How to Fix the CORS Error: MethodDisallowedByPreflightResponse in ASP.NET C#

Описание: Learn how to resolve the CORS error `MethodDisallowedByPreflightResponse` when making PUT requests to your ASP.NET Core API server. This guide offers step-by-step instructions and helpful tips.
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Understanding and Resolving CORS Errors in ASP.NET C#

When developing web applications that interact with APIs, you may encounter Cross-Origin Resource Sharing (CORS) issues. One such error is MethodDisallowedByPreflightResponse, which can occur during HTTP requests, particularly with the PUT method. In this post, we'll explore the details of this issue and guide you through the solution to resolve it in your ASP.NET Core application.

The Problem

While working with an ASP.NET Core API, you might face a CORS error message that reads MethodDisallowedByPreflightResponse. This usually occurs when sending a PUT request, which differs from the POST and GET requests that function without any issues. The error typically signals that your server's CORS configuration isn't allowing the HTTP method you're attempting to use.

Here's what the situation looked like for a developer experiencing this issue:

Research: Despite extensive Googling, there were no clear references specifically addressing this error.

Debugging: Checking the DevTools Network tab revealed no indications about allowed methods in the preflight request/response headers.

Key Insight:

This hints that your server's CORS settings need adjustment to permit the PUT method.

The Solution

To fix the MethodDisallowedByPreflightResponse error, we need to ensure that the server's CORS policy is configured correctly to allow the necessary HTTP methods.

Step-by-Step Breakdown:

Locate the CORS Configuration:
Make sure to find your CORS settings in the Startup.cs file of your ASP.NET Core application. This is typically where you'll manage services and middleware configurations.

Use AllowAnyMethod():
Add the AllowAnyMethod() method to your CORS configuration to explicitly permit all HTTP methods, including PUT. Here’s how to implement it:

[[See Video to Reveal this Text or Code Snippet]]

Test Your Implementation:
After configuring the CORS policy as shown, try sending your PUT request again. The error should be resolved, and you should be able to communicate with your ASP.NET Core API without issues.

Conclusion

Dealing with CORS errors can be frustrating, especially when they stem from seemingly obscure issues like MethodDisallowedByPreflightResponse. However, by ensuring your server correctly allows the necessary HTTP methods through proper CORS configuration, you can eliminate this error and enhance your API's interoperability with client applications.

For further insights or if you encounter other CORS-related issues, don’t hesitate to reach out or consult additional resources on ASP.NET Core's CORS policies.