13:
14: public void OnActionExecuted(ActionExecutedContext context)
15: {
16: // do something after the action executes
17: }
18: }
19: #endregion
20: }
Asynchronous filters define a single OnStageExecutionAsync method. This method takes a FilterTypeExecutionDelegate delegate which executes the filter’s pipeline stage. For example, ActionExecutionDelegate calls the action method, and you can execute code before and after you call it.
[!code-csharpMain]
1: using System.Threading.Tasks;
2: using Microsoft.AspNetCore.Mvc.Filters;
3:
4: namespace FiltersSample.Filters
5: {
6: public class SampleAsyncActionFilter : IAsyncActionFilter
7: {
8: public async Task OnActionExecutionAsync(
9: ActionExecutingContext context,
10: ActionExecutionDelegate next)
11: {
12: // do something before the action executes
13: var resultContext = await next();
14: // do something after the action executes; resultContext.Result will be set
15: }
16: }
17: }
You can implement interfaces for multiple filter stages in a single class. For example, the ActionFilterAttribute abstract class implements both IActionFilter and IResultFilter, as well as their async equivalents.
[!NOTE] Implement either the synchronous or the async version of a filter interface, not both. The framework 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 you were to implement both interfaces on one class, only the async method would be called. When using abstract classes like ActionFilterAttribute you would override only the synchronous methods or the async method for each filter type.
IFilterFactory
IFilterFactory implements IFilter. Therefore, an IFilterFactory instance can be used as an IFilter instance anywhere in the filter pipeline. When the framework 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 IFilter instance that will be invoked. This provides a very flexible design, since the precise filter pipeline does not need to be set explicitly when the application starts.
You can implement IFilterFactory on your own attribute implementations as another approach to creating filters:
[!code-csharpMain]
1: using System;
2: using Microsoft.AspNetCore.Mvc.Filters;
3:
4: namespace FiltersSample.Filters
5: {
6: #region snippet_IFilterFactory
7: public class AddHeaderWithFactoryAttribute : Attribute, IFilterFactory
8: {
9: // Implement IFilterFactory
10: public IFilterMetadata CreateInstance(IServiceProvider serviceProvider)
11: {
12: return new InternalAddHeaderFilter();
13: }
14:
15: private class InternalAddHeaderFilter : IResultFilter
16: {
17: public void OnResultExecuting(ResultExecutingContext context)
18: {
19: context.HttpContext.Response.Headers.Add(
20: "Internal", new string[] { "Header Added" });
21: }
22:
23: public void OnResultExecuted(ResultExecutedContext context)
24: {
25: }
26: }
27:
28: public bool IsReusable
29: {
30: get
31: {
32: return false;
33: }
34: }
35: }
36: #endregion
37: }
Built-in filter attributes
The framework includes built-in attribute-based filters that you can subclass and customize. For example, the following Result filter adds a header to the response.
[!code-csharpMain]
1: using Microsoft.AspNetCore.Mvc.Filters;
2:
3: namespace FiltersSample.Filters
4: {
5: public class AddHeaderAttribute : ResultFilterAttribute
6: {
7: private readonly string _name;
8: private readonly string _value;
9:
10: public AddHeaderAttribute(string name, string value)
11: {
12: _name = name;
13: _value = value;
14: }
15:
16: public override void OnResultExecuting(ResultExecutingContext context)
17: {
18: context.HttpContext.Response.Headers.Add(
19: _name, new string[] { _value });
20: base.OnResultExecuting(context);
21: }
22: }
23: }
Attributes allow filters to accept arguments, as shown in the example above. You would add this attribute to a controller or action method and specify the name and value of the HTTP header:
[!code-csharpMain]
1: using FiltersSample.Filters;
2: using Microsoft.AspNetCore.Mvc;
3:
4: namespace FiltersSample.Controllers
5: {
6: #region snippet_AddHeader
7: [AddHeader("Author", "Steve Smith @ardalis")]
8: public class SampleController : Controller
9: {
10: public IActionResult Index()
11: {
12: return Content("Examine the headers using developer tools.");
13: }
14:
15: [ShortCircuitingResourceFilter]
16: public IActionResult SomeResource()
17: {
18: return Content("Successful access to resource - header should be set.");
19: }
20: #endregion
21:
22: [AddHeaderWithFactory]
23: public IActionResult HeaderWithFactory()
24: {
25: return Content("Examine the headers using developer tools.");
26: }
27: }
28: }
The result of the Index action is shown below - the response headers are displayed on the bottom right.
