How to use IHttpClientFactory in class library

Question

I am trying to convert some code from net core api to class library.

I am stuck how to use HttpClientfactory. Normally the httpclientfactory can be configured in program.cs or Startup like

services.AddHttpClient("abc", xxx config). 

How to do configurations in class library for Httpclientfactory.

Answer 1

In your library add an extension method for IServiceCollection to "enable" it in the main project.

In the library:

public static class ServiceCollectionExt
{
    public static void AddYourStaff(this IServiceCollection services)
    {
        services.AddHttpClient("xxx", client =>
        {
            //your staff here
        });
        services.AddSingleton<ISomethingElse, SomethingElse>();
    }
} 

Then in your Startup just call it:

services.AddYourStaff();

UPDATE: As the author described, he's working on the plugin based application. In that case you need some kind of convention, for instance:

1. each plugin library must have a static class called Registration with the method Invoke(IServiceCollection sc, IConfiguration config)
2. Then in your Startup you can iterate through all plugin libraries and call their Registration.Invoke(sc, config) using reflection:

foreach(var pluginAssembly in plugins)
{
    pluginAssembly
        .GetType("Registration")
        .GetMethod("Invoke")
        .Invoke(null, new object[] {services, Configuration});
}

Answer 2

You could try as below:

public class HttpClientUtil
    {
        private static IServiceProvider serviceProvider { get; set; } 

        public static void Initial(IServiceProvider Provider)
        {
            if (Provider == null)
            {
                IHostBuilder builder = Host.CreateDefaultBuilder();
                builder.ConfigureServices(services =>
                {
                    services.AddHttpClient("client_1", config =>
                    {
                        config.BaseAddress = new Uri("http://client_1.com");
                        config.DefaultRequestHeaders.Add("header_1", "header_1");
                    });
                    services.AddHttpClient("client_2", config =>
                    {
                        config.BaseAddress = new Uri("http://client_2.com");
                        config.DefaultRequestHeaders.Add("header_2", "header_2");
                    });
                });
                serviceProvider = builder.Build().Services;
            }
            else
            {
                serviceProvider = Provider;
            } 
        }

        public static IHttpClientFactory GetHttpClientFactory()
        {
            if(serviceProvider==null)
            {
                Initial(serviceProvider);
            }
            return (IHttpClientFactory)serviceProvider.GetServices<IHttpClientFactory>();
        }

    }

you could get the instance of httpclinetfactory through the interface