Use Cors based on an appSettings in .Net Core

Question

I am updating a .net 4.5.2 project to .Net core web api. Right now, the Cors is setup as below based on an appSetting value CorsAllowAll:

if ((ConfigurationManager.AppSettings["CorsAllowAll"] ?? "false") == "true")
{
    appBuilder.UseCors(CorsOptions.AllowAll);
}
else
{
    ConfigureCors(appBuilder);
}

private void ConfigureCors(IAppBuilder appBuilder)
{
    appBuilder.UseCors(new CorsOptions
    {
    PolicyProvider = new CorsPolicyProvider
    {
        PolicyResolver = context =>
        {
           var policy = new CorsPolicy();
           policy.Headers.Add("Content-Type");
           policy.Headers.Add("Accept");
           policy.Headers.Add("Auth-Token");
           policy.Methods.Add("GET");
           policy.Methods.Add("POST");
           policy.Methods.Add("PUT");
           policy.Methods.Add("DELETE");
           policy.SupportsCredentials = true;
           policy.PreflightMaxAge = 1728000;
           policy.AllowAnyOrigin = true;
           return Task.FromResult(policy);
        }
    }
    });
}

How can I achieve the same in .net core? Unfortunately, I won't be knowing the URLs of each environment. But I do know that for Local, DEV and QA environments, the appSetting CorsAllowAll is true. But the UAT and PROD environments it would be false.

UPDATE My appSettings.json is like below:

"AppSettings": {
    ...
    "CorsAllowAll": true 
    ...
  }

lastlink · Accepted Answer · 2021-04-27 19:42:13Z

This method works great. WithOrigins accepts a string [] so you can just split an appsettings value by ; or something else.

appsettings.json


  {
  "AllowedOrigins": "http://localhost:8080;http://localhost:3000"
  }

startup.cs

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ApplicationDbContext dbContext, IOptions<AppSettings> appSettings)

if (!String.IsNullOrEmpty(_appSettings.AllowedOrigins))
       {
          var origins = _appSettings.AllowedOrigins.Split(";");
          app.UseCors(x => x
                    .WithOrigins(origins)
                    .AllowAnyMethod()
                    .AllowCredentials()
                    .AllowAnyHeader());
       }

The main reason for this semi colon format is because it is similar to Application\Properties\launchSettings.json

...
"profiles": {
        "IIS Express": {
            "commandName": "IISExpress",
            "launchBrowser": true,
            "launchUrl": "api/values",
            "environmentVariables": {
                "ASPNETCORE_ENVIRONMENT": "Development"
            }
        },
        "Application": {
            "commandName": "Project",
            "launchBrowser": true,
            "launchUrl": "api/values",
            "applicationUrl": "http://localhost:5000;http://192.168.50.20:5000",
            "environmentVariables": {
                "ASPNETCORE_ENVIRONMENT": "Development"
            }
        }
    }
...

You can also just use an array as well: json { "AllowedOrigins": ["http://localhost:8080", "http://localhost:3000"] }
@Tyrrrz boy did your comment send me on a wild goose chase. Turns out you can't just Configuration.GetValue<string[]>("AllowedOrigins") and you have to jump through some hoops to do it.

Yared · Accepted Answer · 2018-06-22 06:21:00Z

In ConfigureServices method, define two policies namely CorsAllowAll and CorsAllowSpecific

services.AddCors(options =>
            {
                options.AddPolicy("CorsAllowAll",
                    builder =>
                    {
                        builder
                        .AllowAnyOrigin() 
                        .AllowAnyMethod()
                        .AllowAnyHeader()
                        .AllowCredentials();
                    });                    

                options.AddPolicy("CorsAllowSpecific",
                    p => p.WithHeaders("Content-Type","Accept","Auth-Token")
                        .WithMethods("POST","PUT","DELETE")
                        .SetPreflightMaxAge(new TimeSpan(1728000))
                        .AllowAnyOrigin()
                        .AllowCredentials()
                    ); 
            });

The setting CorsAllowAll value can be accessed from IConfiguration in Startup.cs. Depending on its value, it is possible to set one of the defined policies globally in Configure method, before calling app.UseMvc().

//Read value from appsettings
var corsAllowAll = Configuration["AppSettings:CorsAllowAll"] ?? "false";
app.UseCors(corsAllowAll == "true"? "CorsAllowAll" : "CorsAllowSpecific");

My appSettings is not at the root level. It is buried inside the element "AppSettings". I edited the question to include that..
Also, my question was not on just how to get the appSettings value. But how the appBuilder.useCors can be used based on the appSetting value? I am unable to use the ConfigureCors method in .net core..
Please edit the answer and use Configuration["AppSettings:CorsAllowAll"]. AppSettings.CorsAllowAll is null always..

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