Several of the filter interfaces have corresponding attributes that can be used as base classes for custom implementations.
Filter attributes:
ActionFilterAttributeExceptionFilterAttributeResultFilterAttributeFormatFilterAttributeServiceFilterAttributeTypeFilterAttribute
TypeFilterAttribute and ServiceFilterAttribute are explained later in this article.
Filter scopes and order of execution
A filter can be added to the pipeline at one of three scopes. You can add a filter to a particular action method or to a controller class by using an attribute. Or you can register a filter globally (for all controllers and actions) by adding it to the MvcOptions.Filters collection in the ConfigureServices method in the Startup class:
[!code-csharpMain]
1: using FiltersSample.Filters;
2: using Microsoft.AspNetCore.Builder;
3: using Microsoft.Extensions.DependencyInjection;
4: using Microsoft.Extensions.Logging;
5:
6: namespace FiltersSample
7: {
8: public class Startup
9: {
10: // This method gets called by the runtime. Use this method to add services to the container.
11: #region snippet_ConfigureServices
12: public void ConfigureServices(IServiceCollection services)
13: {
14: services.AddMvc(options =>
15: {
16: options.Filters.Add(new AddHeaderAttribute("GlobalAddHeader",
17: "Result filter added to MvcOptions.Filters")); // an instance
18: options.Filters.Add(typeof(SampleActionFilter)); // by type
19: options.Filters.Add(new SampleGlobalActionFilter()); // an instance
20: });
21:
22: services.AddScoped<AddHeaderFilterWithDi>();
23: }
24: #endregion
25:
26: // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
27: public void Configure(IApplicationBuilder app, ILoggerFactory loggerFactory)
28: {
29: loggerFactory.AddConsole(minLevel: LogLevel.Debug)
30: .AddDebug(minLevel: LogLevel.Debug);
31:
32: app.UseMvcWithDefaultRoute();
33: }
34: }
35: }
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. This is sometimes referred to as “Russian doll” nesting, as each increase in scope is wrapped around the previous scope, like a nesting doll. You generally get the desired overriding behavior without having to explicitly determine ordering.
As a result of this nesting, the after code of filters runs in the reverse order of the before code. The sequence looks like this:
- The before code of filters applied globally
- The before code of filters applied to controllers
- The before code of filters applied to action methods
- The after code of filters applied to action methods
- The after code of filters applied to controllers
- The before code of filters applied to controllers
- The after code of filters applied globally
Here’s an 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 that the method filter is nested within the controller filter, and the controller filter is nested within the global filter. To put it another way, if you’re inside an async filter’s OnStageExecutionAsync method, all of the filters with a tighter scope run while your code is on the stack.
[!NOTE] Every controller that inherits from the
Controllerbase class includesOnActionExecutingandOnActionExecutedmethods. These methods wrap the filters that run for a given action:OnActionExecutingis called before any of the filters, andOnActionExecutedis called after all of the filters.
Overriding the default order
You can override the default sequence of execution by implementing IOrderedFilter. This interface exposes an Order property that takes precedence over scope to determine the order of execution. A filter with a lower Order value will have its before code executed before that of a filter with a higher value of Order. A filter with a lower Order value will have its after code executed after that of a filter with a higher Order value. You can set the Order property by using a constructor parameter:
If you have the same 3 Action filters shown in the preceding example but set the Order property of the controller and global filters to 1 and 2 respectively, the order of execution would be 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 trumps scope when determining the order in which filters will 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, so scope determines order unless you set Order to a non-zero value.
Cancellation and short circuiting
You can short-circuit the filter pipeline at any point by setting the Result property on the context parameter provided to the filter method. For instance, the following Resource filter prevents the rest of the pipeline from executing.
