如何在C＃WebApi中将依赖项注入静态框架类中

Question

In my WebApiConfig.cs class for a .NET 4.6.1 WebApi project, I want to enable CORS only if an app setting in web.config is set to true. I generally read web.config AppSettings using an AppSettings class that converts the values from string to a more appropriate data type. I declare an IAppSettings interface and register the type with the Autofac DI container I'm using.

However, in this case, WebApiConfig is a static class, and its Register method is called as follows: GlobalConfiguration.Configure(WebApiConfig.Register); I can't change the signature of the GlobalConfiguration.Configure class, so I don't see how to inject the IAppSettings object to make it accessible in the Register method. Of course I can access ConfigurationManager, but that seems very hacky. Of course I could also declare AppSettings as a static class, but that would make unit testing difficult. Is there some cleaner way to do this?

Here's the relevant code - the ConfigurationManager line is the one I want to replace with a call to an appSettings class:

    public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        // Web API configuration and services
        if (System.Configuration.ConfigurationManager.AppSettings.Get("IsCorsEnabled", false))
        {
            config.EnableCors();
        }
        // Web API routes
        config.MapHttpAttributeRoutes();

        config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "api/{controller}/{id}",
            defaults: new { id = RouteParameter.Optional }
        );
    }
}

Many thanks for any assistance!!

Answer 1

Say you have an interface like this for your appsettings (sort of):

public interface IAppSettings
{
    T Get<T>(string name);
}

And a default implementation of this which just wraps around ConfigurationManager :

public class DefaultAppSettings : IAppSettings
{
    public T Get<T>(string name)
    {
        return (T)Convert.ChangeType(ConfigurationManager.AppSettings[name], typeof(T));
    }
}

If you then define a class like this:

public static class CurrentAppSettings
{
    public static Func<IAppSettings> Instance = () => new DefaultAppSettings();
}

You could use this in your register method:

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        // Web API configuration and services
        if (CurrentAppSettings.Instance().Get<bool>("IsCorsEnabled"))
        {
            config.EnableCors();
        }
        // Web API routes
        config.MapHttpAttributeRoutes();

        config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "api/{controller}/{id}",
            defaults: new { id = RouteParameter.Optional }
        );
    }
}

If you want to inject some other implementation of this, like in a test, you just set the CurrentAppSettings instance to something else:

[Test]
public void SomeTest()
{
    CurrentAppSettings.Instance = () => new SimpleKeyValueAppSettings(new Dictionary<string, string>());

    // ...
}