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Why isn't my C# .Net Core Rest API route finding my method?

I am working on an API.
I have an "AbstractController.cs" and I am having difficulties calling a GET with two parameters.
[Route("api/[controller]")]
[ApiController]
public class AbstractController : ControllerBase
{
// GET: api/Abstract
[HttpGet]
public IEnumerable<string> Get()
{
return new string[] { "missing implementation" };
}
Going to https://localhost:44363/api/abstract/ generates this.
["missing implementation"]
Awesome!
Now I need to make the Get method that passes in a show year, and code to a SQL query. Easy enough?
// GET: api/Abstract?ShowYear=2019&ShowCode=248621
[Route("api/{controller}/{ShowYear}/{ShowCode}")]
[HttpGet]
public Abstract Get(int ShowYear, int ShowCode) // GetAbstractByYearAndSHCode
{
string x = "";
return new Abstract();
}
No matter what I do I can't get this method to breakpoint/enter execution!
I'm guessing it's a routing issue, but I've tried most tenable ways of calling the endpoint.
Now I check the MS Documentation like any self-respecting-learning-programmer would.
https://learn.microsoft.com/en-us/aspnet/core/mvc/controllers/routing?view=aspnetcore-3.1
Endpoint routing in ASP.NET Core 3.0 and later:
Doesn't have a concept of routes. Doesn't provide ordering guarantees
for the execution of extensibility, all endpoints are processed at
once.
GULP Sounds scary.
I haven't touched my middleware at this point.
All examples I've looked at don't have my "MapControllers();" method in their Startup.cs.
This is because it's new to .NET Core 3.0/3.1
The method: "Adds endpoints for controller actions to the IEndpointRouteBuilder without specifying any routes."
OK, so do I have to manually specify the route here still?
I did this, and it sorta works.
Well, it breakpoints in the startup.cs now when I go to https://localhost:44363/api/abstract/2019/287
Wait, it's not doing ANYTHING with my controller code! Why?
The following (also above) code ends up declaring as a null in the startup.cs
var controller = context.Request.RouteValues["Abstract"];
Hoping to learn what I'm doing wrong.
MapGet("/api/abstract/{showYear}/{showCode}", ...);
Isn't this code responsible for mapping that route with the controller named AbstractController.cs in my Controllers folder? Not hitting any breakpoints.
Edit:
Reading through this which compares the differences in the Startup.cs from .Net Core 2, 2.2, MVC projects vs 3.0
I have a good feeling reading all of this, I will find the issue.
https://andrewlock.net/comparing-startup-between-the-asp-net-core-3-templates/
Edit.. nevermind didn't find a resolution.
Edit 2:
Completely commenting out this troublesome code in startup.cs:
endpoints.MapGet("/api/abstract/{showYear}/{showCode}", async context =>
{
var controller = context.Request.RouteValues["Abstract"];
var showYear = context.Request.RouteValues["showYear"];
var showCode = context.Request.RouteValues["showCode"]; // just an example.
//await context.Response.WriteAsync($"Hello {showYear} is your year, {showCode} for the code!");
//GetAbstractByYearAndSHCode();
});
endpoints.MapGet("/api/abstract/{showYear}", async context =>
{
var name = context.Request.RouteValues["name"];
await context.Response.WriteAsync($"Hello {name}!");
});
Resolves my issue of not being able to reach the controller.
https://localhost:44363/api/abstract/2019 hits, and the id value is 2019. Great.
https://i.imgur.com/rmHyDPg.png the output looks great.
I am still not able to use > 1 parameter. How do I simply use the Year, and ShowCode paramaters? What's the syntax?
https://localhost:44363/api/abstract/2019

Just add the parameters to your attribute
[HttpGet("{ShowYear}/{ShowCode}")]

The "api/abstract" route is already used by the first method.
You cannot use it again for other actions. Create a new one as shown inline.
[HttpGet("{GetAbstractByYearAndSHCode}",Name = "GetAbstractByYearAndSHCode")]
public Abstract Get(int ShowYear, int ShowCode) // GetAbstractByYearAndSHCode
{
string x = "";
return new Abstract();
}
And call the url as shown:
https://localhost/api/abstract/GetAbstractByYearAndSHCode?ShowYear=1&ShowCode=5

