How to update values into appsetting.json?

At the time of writing this answer it seemed that there is no component provided by the Microsoft.Extensions.Options package that has functionality to write configuration values back to appsettings.json.

In one of my ASP.NET Core projects I wanted to enable the user to change some application settings – and those setting values should be stored in appsettings.json, more precisly in an optional appsettings.custom.json file, that gets added to the configuration if present.

Like this…

public Startup(IHostingEnvironment env)
{
    IConfigurationBuilder builder = new ConfigurationBuilder()
        .SetBasePath(env.ContentRootPath)
        .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
        .AddJsonFile("appsettings.custom.json", optional: true, reloadOnChange: true)
        .AddEnvironmentVariables();

    this.Configuration = builder.Build();
}

I declared the IWritableOptions<T> interface that extends IOptions<T>; so I can just replace IOptions<T> by IWritableOptions<T> whenever I want to read and write settings.

public interface IWritableOptions<out T> : IOptions<T> where T : class, new()
{
    void Update(Action<T> applyChanges);
}

Also, I came up with IOptionsWriter, which is a component that is intended to be used by IWritableOptions<T> to update a configuration section. This is my implementation for the beforementioned interfaces…

class OptionsWriter : IOptionsWriter
{
    private readonly IHostingEnvironment environment;
    private readonly IConfigurationRoot configuration;
    private readonly string file;

    public OptionsWriter(
        IHostingEnvironment environment, 
        IConfigurationRoot configuration, 
        string file)
    {
        this.environment = environment;
        this.configuration = configuration;
        this.file = file;
    }

    public void UpdateOptions(Action<JObject> callback, bool reload = true)
    {
        IFileProvider fileProvider = this.environment.ContentRootFileProvider;
        IFileInfo fi = fileProvider.GetFileInfo(this.file);
        JObject config = fileProvider.ReadJsonFileAsObject(fi);
        callback(config);
        using (var stream = File.OpenWrite(fi.PhysicalPath))
        {
            stream.SetLength(0);
            config.WriteTo(stream);
        }

        this.configuration.Reload();
    }
}

Since the writer is not aware about the file structure, I decided to handle sections as JObject objects. The accessor tries to find the requested section and deserializes it to an instance of T, uses the current value (if not found), or just creates a new instance of T, if the current value is null. This holder object is than passed to the caller, who will apply the changes to it. Than the changed object gets converted back to a JToken instance that is going to replace the section…

class WritableOptions<T> : IWritableOptions<T> where T : class, new()
{
    private readonly string sectionName;
    private readonly IOptionsWriter writer;
    private readonly IOptionsMonitor<T> options;

    public WritableOptions(
        string sectionName, 
        IOptionsWriter writer, 
        IOptionsMonitor<T> options)
    {
        this.sectionName = sectionName;
        this.writer = writer;
        this.options = options;
    }

    public T Value => this.options.CurrentValue;

    public void Update(Action<T> applyChanges)
    {
        this.writer.UpdateOptions(opt =>
        {
            JToken section;
            T sectionObject = opt.TryGetValue(this.sectionName, out section) ?
                JsonConvert.DeserializeObject<T>(section.ToString()) :
                this.options.CurrentValue ?? new T();

            applyChanges(sectionObject);

            string json = JsonConvert.SerializeObject(sectionObject);
            opt[this.sectionName] = JObject.Parse(json);
        });
    }
}

Finally, I implemented an extension method for IServicesCollection allowing me to easily configure a writable options accessor…

static class ServicesCollectionExtensions
{
    public static void ConfigureWritable<T>(
        this IServiceCollection services, 
        IConfigurationRoot configuration, 
        string sectionName, 
        string file) where T : class, new()
    {
        services.Configure<T>(configuration.GetSection(sectionName));

        services.AddTransient<IWritableOptions<T>>(provider =>
        {
            var environment = provider.GetService<IHostingEnvironment>();
            var options = provider.GetService<IOptionsMonitor<T>>();
            IOptionsWriter writer = new OptionsWriter(environment, configuration, file);
            return new WritableOptions<T>(sectionName, writer, options);
        });
    }
}

Which can be used in ConfigureServices like…

services.ConfigureWritable<CustomizableOptions>(this.Configuration, 
    "MySection", "appsettings.custom.json");

In my Controller class I can just demand an IWritableOptions<CustomizableOptions> instance, that has the same characteristics as IOptions<T>, but also allows to change and store configuration values.

private IWritableOptions<CustomizableOptions> options;

...

this.options.Update((opt) => {
    opt.SampleOption = "...";
});