问题
I had my working project written in asp.net core 2.1 for a long time, but yesterday, I was forced to upgrade it to .net core 3.0 (due to 2.1 cannot call Dll' s which are written in 3.0 already).
With that, a lot of functions were obsolete or already removed. I fixed almost all of it, but one problem with CORS.
Like many people before me, I used:
app.UseCors(x => x
.AllowAnyOrigin()
.AllowAnyMethod()
.AllowAnyHeader()
.AllowCredentials());
in Configure function. And services.AddCors() in ConfigureServices function.
I was able to fixed this quite easily with setting WithOrigins() or .SetIsOriginAllowed(_ => true) instead of AllowAnyOrigin() which does not work anymore with AllowCredentials().
After that, I was able to start the application and I thought everything is fine, but then I get stuck until now with problem I do not know, how to fix.
I have DB relation N:N and relation table which handle that, that means I have Admin entity with AdminProject list property, then I have AdminProject entity with Admin list and Project list properties and Project entity with AdminProject list property once again.
When I am listing my projects of certain admin, I am returning in Controller this return Ok(projects), where I just use getAll on AdminProject entity and then with Select return only project.
For that, I have to use[JsonIgnore] in project/admin for properties which I do not need to avoid cycling when creating json.
With that said: NOW IN .NET CORE 3.0 AND CORS SETTINGS IT DOES NOT WORK.
I am getting an error:
System.Text.Json.JsonException: A possible object cycle was detected which is not supported. This can either be due to a cycle or if the object depth is larger than the maximum allowed depth of 32.
when debugging in console and error Access to XMLHttpRequest at 'http://localhost:5000/api/project/adminlist/1' from origin 'http://localhost:8080' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. in WEB browser
I think I tried almost everything with Cors settings etc and I do not know why is this happening now. I also tried to JsonConvert.SerializeObject() before return it ---> return Ok(JsonConvert.SerializeObject(projects)) and this is working, but I am not able (mentally) to do this in every single controllers functions.
Please help! Thanks a lot!
回答1:
The problem was occurring because in .NET Core 3 they change little bit the JSON politics. Json.Net is not longer supported and if you want to used all Json options, you have to download this Nuget: Microsoft.AspNetCore.Mvc.NewtonsoftJson.
After that in your Startup.cs file change/fix/add line where you are adding MVC (in the ConfigureServices method.
So: here is what I did and what fixed my issue:
services.AddMvc(option => option.EnableEndpointRouting = false)
.SetCompatibilityVersion(CompatibilityVersion.Version_3_0)
.AddNewtonsoftJson(opt => opt.SerializerSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore);
I hope it will help somebody else. Cheers!
回答2:
A couple other things have changed in .net core 3 and now instead of using addMVC you can use addControllers. So your code might look like the follow:
services.AddControllers().AddNewtonsoftJson(x => x.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore);
来源：https://stackoverflow.com/questions/57912012/net-core-3-upgrade-cors-and-jsoncycle-xmlhttprequest-error