ASP.NET Core 8 Method PATCH is not allowed by Access-Control-Allow-Methods in preflight response.

Ben_Pierce 

I have tried to solve and summarise your issue for the PATCH method not being allowed in Blazor due to CORS restrictions, let me know if this helps.

Understanding the Issue:

• CORS (Cross-Origin Resource Sharing): A browser security mechanism that restricts web pages from making requests to a different domain than the one that served the page.
• Preflight Requests (for non-standard methods like PATCH): Browsers send an initial OPTIONS request to check allowed methods before making the actual PATCH request.
• Access-Control-Allow-Methods Header: The server must explicitly list PATCH in this response header to grant permission.

Resolving the Issue in Blazor:

1. Configure CORS in Startup.cs:

   • Add the CORS middleware:

services.AddCors(options =>
{
    options.AddPolicy("AllowPATCH",
        builder => builder.WithOrigins("http://localhost:5000") // Adjust origins as needed
                           .AllowAnyMethod() // Or explicitly list PATCH
                           .AllowAnyHeader();
});

• Apply the policy in the app's middleware pipeline:

app.UseCors("AllowPATCH");

• Enable CORS in ASP.NET Core Web API (if applicable):

  • Use EnableCors attribute on controllers or actions:

[EnableCors("AllowPATCH")]
[HttpPatch]
public IActionResult PatchData()
{
    // ...
}