How to update values into appsetting.json?

Question

I am using the IOptions pattern as described in the official documentation.

This works fine when I am reading values from appsetting.json,

Answer 1

Simplified version of Matze's answer:

public interface IWritableOptions : IOptionsSnapshot where T : class, new()
{
    void Update(Action applyChanges);
}

public class WritableOptions : IWritableOptions where T : class, new()
{
    private readonly IHostingEnvironment _environment;
    private readonly IOptionsMonitor _options;
    private readonly string _section;
    private readonly string _file;

    public WritableOptions(
        IHostingEnvironment environment,
        IOptionsMonitor options,
        string section,
        string file)
    {
        _environment = environment;
        _options = options;
        _section = section;
        _file = file;
    }

    public T Value => _options.CurrentValue;
    public T Get(string name) => _options.Get(name);

    public void Update(Action applyChanges)
    {
        var fileProvider = _environment.ContentRootFileProvider;
        var fileInfo = fileProvider.GetFileInfo(_file);
        var physicalPath = fileInfo.PhysicalPath;

        var jObject = JsonConvert.DeserializeObject(File.ReadAllText(physicalPath));
        var sectionObject = jObject.TryGetValue(_section, out JToken section) ?
            JsonConvert.DeserializeObject(section.ToString()) : (Value ?? new T());

        applyChanges(sectionObject);

        jObject[_section] = JObject.Parse(JsonConvert.SerializeObject(sectionObject));
        File.WriteAllText(physicalPath, JsonConvert.SerializeObject(jObject, Formatting.Indented));
    }
}

public static class ServiceCollectionExtensions
{
    public static void ConfigureWritable(
        this IServiceCollection services,
        IConfigurationSection section,
        string file = "appsettings.json") where T : class, new()
    {
        services.Configure(section);
        services.AddTransient>(provider =>
        {
            var environment = provider.GetService();
            var options = provider.GetService>();
            return new WritableOptions(environment, options, section.Key, file);
        });
    }
}

Usage:

services.ConfigureWritable(Configuration.GetSection("MySection"));

Then:

private readonly IWritableOptions _options;

public MyClass(IWritableOptions options)
{
    _options = options;
}

To save the changes to the file:

_options.Update(opt => {
    opt.Field1 = "value1";
    opt.Field2 = "value2";
});

And you can pass a custom json file as optional parameter (it will use appsettings.json by default):

services.ConfigureWritable(Configuration.GetSection("MySection"), "appsettings.custom.json");