[!code-csharpMain]
1: using System;
2: using Microsoft.AspNetCore.Mvc;
3: using Microsoft.AspNetCore.Mvc.Filters;
4:
5: namespace FiltersSample.Filters
6: {
7: public class ShortCircuitingResourceFilterAttribute : Attribute,
8: IResourceFilter
9: {
10: public void OnResourceExecuting(ResourceExecutingContext context)
11: {
12: context.Result = new ContentResult()
13: {
14: Content = "Resource unavailable - header should not be set"
15: };
16: }
17:
18: public void OnResourceExecuted(ResourceExecutedContext context)
19: {
20: }
21: }
22: }
In the following code, both the ShortCircuitingResourceFilter and the AddHeader filter target the SomeResource action method. However, because the ShortCircuitingResourceFilter runs first (because it is a Resource Filter and AddHeader is an Action Filter) and short-circuits the rest of the pipeline, 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 (because of its filter type, or explicit use of Order property, for instance).
[!code-csharpMain]
1: using FiltersSample.Filters;
2: using Microsoft.AspNetCore.Mvc;
3:
4: namespace FiltersSample.Controllers
5: {
6: #region snippet_AddHeader
7: [AddHeader("Author", "Steve Smith @ardalis")]
8: public class SampleController : Controller
9: {
10: public IActionResult Index()
11: {
12: return Content("Examine the headers using developer tools.");
13: }
14:
15: [ShortCircuitingResourceFilter]
16: public IActionResult SomeResource()
17: {
18: return Content("Successful access to resource - header should be set.");
19: }
20: #endregion
21:
22: [AddHeaderWithFactory]
23: public IActionResult HeaderWithFactory()
24: {
25: return Content("Examine the headers using developer tools.");
26: }
27: }
28: }
Dependency injection
Filters can be added by type or by instance. If you add an instance, that instance will be used for every request. If you add a type, it will be type-activated, meaning an instance will be created for each request and any constructor dependencies will be populated by dependency injection (DI). Adding a filter by type is equivalent to filters.Add(new TypeFilterAttribute(typeof(MyFilter))).
Filters that are implemented as attributes and added directly to controller classes or action methods cannot have constructor dependencies provided by dependency injection (DI). This is because attributes must have their constructor parameters supplied where they are applied. This is a limitation of how attributes work.
If your filters have dependencies that you need to access from DI, there are several supported approaches. You can apply your filter to a class or action method using one of the following:
ServiceFilterAttributeTypeFilterAttributeIFilterFactoryimplemented on your attribute
[!NOTE] One dependency you might want to get from DI is a logger. However, avoid creating and using filters purely for logging purposes, since the (xref:)built-in framework logging features may already provide what you need. If you’re going to add logging to your filters, it should focus on business domain concerns or behavior specific to your filter, rather than MVC actions or other framework events.
ServiceFilterAttribute
A ServiceFilter retrieves an instance of the filter from DI. You add the filter to the container in ConfigureServices, and reference it in a ServiceFilter attribute
[!code-csharpMain]
1: using FiltersSample.Filters;
2: using Microsoft.AspNetCore.Builder;
3: using Microsoft.Extensions.DependencyInjection;
4: using Microsoft.Extensions.Logging;
5:
6: namespace FiltersSample
7: {
8: public class Startup
9: {
10: // This method gets called by the runtime. Use this method to add services to the container.
11: #region snippet_ConfigureServices
12: public void ConfigureServices(IServiceCollection services)
13: {
14: services.AddMvc(options =>
15: {
16: options.Filters.Add(new AddHeaderAttribute("GlobalAddHeader",
17: "Result filter added to MvcOptions.Filters")); // an instance
18: options.Filters.Add(typeof(SampleActionFilter)); // by type
19: options.Filters.Add(new SampleGlobalActionFilter()); // an instance
20: });
21:
22: services.AddScoped<AddHeaderFilterWithDi>();
23: }
24: #endregion
25:
26: // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
27: public void Configure(IApplicationBuilder app, ILoggerFactory loggerFactory)
28: {
29: loggerFactory.AddConsole(minLevel: LogLevel.Debug)
30: .AddDebug(minLevel: LogLevel.Debug);
31:
32: app.UseMvcWithDefaultRoute();
33: }
34: }
35: }
[!code-csharpMain]
1: using FiltersSample.Filters;
2: using Microsoft.AspNetCore.Mvc;
3: using System.Globalization;
4:
5: namespace FiltersSample.Controllers
6: {
7: public class HomeController : Controller
8: {
9: #region snippet_ServiceFilter
