Скачать или смотреть Understanding UrlRouting Behavior in ASP.NET Core: 405 vs. 404 Errors Explained

Explore the intricacies of `UrlRouting` in ASP.NET Core. Learn why you're receiving 405 errors instead of expected 404s and how it relates to route matching.
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Understanding UrlRouting Behavior in ASP.NET Core: 405 vs. 404 Errors Explained

When working with ASP.NET Core, many developers encounter issues with UrlRouting, especially when dealing with complex route segments. A common confusion arises between the 405 Method Not Allowed and 404 Not Found status codes during route matching. In this guide, we'll unpack this issue by describing a user's experience and clarifying how ASP.NET Core manages routing.

The Problem at Hand

A developer was experimenting with a basic ASP.NET Core Web API, specifically the out-of-the-box "Weather Forecast" example, but made a modification. They changed the controller to only define a single endpoint:

Endpoint defined: POST foo/{id}:bar

The intended behavior was clear: if any request variation did not match this specific route, the appropriate response should be a 404 Not Found. However, the developer encountered 405 statuses for unexpected requests and found themselves questioning whether it was a bug or a misunderstanding of how UrlRouting worked.

Here's a quick glance at the results they measured using Postman:

Method/foo/123:bar/foo/literalGET405 (expected)405 (why?)POST200 (expected)404 (expected)DELETE405 (expected)405 (why?)The expectation for all verbs on /foo/literal was to return 404, given that it doesn’t fit the defined route. So, why do some requests return a 405 instead?

Analyzing the Response

To gain clarity on this issue, the developer reached out to the dotnet/aspnetcore repository and received an insightful explanation:

Automatic Method Evaluation: The framework checks the HTTP Method before evaluating the route's constraints. As a result, it produces a 405 error. This is because certain routes can still match the structure of the URL, even if the method does not apply.

Key Takeaways from Their Response

Complex Segments Are Treated as Parameters: From the perspective of the routing mechanism, complex segments like {id}:bar are handled as parameters subject to the constraints defined in the route.

Method Matching Precedes Constraint Evaluation: The routing engine's design intentionally delays constraint evaluations because they can be resource-intensive. This can lead to situations where it determines that there are other potential matches for the same endpoint, resulting in a 405.

Clarifying the Misunderstanding

To summarize, the confusion around receiving 405 errors instead of the expected 404s is not a bug, but rather a fundamental aspect of how ASP.NET Core works with routing and constraints:

405 Method Not Allowed: This occurs when the request's method is not permitted for the matched route. In your case, while /foo/literal partially matches the route pattern, the HttpMethodMatcherPolicy establishes that no valid method aligns — hence the 405.

404 Not Found: This would result when a complete route match is not found at all. In this case, POST works with the defined pattern, returning a 200 on valid requests, while any other method still yields the 405.

Conclusion

Understanding UrlRouting in ASP.NET Core can sometimes present challenges, especially around 405 and 404 status codes. By acknowledging how the framework handles complex segments and method constraints efficiently, developers can create more robust and predictable routing in their applications.

If you're running into similar issues, it's crucial to remember that while the routing system might be complex, it ultimately follows a logical approach dictated by defined endpoints and methods.

Feel free to share your own thoughts or experiences with routing in ASP.NET Core in the comments below!