使用 UseSetting 覆盖配置

Question

I am using .NET Core 2.2, default template and trying to override configuration using UseSetting, however I can't make it work. Test value set in config to File, I would like to override it with same value in Code, and then in Startup I want to get overridden value.

(Originally I was trying to add AzureKeyVaultProvider, but it was not working for me and I've ended with this example)

Configuration:

{
  "Logging": {
    "LogLevel": {
      "Default": "Warning"
    }
  },
  "AllowedHosts": "*",
  "Test": "file" 
}

Program.cs:

public class Program
{
    public static void Main(string[] args)
    {
        CreateWebHostBuilder(args).Build().Run();
    }

    public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .UseStartup<Startup>()
            .UseSetting("Test", "code");
}

Startup.cs:

public class Startup
{
    private readonly IConfiguration _configuration;

    public Startup(IConfiguration configuration)
    {
        _configuration = configuration;
    }

    public void ConfigureServices(IServiceCollection services)
    {
        var value = _configuration.GetValue<string>("Test");
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        app.Run(async (context) =>
        {
            await context.Response.WriteAsync("Hello World!");
        });
    }
}

Answer 1

UseSetting applies to the host configuration, which is applied before the application's configuration. In your example, it's the JSON value that overrides the UseSetting value. UseSetting sets the value in the host configuration, which gets copied into the application configuration and then overridden by the JSON value.

Use ConfigureAppConfiguration and AddInMemoryCollection to achieve the desired result:

WebHost.CreateDefaultBuilder(args)
    .UseStartup<Startup>()
    .ConfigureAppConfiguration((ctx, configurationBuilder) =>
    {
        configurationBuilder.AddInMemoryCollection(new Dictionary<string, string>
        {
            ["Test"] = "code"
        });
    });

With this setup, values used in the call to AddInMemoryCollection override all other values in JSON, env, etc.