10: [ServiceFilter(typeof(AddHeaderFilterWithDi))]
11: public IActionResult Index()
12: {
13: return View();
14: }
15: #endregion
16:
17: [AddHeader("Author", "Steve Smith @ardalis")]
18: public IActionResult Hello(string name)
19: {
20: return Content($"Hello {name}");
21: }
22:
23: #region snippet_TypeFilter
24: [TypeFilter(typeof(AddHeaderAttribute),
25: Arguments = new object[] { "Author", "Steve Smith (@ardalis)" })]
26: public IActionResult Hi(string name)
27: {
28: return Content($"Hi {name}");
29: }
30: #endregion
31:
32: #region snippet_MiddlewareFilter
33: [Route("{culture}/[controller]/[action]")]
34: [MiddlewareFilter(typeof(LocalizationPipeline))]
35: public IActionResult CultureFromRouteData()
36: {
37: return Content($"CurrentCulture:{CultureInfo.CurrentCulture.Name},"
38: + "CurrentUICulture:{CultureInfo.CurrentUICulture.Name}");
39: }
40: #endregion
41: }
42: }
Using ServiceFilter without registering the filter type results in an exception:
System.InvalidOperationException: No service for type
'FiltersSample.Filters.AddHeaderFilterWithDI' has been registered.ServiceFilterAttribute implements IFilterFactory, which exposes a single method for creating an IFilter instance. In the case of ServiceFilterAttribute, the IFilterFactory interface’s CreateInstance method is implemented to load the specified type from the services container (DI).
TypeFilterAttribute
TypeFilterAttribute is very similar to ServiceFilterAttribute (and also implements IFilterFactory), but its type is not resolved directly from the DI container. Instead, it instantiates the type by using Microsoft.Extensions.DependencyInjection.ObjectFactory.
Because of this difference, types that are referenced using the TypeFilterAttribute do not need to be registered with the container first (but they will still have their dependencies fulfilled by the container). Also, TypeFilterAttribute can optionally accept constructor arguments for the type in question. The following example demonstrates how to pass arguments to a type using TypeFilterAttribute:
[!code-csharpMain]
1: using FiltersSample.Filters;
2: using Microsoft.AspNetCore.Mvc;
3: using System.Globalization;
4:
5: namespace FiltersSample.Controllers
6: {
7: public class HomeController : Controller
8: {
9: #region snippet_ServiceFilter
10: [ServiceFilter(typeof(AddHeaderFilterWithDi))]
11: public IActionResult Index()
12: {
13: return View();
14: }
15: #endregion
16:
17: [AddHeader("Author", "Steve Smith @ardalis")]
18: public IActionResult Hello(string name)
19: {
20: return Content($"Hello {name}");
21: }
22:
23: #region snippet_TypeFilter
24: [TypeFilter(typeof(AddHeaderAttribute),
25: Arguments = new object[] { "Author", "Steve Smith (@ardalis)" })]
26: public IActionResult Hi(string name)
27: {
28: return Content($"Hi {name}");
29: }
30: #endregion
31:
32: #region snippet_MiddlewareFilter
33: [Route("{culture}/[controller]/[action]")]
34: [MiddlewareFilter(typeof(LocalizationPipeline))]
35: public IActionResult CultureFromRouteData()
36: {
37: return Content($"CurrentCulture:{CultureInfo.CurrentCulture.Name},"
38: + "CurrentUICulture:{CultureInfo.CurrentUICulture.Name}");
39: }
40: #endregion
41: }
42: }
If you have a filter that doesn’t require any arguments, but which has constructor dependencies that need to be filled by DI, you can use your own named attribute on classes and methods instead of [TypeFilter(typeof(FilterType))]). The following filter shows how this can be implemented:
[!code-csharpMain]
1: using Microsoft.AspNetCore.Mvc;
2: using Microsoft.AspNetCore.Mvc.Filters;
3: using Microsoft.Extensions.Logging;
4:
5: namespace FiltersSample.Filters
6: {
7: #region snippet_TypeFilterAttribute
8: public class SampleActionFilterAttribute : TypeFilterAttribute
9: {
10: public SampleActionFilterAttribute():base(typeof(SampleActionFilterImpl))
11: {
12: }
13:
14: private class SampleActionFilterImpl : IActionFilter
15: {
16: private readonly ILogger _logger;
17: public SampleActionFilterImpl(ILoggerFactory loggerFactory)
18: {
19: _logger = loggerFactory.CreateLogger<SampleActionFilterAttribute>();
20: }
21:
22: public void OnActionExecuting(ActionExecutingContext context)
23: {
24: _logger.LogInformation("Business action starting...");
25: // perform some business logic work
26:
27: }
28:
29: public void OnActionExecuted(ActionExecutedContext context)
30: {
31: // perform some business logic work
32: _logger.LogInformation("Business action completed.");
33: }
34: }
35: }
36: #endregion
37: }
This filter can be applied to classes or methods using the [SampleActionFilter] syntax, instead of having to use [TypeFilter] or [ServiceFilter].