AspNetCore 3.0 (upgraded from 2.2) routes seem to break when Controller Method has Async suffix

I upgraded my project to Core 3.0 from 2.2 and everything seems to work well, except one get JSON request. I have the following Js code doing an Ajax request to the Home controller:
var isLastPage = false;
var incidentModel = {
incidents: ko.observableArray([]),
getIncidents: function(a) {
var self = this;
//var $incdiv = $('#incidentsList');
$.getJSON('#Url.Action("AjaxPageAsync", "Home")', { page: page++, user: user, type: type }, function(data) {
//console.log(data);
self.incidents(self.incidents().concat(data));
if (data[0].IsLastPage) {
isLastPage = true;
}
a();
});
}
}
The Home controller is like this:
public async Task<ActionResult> AjaxPageAsync([FromQuery] string type, [FromQuery] string user, [FromQuery] int? page)
{
//Get some json data
return Json(Incident);
}
As you may tell, I'm using knockout (version 3.5.0 & jquery 3.3.0) which both are working fine on other pages. However, the getJSON request in the js code above is returning a 404:
GET https://localhost:44366/Home/AjaxPageAsync?page=1&user=&type=&_=1575386778917 404
The debug output looks similar:
Microsoft.AspNetCore.Hosting.Diagnostics: Information: Request starting HTTP/2.0 GET https://localhost:44366/Home/AjaxPageAsync?page=1&user=&type=&_=1575386536876
The 2.2 version is identical and works fine. I thought it might be the number appended onto the end of the url, but that exists in 2.2 as well.
At this point, I'm thinking it's a problem with the syntax of my AjaxPageAsync task in the controller, but not sure what it should be.
Anyone know where I'm going wrong here? Thank you.

According to https://github.com/aspnet/AspNetCore/issues/8998, in .NET Core 3.0 Async is trimmed from Action name. Your endpoint is available at /Home/AjaxPage. You can change this behaviour by replacing
services.AddControllers();
with
services.AddControllers(options => options.SuppressAsyncSuffixInActionNames = false);
in ConfigureServices method, or just use the new routes

I would start with an elimination of getJson and what not, run a manual Postman or Curl against the endpoint to create the simplest reproducible use case to troubleshoot.
If it's still 404, you know its the configuration side of AspNetCore.
If it's not 404, you have a problem specific with an Ajax call to NetCore 3.0. That can narrow the scope of what you are searching for (and thereby your Google parameters).
My guess to the actual problem is the routing you have setup is not working or configuration has shifted in NetCore3. This usually happens when you had a routing setup in NetCore 1.0/1.2, years later migrated an example to NetCore 2.0 which was backwards compatible, years later again, bumped to NetCore 3.0, and NetCore 3 may not exactly support that routing configuration without some additional hoops to jump through.
Try creating a simple helloworld in the same controller and wiring it up. If that works you have essentially isolated the issues to the controller/method and the routing.
If didn't support it at all, it wouldn't compile though...
...so swings and roundabouts!
Also, it's Task but you are only returning Result... this is probably a simplified example but make sure you are returning/awaiting correctly.
Looking at specifically Async method tags with AspNetCore3 and there is an MvcOption that SuppressesAsyncSuffixActionName and is true by default in AspNetCore 3.0 (looks like Piotr found it first!)
https://learn.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.mvc.mvcoptions.suppressasyncsuffixinactionnames?view=aspnetcore-3.0

