Reloading Options with reloadOnChange in ASP.NET Core

In my ASP.NET Core application I bind the appsettings.json to a strongly typed class AppSettings.

public Startup(IHostingEnvironment environment)
{
var builder = new ConfigurationBuilder()
.SetBasePath(environment.ContentRootPath)
.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
.AddJsonFile($"appsettings.{environment.EnvironmentName}.json", optional: true, reloadOnChange: true)
.AddEnvironmentVariables();

Configuration = builder.Build();
}

public void ConfigureServices(IServiceCollection services)
{
services.Configure<AppSettings>(Configuration);
//...
}


In a singleton class I wrap this AppSettings class like this:

public class AppSettingsWrapper : IAppSettingsWrapper
{
private readonly IOptions<AppSettings> _options;

public AppSettingsAdapter(IOptions<AppSettings> options)
{
_options = options ?? throw new ArgumentNullException("Options cannot be null");
}

public SomeObject SomeConvenienceGetter()
{
//...
}
}


Now I'm struggling with reloading the AppSettings if the json file changes. I read somewhere that the class IOptionsMonitor can detect changes but it doesn't work in my case.

I tried calling the OnChange event like this for testing purposes:

public void Configure(IApplicationBuilder applicationBuilder, IOptionsMonitor<AppSettings> optionsMonitor)
{
applicationBuilder.UseStaticFiles();
applicationBuilder.UseMvc();

optionsMonitor.OnChange<AppSettings>(vals =>
{
System.Diagnostics.Debug.WriteLine(vals);
});
}


The event is never triggered when I change the json file. Has someone an idea what I can change to get the reloading mechanic to work in my scenario?

1 Answer
1

You need to inject IOptionsSnapshot<AppSettings> to get the reload working.

IOptionsSnapshot<AppSettings>

Unfortunately you cannot load the IOptionsSnapshot into a Singleton service. IOptionsSnapshot is a Scoped service so you can only reference it in a Scoped or Transient registered class.

IOptionsSnapshot

IOptionsSnapshot

But, if think about it, that makes sense. The settings need to be reloaded when they change so if you inject them into a Singleton then the class will never get the updated settings because the constructor will not be called again for a Singleton.

I thought that I maybe could rebind the AppSettingsWrapper with the new AppSettings in the "OnChange" event. It is unfortunate that I have to change the lifetime of my wrapper class because besides the rare reloading it never changes.
– Shamshiel
Jun 29 at 11:44

Unless you have a really compelling reason to keep it as a singleton e.g. real time performance, switching to a Scoped service should give you what you want without much penalty, after all you are still injecting the same class, only the registration has changed.
– Simply Ged
Jun 29 at 13:15

Scoped

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