Authorization filters
Authorization filters control access to action methods and are the first filters to be executed within the filter pipeline. They have only a before method, unlike most filters that support before and after methods. You should only write a custom authorization filter if you are writing your own authorization framework. Prefer configuring your authorization policies or writing a custom authorization policy over writing a custom filter. The built-in filter implementation is just responsible for calling the authorization system.
Note that you should not throw exceptions within authorization filters, since nothing will handle the exception (exception filters won’t handle them). Instead, issue a challenge or find another way.
Learn more about Authorization.
Resource filters
Resource filters implement either the IResourceFilter or IAsyncResourceFilter interface, and their execution wraps most of the filter pipeline. (Only Authorization filters run before them.) Resource filters are especially useful if you need to short-circuit most of the work a request is doing. For example, a caching filter can avoid the rest of the pipeline if the response is already in the cache.
The short circuiting resource filter shown earlier is one example of a resource filter. Another example is DisableFormValueModelBindingAttribute, which prevents model binding from accessing the form data. It’s useful for cases where you know that you’re going to receive large file uploads and want to prevent the form from being read into memory.
Action filters
Action filters implement either the IActionFilter or IAsyncActionFilter interface, and their execution surrounds the execution of action methods.
Here’s a sample action filter:
[!code-csharpMain]
1: using FiltersSample.Helper;
2: using Microsoft.AspNetCore.Mvc.Filters;
3:
4: namespace FiltersSample.Filters
5: {
6: #region snippet_ActionFilter
7: public class SampleActionFilter : IActionFilter
8: {
9: public void OnActionExecuting(ActionExecutingContext context)
10: {
11: // do something before the action executes
12: }
13:
14: public void OnActionExecuted(ActionExecutedContext context)
15: {
16: // do something after the action executes
17: }
18: }
19: #endregion
20: }
The ActionExecutingContext provides the following properties:
ActionArguments- lets you manipulate the inputs to the action.Controller- lets you manipulate the controller instance.Result- setting this short-circuits execution of the action method and subsequent action filters. Throwing an exception also prevents execution of the action method and subsequent filters, but is treated as a failure instead of a successful result.
The ActionExecutedContext provides Controller and Result plus the following properties:
Canceled- will be true if the action execution was short-circuited by another filter.Exception- will be non-null if the action or a subsequent action filter threw an exception. Setting this property to null effectively ‘handles’ an exception, andResultwill be executed as if it were returned from the action method normally.
For an IAsyncActionFilter, a call to the ActionExecutionDelegate executes any subsequent action filters and the action method, returning an ActionExecutedContext. To short-circuit, assign ActionExecutingContext.Result to some result instance and do not call the ActionExecutionDelegate.
The framework provides an abstract ActionFilterAttribute that you can subclass.
You can use an action filter to automatically validate model state and return any errors if the state is invalid:
[!code-csharpMain]
1: using Microsoft.AspNetCore.Mvc;
2: using Microsoft.AspNetCore.Mvc.Filters;
3:
4: namespace FiltersSample.Filters
5: {
6: public class ValidateModelAttribute : ActionFilterAttribute
7: {
8: public override void OnActionExecuting(ActionExecutingContext context)
9: {
10: if (!context.ModelState.IsValid)
11: {
12: context.Result = new BadRequestObjectResult(context.ModelState);
13: }
14: }
15: }
16: }
The OnActionExecuted method runs after the action method and can see and manipulate the results of the action through the ActionExecutedContext.Result property. ActionExecutedContext.Canceled will be set to true if the action execution was short-circuited by another filter. ActionExecutedContext.Exception will be set to a non-null value if the action or a subsequent action filter threw an exception. Setting ActionExecutedContext.Exception to null effectively ‘handles’ an exception, and ActionExectedContext.Result will then be executed as if it were returned from the action method normally.