ASP.NET Core - Swashbuckle not creating swagger.json file

I am having trouble getting the Swashbuckle.AspNetCore (1.0.0) package to generate any output. I read the swagger.json file should be written to '~/swagger/docs/v1'. However, I am not getting any output.
I started with a brand new ASP.NET Core API project. I should mention this is ASP.NET Core 2. The API works, and I am able to retrieve values from the values controller just fine.
My startup class has the configuration exactly as described in this article (Swashbuckle.AspNetCore on GitHub).
public class Startup
{
public Startup(IConfiguration configuration)
{
Configuration = configuration;
}
public IConfiguration Configuration { get; }
// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
services.AddMvc();
services.AddSwaggerGen(c =>
{
c.SwaggerDoc("v1", new Info { Title = "My API", Version = "v1" });
});
}
// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
// Enable middleware to serve generated Swagger as a JSON endpoint.
app.UseSwagger();
app.UseSwaggerUI(c =>
{
c.SwaggerEndpoint("/swagger/v1/swagger.json", "MyAPI V1");
});
}
else
{
app.UseExceptionHandler();
}
app.UseStatusCodePages();
app.UseMvc();
//throw new Exception();
}
}
You can see the NuGet references...
Again, this is all the default template, but I include the ValuesController for reference...
[Route("api/[controller]")]
public class ValuesController : Controller
{
// GET api/values
[HttpGet]
public IEnumerable<string> Get()
{
return new string[] { "value1", "value2" };
}
// GET api/values/5
[HttpGet("{id}")]
public string Get(int id)
{
return "value";
}
// POST api/values
[HttpPost]
public void Post([FromBody]string value)
{
}
// PUT api/values/5
[HttpPut("{id}")]
public void Put(int id, [FromBody]string value)
{
}
// DELETE api/values/5
[HttpDelete("{id}")]
public void Delete(int id)
{
}
}

I had the same problem. Check http://localhost:XXXX/swagger/v1/swagger.json. If you get any a errors, fix them.
For example, I had an ambiguous route in a base controller class and I got the error: "Ambiguous HTTP method for action. Actions require an explicit HttpMethod binding for Swagger 2.0.".
If you use base controllers make sure your public methods use the HttpGet/HttpPost/HttpPut/HttpDelete OR Route attributes to avoid ambiguous routes.
Then, also, I had defined both HttpGet("route") AND Route("route") attributes in the same method, which was the last issue for swagger.

I believe you missed these two lines on your Configure:
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
// Enable middleware to serve generated Swagger as a JSON endpoint.
app.UseSwagger();
app.UseSwaggerUI(c =>
{
c.SwaggerEndpoint("v1/swagger.json", "MyAPI V1");
});
}
To access Swagger UI, the URL should be: http://localhost:XXXX/swagger/
The json can be found at the top of Swagger UI:

If your application is hosted on IIS/IIS Express try the following:
c.SwaggerEndpoint("../swagger/v1/swagger.json", "MyAPI V1");

I was running into a similar, but not exactly the same issue with swagger. Hopefully this helps someone else.
I was using a custom document title and was not changing the folder path in the SwaggerEndPoint to match the document title. If you leave the endpoint pointing to swagger/v1/swagger.json it won't find the json file in the swagger UI.
Example:
services.AddSwaggerGen(swagger =>
{
swagger.SwaggerDoc("AppAdministration", new Info { Title = "App Administration API", Version = "v1.0" });
});
app.UseSwaggerUI(c =>
{
c.SwaggerEndpoint("/swagger/AppAdministration/swagger.json", "App Administration");
});

#if DEBUG
// For Debug in Kestrel
c.SwaggerEndpoint("/swagger/v1/swagger.json", "Web API V1");
#else
// To deploy on IIS
c.SwaggerEndpoint("/webapi/swagger/v1/swagger.json", "Web API V1");
#endif
When deployed to IIS webapi(base URL) is the Application Alias. You need to keep Application Alias(base URL) same for all IIS deployments because swagger looks for swagger.json at "/swagger/v1/swagger.json" location but wont prefix application Alias(base URL) that is the reason it wont work.
For Example:
localhost/swagger/v1/swagger.json - Couldn't find swagger.json

You must conform to 2 rules:
Decorate all actions with explicit Http Verbs like[HttpGet("xxx")], [HttpPost("xxx")], ... instead of [Route("xxx")].
Decorate public methods in controllers with [NonAction] Attribute.
Note that http://localhost:XXXX/swagger/ page requests for http://localhost:XXXX/swagger/v1/swagger.json file, but an Exception will occur from Swagger if you wouldn't conform above rules.

