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No Access-Control-Allow-Origin header is present on the requested resource while calling a .NET web api from React app
原标题:

I have created a .Net Core 7 Web API and I m trying to call it from a React app. I ve added CORS policy in my backend code in the following manner -

builder.Services.AddCors(p => p.AddPolicy("corspolicy", builder =>
{
    builder
    .WithOrigins("http://localhost:3000/")
    //.AllowAnyOrigin()
    .AllowAnyMethod()
    .AllowAnyHeader();
}));

app.UseCors("corspolicy");

However, I m still getting the following error in my React app when I log the response -

Access to fetch at  https://localhost:7063/api/CloneAPI  from origin  http://localhost:3000  has been blocked by CORS policy: Response to preflight request doesn t pass access control check: No  Access-Control-Allow-Origin  header is present on the requested resource. If an opaque response serves your needs, set the request s mode to  no-cors  to fetch the resource with CORS disabled.

Access to fetch at  https://localhost:7063/api/CloneAPI  from origin  http://localhost:3000  has been blocked by CORS policy: No  Access-Control-Allow-Origin  header is present on the requested resource. If an opaque response serves your needs, set the request s mode to  no-cors  to fetch the resource with CORS disabled.

If I allow all origins in my backend code like this -

builder.Services.AddCors(p => p.AddPolicy("corspolicy", builder =>
{
    builder
    //.WithOrigins("http://localhost:3000/")
    .AllowAnyOrigin()
    .AllowAnyMethod()
    .AllowAnyHeader();
}));

app.UseCors("corspolicy");

then I am able to fetch all the data successfully. However, I don t want to allow all origins due to safety reasons. Can someone please help me out on why I am getting this error despite setting up CORS and the URLs are correct too?

My frontend code -

  const [posts, setPosts] = useState([]);

  useEffect(() => {
    fetch("https://localhost:7063/api/CloneAPI")
      .then((response) => response.json())
      .then((data) => {
        console.log(data);
        setPosts(data);
      })
      .catch((err) => {
        console.log(err.message);
      });
  }, []);
问题回答

Refer to Enable Cross-Origin Requests (CORS) in ASP.NET Core

Note: The specified URL must not contain a trailing slash (/). If the URL terminates with /, the comparison returns false and no header is returned.

Please remove the trailing slash in .WithOrigins(), just configure like:

.WithOrigins("http://localhost:3000")

then you can access your endpoint correctly without Cors error.





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