如何将 appsettings 中的默认值设置为 API Model 上的属性

Question

How can I set a default value to a property, reading it from my appsettings.json file, on a model that is instantiated by the .NET Core 3 framework?

I've created a repo (a completely new .NET Core 3 project) where I try to illustrate the problem: https://github.com/NelsonPRSousa/dependency-injection-default-constructor

API Action:

[HttpGet]
public IEnumerable<WeatherForecast> Get([FromQuery] FilteringRequestModel request)
{

    var defaultType = request.Type;

    var rng = new Random();
    return Enumerable.Range(1, 5).Select(index => new WeatherForecast
    {
        Date = DateTime.Now.AddDays(index),
        TemperatureC = rng.Next(-20, 55),
        Summary = Summaries[rng.Next(Summaries.Length)]
    })
    .ToArray();
}

Model:

public class FilteringRequestModel
{
    /* Please note that we must have a parameterless constructor, since it's the framework responsability to instatiate this object
     * when invoking this action: IEnumerable<WeatherForecast> Get([FromQuery] FilteringRequestModel request)
     */

    public FilteringRequestModel()
    {
        //Type = System.Configuration.ConfigurationManager.AppSettings["DefaultTypeTopRated"];
    }

    public string Type { get; set; } = "top_rated"; // TODO: Read from appsettings
}

Answer 1

You could use IOptions with Dependecy Injection

Update FilteringRequestModel class

public class FilteringRequestModel
{
    public FilteringRequestModel()
    {
    }

    public void Initialize(IOptions<FilteringRequestSettings> settings)
    {
        if (string.IsNullOrEmpty(this.Type))
        {
            this.Type = settings.Value.Type;
        }
    }

    public string Type { get; set; }
}

Add a FilteringRequestSettings class

public class FilteringRequestSettings
{
    public string Type { get; set; }
}

Update appsettings.Development.json like this

{
  "Logging": {
    "LogLevel": {
      "Default": "Debug",
      "System": "Information",
      "Microsoft": "Information"
    }
  },
  "FilteringRequest": {
    "Type":  "foo"
  }
}

Update ConfigureServices method in Startup class like this

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<FilteringRequestSettings>(this.Configuration.GetSection("FilteringRequest"));

    services.AddControllers();
}

Finally, update WeatherForecastController class like this

[HttpGet]
public IEnumerable<WeatherForecast> Get([FromQuery] FilteringRequestModel request)
{
    request.Initialize(_settings);

    var defaultType = request.Type;

    var rng = new Random();
    return Enumerable.Range(1, 5).Select(index => new WeatherForecast
    {
        Date = DateTime.Now.AddDays(index),
        TemperatureC = rng.Next(-20, 55),
        Summary = Summaries[rng.Next(Summaries.Length)]
    })
    .ToArray();
}

Explanation

• You will load parameter from appsettings into IOptions<FilteringRequestSettings>
• IOptions<FilteringRequestSettings> will be injected into WeatherForecastController
• You will then call the Initialize method your request parameter with IOptions<FilteringRequestSettings>
• The Initialize method verify if the Type property has not already been set (from query / request). If not, it will use the value from appsettings (through IOptions<FilteringRequestSettings>