After watching the answers and checking the recommendations, I end up having no clue what was going wrong.
I literally tried everything. So if you end up in the same situation, understand that the issue might be something else, completely irrelevant from swagger.
In my case was a OData exception.
Here's the procedure:
1) Navigate to the localhost:xxxx/swagger
2) Open Developer tools
3) Click on the error shown in the console and you will see the inner exception that is causing the issue.

I am moving my comment to an answer since it appears to be helpful.
To avoid issues with IIS aliases, remove /swagger/ from the URL path. It should look like this:
app.UseSwaggerUI(c => { c.SwaggerEndpoint("v1/swagger.json", "API name"); });

I don't know if this is useful for someone, but in my case the problem was that the name had different casing.
V1 in the service configuration - V capital letter
v1 in Settings -- v lower case
The only thing I did was to use the same casing and it worked.

If you have any issues in your controller to map to an unique URL you get this error.
The best way to find the cause of issue is exclude all controllers from project. Then try running the app by enabling one controller or one or more methods in a controller at a time to find the controllers/ controller method(S) which have an issue. Or you could get smart and do a binary search logic to find the disable enable multiple controller/methods to find the faulty ones.
Some of the causes is
Having public methods in controller without HTTP method attributes
Having multiple methods with same Http attributes which could map to same api call if you are not using "[action]" based mapping
If you are using versioning make sure you have the method in all the controller versions (if using inheritance even though you use from base)

A common error that we make when use Swagger is to give the same name to(NET ASP) two or more routes. this cause that swagger cannot generate the JSON file. for example, this is a wrong way
[HttpPost, Route("Start")]
public async Task<TransactionResult> WipStart(BodyWipStartDTO data)
{
return await _wipServices.WipStart(data);
}
Other action with the same route name but different action name
[HttpPost, Route("Start")]
public async Task<TransactionResult> WipAbort(BodyWipStartDTO data)
{
return await _wipServices.WipAbort(data);
}
This a correct way
[HttpPost, Route("Start")]
public async Task<TransactionResult> WipStart(BodyWipStartDTO data)
{
return await _wipServices.WipStart(data);
}
[HttpPost, Route("Abort")]
public async Task<TransactionResult> WipAbort(BodyWipStartDTO data)
{
return await _wipServices.WipAbort(data);
}

You actually just need to fix the swagger url by removing the starting backslash just like this :
c.SwaggerEndpoint("swagger/v1/swagger.json", "MyAPI V1");
instead of :
c.SwaggerEndpoint("/swagger/v1/swagger.json", "MyAPI V1");

Be aware that in Visual Studio 2022 and .NetCore 6 if you create a new ASP.NET Core Web App, Program.cs has the oposite check for Development environment.
instead of
if (app.Environment.IsDevelopment())
{
app.UseSwagger();
app.UseSwaggerUI();
}
you will find
if (!app.Environment.IsDevelopment())
{
app.UseExceptionHandler("/Home/Error");
}
// You shoukd add swagger calls here
app.UseSwagger();
app.UseSwaggerUI();
If you create a new project by selecting the template ASP.NET Core Web API and check "Enable OpenAPI support" you will have different Program.cs with preinstalled swagger package and related code.
This took some time for me to find, hope to help someone.

Adding a relative path worked for me:
app.UseSwaggerUI(c =>
{
c.SwaggerEndpoint("../swagger/v1/swagger.json", "My App");
});

Personally I had the same issue and when I tried again today after a while I found in the new version (2.5.0) that going in the json I could see an explanation of the error that was in here.
Also another thing that helped to fix it to me was removing the hosting information connected to the website that is hold inside "..vs\config\applicationhost.config" at the root of the solution folder
I removed the element that was configuring the website.
<site name="**" id="9">
<application path="/" applicationPool=""></application>
<bindings></bindings>
</site>

I had this problem when I used a inner class in Post parameters
[HttpPost]
public async Task Post([FromBody] Foo value)
{
}
Where Foo is
public class Foo
{
public IEnumerable<Bar> Bars {get;set;}
public class Bar
{
}
}

Try to follow these steps, easy and clean.
Check your console are you getting any error like "Ambiguous HTTP method for action. Actions require an explicit HttpMethod binding for Swagger 2.0."
If YES:
Reason for this error: Swagger expects
each endpoint should have the method (get/post/put/delete)
.
Solution:
Revisit your each and every controller and make sure you have added
expected method.
(or you can just see in console error which controller causing ambiguity)
If NO. Please let us know your issue and solution if you have found any.