Exception filters
Exception filters implement either the IExceptionFilter or IAsyncExceptionFilter interface. They can be used to implement common error handling policies for an app.
The following sample exception filter uses a custom developer error view to display details about exceptions that occur when the application is in development:
[!code-csharpMain]
1: using Microsoft.AspNetCore.Hosting;
2: using Microsoft.AspNetCore.Mvc;
3: using Microsoft.AspNetCore.Mvc.Filters;
4: using Microsoft.AspNetCore.Mvc.ModelBinding;
5: using Microsoft.AspNetCore.Mvc.ViewFeatures;
6:
7: namespace FiltersSample.Filters
8: {
9: #region snippet_ExceptionFilter
10: public class CustomExceptionFilterAttribute : ExceptionFilterAttribute
11: {
12: private readonly IHostingEnvironment _hostingEnvironment;
13: private readonly IModelMetadataProvider _modelMetadataProvider;
14:
15: public CustomExceptionFilterAttribute(
16: IHostingEnvironment hostingEnvironment,
17: IModelMetadataProvider modelMetadataProvider)
18: {
19: _hostingEnvironment = hostingEnvironment;
20: _modelMetadataProvider = modelMetadataProvider;
21: }
22:
23: public override void OnException(ExceptionContext context)
24: {
25: if (!_hostingEnvironment.IsDevelopment())
26: {
27: // do nothing
28: return;
29: }
30: var result = new ViewResult {ViewName = "CustomError"};
31: result.ViewData = new ViewDataDictionary(_modelMetadataProvider,context.ModelState);
32: result.ViewData.Add("Exception", context.Exception);
33: // TODO: Pass additional detailed data via ViewData
34: context.Result = result;
35: }
36: }
37: #endregion
38: }
Exception filters do not have two events (for before and after) - they only implement OnException (or OnExceptionAsync).
Exception filters handle unhandled exceptions that occur in controller creation, model binding, action filters, or action methods. They won’t catch exceptions that occur in Resource filters, Result filters, or MVC Result execution.
To handle an exception, set the ExceptionContext.ExceptionHandled property to true or write a response. This stops propagation of the exception. Note that an Exception filter can’t turn an exception into a “success”. Only an Action filter can do that.
[!NOTE] In ASP.NET 1.1, the response is not sent if you set
ExceptionHandledto true and write a response. In that scenario, ASP.NET Core 1.0 does send the response, and ASP.NET Core 1.1.2 will return to the 1.0 behavior. For more information, see issue #5594 in the GitHub repository.
Exception filters are good for trapping exceptions that occur within MVC actions, but they’re not as flexible as error handling middleware. Prefer middleware for the general case, and use filters only where you need to do error handling differently based on which MVC action was chosen. For example, your 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.
The framework provides an abstract ExceptionFilterAttribute that you can subclass.
Result filters
Result filters implement either the IResultFilter or IAsyncResultFilter interface, and their execution surrounds the execution of action results.
Here’s an example of a Result filter that adds an HTTP header.
[!code-csharpMain]
1: using Microsoft.AspNetCore.Mvc.Filters;
2: using Microsoft.Extensions.Logging;
3:
4: namespace FiltersSample.Filters
5: {
6: /// <summary>
7: /// https://github.com/aspnet/Mvc/blob/master/test/WebSites/FiltersWebSite/Filters/AddHeaderAttribute.cs
8: /// </summary>
9: #region snippet_ResultFilter
10: public class AddHeaderFilterWithDi : IResultFilter
11: {
12: private ILogger _logger;
13: public AddHeaderFilterWithDi(ILoggerFactory loggerFactory)
14: {
15: _logger = loggerFactory.CreateLogger<AddHeaderFilterWithDi>();
16: }
17:
18: public void OnResultExecuting(ResultExecutingContext context)
19: {
20: var headerName = "OnResultExecuting";
21: context.HttpContext.Response.Headers.Add(
22: headerName, new string[] { "ResultExecutingSuccessfully" });
23: _logger.LogInformation($"Header added: {headerName}");
24: }
25:
26: public void OnResultExecuted(ResultExecutedContext context)
27: {
28: // Can't add to headers here because response has already begun.
