How To Register App.Config and appsettings.json -> Config Manager for .Net 6+ and ASP.NET Core 6+

App.config

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <appSettings>
    <add key="Bla:Foo:Zoo" value="SampleValue" />
  </appSettings>
</configuration>

appsettings.json

{  
  "ConnectionStrings": {
    "Default": "123456"
  },
  "SampleInt": 1234567,
  "SampleStringInt": "1234567"
}

ConfigController.cs

[ApiController]
[Route("[Controller]/[Action]")]
public class ConfigController : Controller
{

  [HttpGet]
  public IList<ConfigKeyValue> GetConfigs()
  {
      // HOW TO READ CONFIGS ON ASP.NET CORE WEB API (IT HAS OWN DI BUT SOME REASON ITS USABLE FOR IT THO)

      return ConfigManager.ConfigurationKeyValues.ToList();
  }
  
}

ConfigManager.cs

public static class ConfigManager
{
    public static IConfiguration Configuration { get; private set; }
    public static IEnumerable<ConfigKeyValue> ConfigurationKeyValues { get => Configuration.AsEnumerable().Select(cfg => new ConfigKeyValue(cfg.Key, cfg.Value)); }


    /// <summary>
    /// Create configuration
    /// </summary>
    /// <param name="configFilePaths">Json configs (appsettings.json)</param>
    /// <param name="nameValueCollections">Xml configs (App.config)</param>
    /// <returns></returns>
    public static IConfiguration CreateConfiguration(List<ConfigFilePaths> configFilePaths, List<NameValueCollection> nameValueCollections = default)
    {
        ConfigurationBuilder configBuilder = new();

        // APPSETTINGS.JSON
        if (configFilePaths is not null && configFilePaths.Count > 0)
        {
            foreach (var configFilePath in configFilePaths)
            {
                configBuilder.AddJsonFile(
                    path: configFilePath.JsonPath,
                    optional: configFilePath.Optional,
                    reloadOnChange: configFilePath.ReloadOnChange);
            }
        }

        // APP.CONFIG
        if (nameValueCollections is not null && nameValueCollections.Count > 0)
        {
            foreach (var nameValueCollection in nameValueCollections)
            {
                var keyValues = nameValueCollection.AllKeys.ToDictionary(key => key, key => nameValueCollection[key]);
                configBuilder.AddInMemoryCollection(keyValues);
            }
        }

        return configBuilder.Build();
    }


    /// <summary>
    /// Create configuration
    /// </summary>
    /// <param name="configFilePaths">Json configs (appsettings.json)</param>
    /// <param name="nameValueCollections">Xml configs (App.config)</param>
    /// <returns></returns>
    public static void SetConfiguration(List<ConfigFilePaths> configFilePaths, List<NameValueCollection> nameValueCollections = default)
    {
        ConfigurationBuilder configBuilder = new();

        // APPSETTINGS.JSON
        if (configFilePaths is not null && configFilePaths.Count > 0)
        {
            foreach (var configFilePath in configFilePaths)
            {
                configBuilder.AddJsonFile(
                    path: configFilePath.JsonPath,
                    optional: configFilePath.Optional,
                    reloadOnChange: configFilePath.ReloadOnChange);
            }
        }

        // APP.CONFIG
        if (nameValueCollections is not null && nameValueCollections.Count > 0)
        {
            foreach (var nameValueCollection in nameValueCollections)
            {
                var keyValues = nameValueCollection.AllKeys.ToDictionary(key => key, key => nameValueCollection[key]);
                configBuilder.AddInMemoryCollection(keyValues);
            }
        }

        Configuration =  configBuilder.Build();
    }

}

public record ConfigFilePaths(string JsonPath, bool Optional = false, bool ReloadOnChange = true);
public record ConfigKeyValue(string Key, string Value);

RestApiProgram.cs

var builder = WebApplication.CreateBuilder(args);

// HOW TO REGISTER ALL CONFIGS TO ASP.NET CORE DI (EITHER APP.CONFIG or APPSETTINGS.JSON)

ConfigManager.SetConfiguration([new("appsettings.json", true, true), new("appsettings.development.json", true, true)])
builder.Configuration.AddInMemoryCollection(ConfigManager.ConfigurationKeyValues.Select(cm => new KeyValuePair<string, string>(cm.Key, cm.Value)));

var app = builder.Build();

app.Run();

WinFormProgram.cs

/// <summary>
/// The main entry point for the application.
/// </summary>
[STAThread]
static void Main()
{
  List<ConfigFilePaths> configFilePaths = [
      new ConfigFilePaths("appsettings.json", true),
      new ConfigFilePaths("Config/appsettings.json"),
      new ConfigFilePaths($"appsettings.development.json", true)
    ];

  List<NameValueCollection> nameValueCollections = [System.Configuration.ConfigurationManager.AppSettings];

  ConfigManager.SetConfiguration(configFilePaths, nameValueCollections);
}

WinFormUsage.cs

public WinFormUsage()
{
    InitializeComponent();
    
    // SOME USE CASES ON WINFORM FORMS

    gridControl1.DataSource = ConfigManager.ConfigurationKeyValues;
    string connString = ConfigManager.Configuration.GetConnectionString("Default");
    int sampleInt = Convert.ToInt32(ConfigManager.Configuration["SampleInt"]);
    int sampleIntWithGetValue = ConfigManager.Configuration.GetValue<int>("SampleInt");
    int sampleInt2 = ConfigManager.Configuration.GetValue<int>("SampleStringInt");
}