Same problem - easy fix for me.
To find the underlying problem I navigated to the actual swagger.json file which gave me the real error
/swagger/v1/swagger.json
The actual error displayed from this Url was
NotSupportedException: Ambiguous HTTP method for action ... Actions require an explicit HttpMethod binding for Swagger/OpenAPI 3.0
The point being
Actions require an explicit HttpMethod
I then decorated my controller methods with [HttpGet]
[Route("GetFlatRows")]
[HttpGet]
public IActionResult GetFlatRows()
{
Problem solved

Make sure you have all the required dependencies, go to the url xxx/swagger/v1/swagger.json you might find that you're missing one or more dependencies.

I was getting this Swagger error when I created Version 2 of my api using version headers instead of url versioning. The workaround was to add [Obsolete] attributes to the Version 1 methods then use SwaggerGeneratorOptions to ignore the obsolete api methods in Startup -> ConfigureServices method.
services.AddSwaggerGen(c =>
{
c.SwaggerGeneratorOptions.IgnoreObsoleteActions = true;
c.SwaggerDoc("v2", new Info { Title = "My API", Version = "v2" });
});

I had the same problem. I was using swagger like below mentioned pattern i.e. "../swagger/v1/swagger.json" because I am using IIS Express.Later than I change it to
"/swagger/v1/swagger.json"and clean,rebuild the solution worked for me.

You might forgetting to include.. StartUp.cs/Configure()
app.UseSwagger();
Check if you forgot to include, you error must be remove.

I'd a similar issue, my Swagger documentation broke after I was adding async version of APIs to existing ones.
I played around the Swagger DLL's by installing / Reinstalling, finally commenting newly added APIs, and it worked.
Then I added different signature in attributes, and bingo!, It worked.
In your case, you are having two API with matching signatures
[HttpGet]
public IEnumerable<string> Get()
{
return new string[] { "value1", "value2" };
}
// GET api/values/5
[HttpGet("{id}")]
public string Get(int id)
{`enter code here`
return "value";
}
Try providing different names in attributes like
[HttpGet("List")]
public IEnumerable<string> Get()
{
return new string[] { "value1", "value2" };
}
// GET api/values/5
[HttpGet("ListById/{id}")]
public string Get(int id)
{
return "value";
}
This should solve the issue.

I have came across the same issue, and noticed that my API has not hosted in the root folder and in an virtual directory.
I moved my API to the root folder in IIS and worked.
More info in this answer

Take a look on Chrome developer tools, sometimes, swagger.json request throws http 500, witch means that there is some inconsistency on your controllers.
For example: In my case, there is an "Ambiguous HTTP method for action":

Also I had an issue because I was versioning the application in IIS level like below:
If doing this then the configuration at the Configure method should append the version number like below:
app.UseSwaggerUI(options =>
{
options.SwaggerEndpoint("/1.0/swagger/V1/swagger.json", "Static Data Service");
});

I was able to fix and understand my issue when I tried to go to the swagger.json URL location:
https://localhost:XXXXX/swagger/v1/swagger.json
The page will show the error and reason why it is not found.
In my case, I saw that there was a misconfigured XML definition of one of my methods based on the error it returned:
NotSupportedException: HTTP method "GET" & path "api/Values/{id}" overloaded by actions - ...
...
...