29: }
30: }
31: #endregion
32: }
The kind of result being executed depends on the action in question. An MVC 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 for successful results - when the action or action filters produce an action result. Result filters are not executed when exception filters handle an exception.
The OnResultExecuting method can short-circuit execution of the action result and subsequent result filters by setting ResultExecutingContext.Cancel to true. You should generally write to the response object when short-circuiting to avoid generating an empty response. Throwing an exception will also prevent execution of the action result and subsequent filters, but will be treated as a failure instead of a successful result.
When the OnResultExecuted method runs, the response has likely been sent to the client and cannot be changed further (unless an exception was thrown). ResultExecutedContext.Canceled will be set to true if the action result execution was short-circuited by another filter.
ResultExecutedContext.Exception will be 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 MVC later in the pipeline. When you’re handling an exception in a result filter, you might not be able to write any data to the response. If the action result throws partway through its execution, and the headers have already been flushed to the client, 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 do not call the ResultExectionDelegate.
The framework provides an abstract ResultFilterAttribute that you can subclass. The AddHeaderAttribute class shown earlier is an example of a result filter attribute.
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 are part of MVC, which means that they have access to MVC context and constructs.
In ASP.NET Core 1.1, you can use middleware in the filter pipeline. You might want to do that if you have a middleware component that needs access to MVC route data, or one that should run only for certain controllers or actions.
To use middleware as a filter, create a type with a Configure method that specifies the middleware that you want to inject into the filter pipeline. Here’s an example that uses the localization middleware to establish the current culture for a request:
[!code-csharpMain]
1: using Microsoft.AspNetCore.Builder;
2: using Microsoft.AspNetCore.Localization;
3: using Microsoft.AspNetCore.Localization.Routing;
4: using System;
5: using System.Collections.Generic;
6: using System.Globalization;
7: using System.Linq;
8: using System.Threading.Tasks;
9:
10:
11: namespace FiltersSample.Filters
12: {
13: #region snippet_MiddlewareFilter
14: public class LocalizationPipeline
15: {
16: public void Configure(IApplicationBuilder applicationBuilder)
17: {
18: var supportedCultures = new[]
19: {
20: new CultureInfo("en-US"),
21: new CultureInfo("fr")
22: };
23:
24: var options = new RequestLocalizationOptions
25: {
26:
27: DefaultRequestCulture = new RequestCulture(culture: "en-US", uiCulture: "en-US"),
28: SupportedCultures = supportedCultures,
29: SupportedUICultures = supportedCultures
30: };
31: options.RequestCultureProviders = new[]
32: { new RouteDataRequestCultureProvider() { Options = options } };
33:
34: applicationBuilder.UseRequestLocalization(options);
35: }
36: }
37: #endregion
38: }
You can then use the MiddlewareFilterAttribute to run the middleware for a selected controller or action or globally:
[!code-csharpMain]
1: using FiltersSample.Filters;
2: using Microsoft.AspNetCore.Mvc;
3: using System.Globalization;
4:
5: namespace FiltersSample.Controllers
6: {
7: public class HomeController : Controller
8: {
9: #region snippet_ServiceFilter
10: [ServiceFilter(typeof(AddHeaderFilterWithDi))]
11: public IActionResult Index()
12: {
13: return View();
14: }
15: #endregion
16:
17: [AddHeader("Author", "Steve Smith @ardalis")]
18: public IActionResult Hello(string name)
19: {
20: return Content($"Hello {name}");
21: }
22:
23: #region snippet_TypeFilter
24: [TypeFilter(typeof(AddHeaderAttribute),
25: Arguments = new object[] { "Author", "Steve Smith (@ardalis)" })]
26: public IActionResult Hi(string name)
27: {
28: return Content($"Hi {name}");
29: }
30: #endregion
31:
32: #region snippet_MiddlewareFilter
33: [Route("{culture}/[controller]/[action]")]
34: [MiddlewareFilter(typeof(LocalizationPipeline))]
35: public IActionResult CultureFromRouteData()
36: {
37: return Content($"CurrentCulture:{CultureInfo.CurrentCulture.Name},"
38: + "CurrentUICulture:{CultureInfo.CurrentUICulture.Name}");
39: }
40: #endregion
41: }
42: }
Middleware filters run at the same stage of the filter pipeline as Resource filters, before model binding and after the rest of the pipeline.
Next actions
To experiment with filters, download, test and modify the sample.
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