【日常排雷】 . Net core 生产环境appsetting读取失败

关键词 System.ArgumentNullException: String reference not set to an instance of a String. (Parameter 's')

1. 问题出现

某年某月某日，把webapi开发完了，也通过了swagger进行了单元测试。

dotnet build
dotnet publish -o publish
dotnet .\publish\xx.Webapi.dll

然后 String reference not set to an instance of a String. (Parameter 's')

fail: Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware[1]
      An unhandled exception has occurred while executing the request.
System.ArgumentNullException: String reference not set to an instance of a String. (Parameter 's')
   at System.Text.Encoding.GetBytes(String s)
   at AliMobilePush.Webapi.Startup.<ConfigureServices>b__5_2(JwtBearerOptions option) in E:\工作事项\code\dev-push\Alipush\AliMobilePush.Webapi\Startup.cs:line 75
   at Microsoft.Extensions.Options.ConfigureNamedOptions`1.Configure(String name, TOptions options)
   at Microsoft.Extensions.Options.OptionsFactory`1.Create(String name)
   at Microsoft.Extensions.Options.OptionsMonitor`1.<>c__DisplayClass11_0.<Get>b__0()
   at System.Lazy`1.ViaFactory(LazyThreadSafetyMode mode)
   at System.Lazy`1.ExecutionAndPublication(LazyHelper executionAndPublication, Boolean useDefaultConstructor)
   at System.Lazy`1.CreateValue()
   at System.Lazy`1.get_Value()
   at Microsoft.Extensions.Options.OptionsCache`1.GetOrAdd(String name, Func`1 createOptions)
   at Microsoft.Extensions.Options.OptionsMonitor`1.Get(String name)
   at Microsoft.AspNetCore.Authentication.AuthenticationHandler`1.InitializeAsync(AuthenticationScheme scheme, HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationHandlerProvider.GetHandlerAsync(HttpContext context, String authenticationScheme)
   at Microsoft.AspNetCore.Authentication.AuthenticationService.AuthenticateAsync(HttpContext context, String scheme)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Swashbuckle.AspNetCore.SwaggerUI.SwaggerUIMiddleware.Invoke(HttpContext httpContext)
   at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
   at Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware.<Invoke>g__Awaited|6_0(ExceptionHandlerMiddleware middleware, HttpContext context, Task task)

配置文件读出来为null，为什么？

2. 难道是非管理员, 权限不够？

切换至管理员，运行上述命令，还是报错。

3. 无意间解决

cd publish
dotnet xx.Webapi.dll

就能读到了，这是为什么，根据上述命令的差异，大概能猜测到应该是路径不同，导致读取 appsetting.json 失败。

4. 原来是这样

dotnet run ，应该会根据 env.ContentRootPath （env 的类型是IHostingEnvironment ）来读取配置文件 appsettings.Production.json 和 appsettings.json 文件， ContentRootPath 属性得到的值为当前启动命令的目录，而不是dll所在的目录，所以应在发布项目 dll 所在的目录执行 dotnet xx.dll ，否则会导致配置文件里面的参数读取不到。请看源码和命令对比：

// Host. CreateDefaultBuilder(args)

//源码Host.cs
builder. ConfigureAppConfiguration((hostingContext, config) =>

            {
                var env = hostingContext.HostingEnvironment;

                config.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
                      .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true, reloadOnChange: true);

                if (env.IsDevelopment() && !string.IsNullOrEmpty(env.ApplicationName))
                {
                    var appAssembly = Assembly.Load(new AssemblyName(env.ApplicationName));
                    if (appAssembly != null)
                    {
                        config.AddUserSecrets(appAssembly, optional: true);
                    }
                }

                config.AddEnvironmentVariables();

                if (args != null)
                {
                    config.AddCommandLine(args);
                }
            })