In my case problem was in method type, should be HttpPOST but there was HttpGET
Once I changed that, everything starts work.
https://c2n.me/44p7lRd.png

You should install the following packages into your project.
5.0.0-rc4 version of Swashbuckle is the minimum. Otherwise, it won't work.
As of now, directly installing it from Nuget, installs the old versions which won't work for Core 3.
I inserted the following lines into .csproj project file like that:
<PackageReference Include="Microsoft.OpenApi" Version="1.1.4" />
<PackageReference Include="Swashbuckle.AspNetCore.Swagger" Version="5.0.0-rc4" />
<PackageReference Include="Swashbuckle.AspNetCore.SwaggerGen" Version="5.0.0-rc4" />
<PackageReference Include="Swashbuckle.AspNetCore.SwaggerUi" Version="5.0.0-rc4" />
After that, Rebuild installs the newer versions.
If not, you can use restore too.
In the Startup.cs, you should configure Swashbuckle like that:
// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
services.AddControllers();
// Register the Swagger generator, defining 1 or more Swagger documents
services.AddSwaggerGen(c =>
{
c.SwaggerDoc("v1", new OpenApiInfo { Title = "My API", Version = "v1" });
});
services.AddMvc();
}
 
// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
}
app.UseHttpsRedirection();
// Enable middleware to serve generated Swagger as a JSON endpoint.
app.UseSwagger();
// Enable middleware to serve swagger-ui (HTML, JS, CSS, etc.),
// specifying the Swagger JSON endpoint.
app.UseSwaggerUI(c =>
{
c.SwaggerEndpoint("/swagger/v1/swagger.json", "My API V1");
//c.RoutePrefix = String.Empty;
});
app.UseRouting();
app.UseAuthorization();
app.UseEndpoints(endpoints =>
{
endpoints.MapControllers();
});
}
Just go to the "https://localhost:5001/swagger/index.html" and you'll see the Swagger UI.
(5001 is my local port, you should change it with yours)
It took a little time for me to figure it out.
I hope it will help others :)

Answer:
If using directories or application with IIS or a reverse proxy,<br/> set the Swagger endpoint to a relative path using the ./ prefix. For example,<br/> ./swagger/v1/swagger.json. Using /swagger/v1/swagger.json instructs the app to<br/>look for the JSON file at the true root of the URL (plus the route prefix, if used). For example, use http://localhost:<br/><br/><port>/<route_prefix>/swagger/v1/swagger.json instead of http://localhost:<br/><port>/<virtual_directory>/<route_prefix>/swagger/v1/swagger.json.<br/>
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
// Enable middleware to serve generated Swagger as a JSON endpoint.
app.UseSwagger();
app.UseSwaggerUI(c =>
{
//c.SwaggerEndpoint("/swagger/v1/swagger.json", "MyAPI V1");
//Add dot in front of swagger path so that it takes relative path in server
c.SwaggerEndpoint("./swagger/v1/swagger.json", "MyAPI V1");
});
}
[Detail description of the swagger integration to web api core 3.0][1]
[1]: https://learn.microsoft.com/en-us/aspnet/core/tutorials/getting-started-with-swashbuckle?view=aspnetcore-3.1&tabs=visual-studio

Mvc.Versioning correct way to build routes

I'm using Microsoft.AspNetCore.Mvc.Versioning and I'm having difficulty setting up the routes correctly. I'm following the info from Hanselman's blog here: http://www.hanselman.com/blog/ASPNETCoreRESTfulWebAPIVersioningMadeEasy.aspx
I want to access my API via URIs like so:
http://me.com/api/v1/foo/bar
http://me.com/api/v1.0/foo/bar
I have the correct attributes on my foo class:
[ApiVersion("1.0")]
[Route("api/v{version:apiVersion}/[controller]")]
The above works OK, but if I type the below (no version):
http://me.com/api/foo/bar
I get a 404 when going to the above (I assume because the route is not setup correctly for no version specified).
I tried adding this to the Startup.cs file:
//Add the versioning
services.AddApiVersioning(o => {
//Everytime a new version is created this must be updated to default to the latest version.
o.AssumeDefaultVersionWhenUnspecified = true;
o.DefaultApiVersion = new ApiVersion(1, 0);
});
But this didn't work either - so I then added the route I wanted to the top of my foo class/controller:
[Route("api/[controller]")]
This gets the desired behavior I want and I can access all routes below:
http://me.com/api/v1/foo/bar
http://me.com/api/v1.0/foo/bar
http://me.com/api/foo/bar
Is this the way it should be done? Why isn't the default version working the way Hanselman describes?

Note that Scott doesn't suggest that the URL path segmenting method will allow you to give a blank version:
To be clear, you have total control, but the result from the outside is quite clean with /api/v[1|2|3]/helloworld
Having default version specified when using URL segment mapping is not supported. See this Github issue for more information.

ASP.NET MVC 6 handling errors based on HTTP status code

I want to display different error messages for each status code e.g:
400 Bad Request
403 Forbidden
500 Internal Server Error
404 Not Found
401 Unauthorized
How can I achieve this in the new ASP.NET MVC 6 applications? Can I do this using the built in UseErrorHandler method?
application.UseErrorHandler("/error");
Also, I noticed that even with the above handler, entering a non-existent URL e.g. /this-page-does-not-exist, causes an ugly 404 Not Found error page from IIS. How can this also be handled?
In MVC 5 we have had to use the system.web customerrors section for ASP.NET and the system.webServer httpErrors section in the web.config file but it was difficult to work with an unwieldy, with lots of very strange behaviour. Does MVC 6 make this a lot simpler?

You could use the StatusCodePagesMiddleware for this. Following is an example:
public void Configure(IApplicationBuilder app, ILoggerFactory loggerFactory)
{
app.UseStatusCodePagesWithReExecute("/StatusCodes/StatusCode{0}");
app.UseMvcWithDefaultRoute();
Controller which handles the status code requests:
public class StatusCodesController : Controller
{
public IActionResult StatusCode404()
{
return View(viewName: "NotFound"); // you have a view called NotFound.cshtml
}
... more actions here to handle other status codes
}
Some Notes:
Check other extension methods like UseStatusCodePagesWithRedirects
and UseStatusCodePages for other capabilities.
I tried having StatusCode as a query string in my example, but looks like this
middleware doesn't handle query strings, but you can take a look at
this code and fix this issue.

How can I achieve this in the new ASP.NET MVC 6 applications? Can I do this using the built in UseErrorHandler method?
Quick answer: Not in an elegant fashion.
Explanation/Alternative: To start lets first look at what the UseErrorHandler method is actually doing: https://github.com/aspnet/Diagnostics/blob/6dbbe831c493e6e7259de81f83a04d1654170137/src/Microsoft.AspNet.Diagnostics/ErrorHandlerExtensions.cs#L25 which adds the following middleware: https://github.com/aspnet/Diagnostics/blob/6dbbe831c493e6e7259de81f83a04d1654170137/src/Microsoft.AspNet.Diagnostics/ErrorHandlerMiddleware.cs Note lines 29-78 (the invoke method)
The invoke method is executed whenever a request comes in (controlled by the location of your application.UseErrorHandler("...") in your Startup.cs). So the UseErrorHandler is a glorified way of adding a custom middleware: middleware = component that can act on an http request.
Now with that background, if we wanted to add our own error middleware that differentiated requests. We could do this by adding a similar middleware that's like the default ErrorHandlerMiddleware by modifying these lines: https://github.com/aspnet/Diagnostics/blob/6dbbe831c493e6e7259de81f83a04d1654170137/src/Microsoft.AspNet.Diagnostics/ErrorHandlerMiddleware.cs#L48-L51 With that approach we could control the redirect path based on the status code.
In MVC 5 we have had to use the system.web customerrors section for ASP.NET and the system.webServer httpErrors section in the web.config file but it was difficult to work with an unwieldy, with lots of very strange behaviour. Does MVC 6 make this a lot simpler?
Answer: It sure does :). Just like the above answer the fix lies in adding middleware. There's a shortcut to adding simple middleware via the IApplicationBuilder in your Startup.cs; at the end of your Configure method you can add the following:
app.Run(async (context) =>
{
await context.Response.WriteAsync("Could not handle the request.");
// Nothing else will run after this middleware.
});
This will work because it means that you reached the end of your http pipeline without the request being handled (since it's at the end of your Configure method in Startup.cs). If you want to add this middleware (in the quick fashion) with the option to execute middleware after you, here's how:
app.Use(async (context, next) =>
{
await context.Response.WriteAsync("Could not handle the request.");
// This ensures that any other middelware added after you runs.
await next();
});
Hope this helps!