Clean way to specify options for a service when doing DI

Question

So I have a service lets say for example it's an email service on ASPNET Core.

When I add my service to the ASPNET DI container I would like to apply the following pattern on my IServiceCollection to setup my service.

public interface IEmailService
{
    void SendMail(string recipient, string message);
}
public void ConfigureServices(IServiceCollection services)
{
    //configures my service
    services.AddEmailService<MyEmailService>(options => options.UseEmailServer(sender, smtpHost, smtpPort, smtpPassword));
}

I would like to know whats the best way to do this if possible. I am sure I would need to make an extension method for the .AddEmailService() method on IServiceCollection however anything beyond that I am not sure where to start or look.

Answer 1

Here's an example application with comments to let you know what the different things are doing:

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        // Add the options stuff. This will allow you to inject IOptions<T>.
        services.AddOptions();

        // This will take care of adding and configuring the email service.
        services.AddEmailService<MyEmailService>(options =>
        {
            options.Host = "some-host.com";
            options.Port = 25;
            options.Sender = "[email protected]";

            options.Username = "email";
            options.Password = "sup4r-secr3t!";
        });
    }

    public void Configure(IApplicationBuilder app, ILoggerFactory loggerFactory)
    {
        // Make sure we add the console logger.
        loggerFactory.AddConsole();

        app.Use(async (context, next) =>
        {
            // Retrieve the email service from the services.
            var emailService = context.RequestServices.GetRequiredService<IEmailService>();

            // Send the email
            await emailService.SendMail("[email protected]", "Hello World!");
        });
    }

    public static void Main(string[] args)
    {
        WebApplication.Run<Startup>(args);
    }
}

public interface IEmailService
{
    Task SendMail(string recipient, string message);
}

public class EmailOptions
{
    public string Sender { get; set; }

    public string Host { get; set; }

    public int Port { get; set; }

    public string Username { get; set; }

    public string Password { get; set; }
}

public class MyEmailService : IEmailService
{
    public MyEmailService(IOptions<EmailOptions> options, ILogger<MyEmailService> logger)
    {
        Options = options; // This contains the instance we configured.
        Logger = logger;
    }

    private IOptions<EmailOptions> Options { get; }

    private ILogger<MyEmailService> Logger { get; }

    public Task SendMail(string recipient, string message)
    {
        // Send the email

        var builder = new StringBuilder();

        builder.AppendLine($"Host: {Options.Value.Host}");
        builder.AppendLine($"Port: {Options.Value.Port}");
        builder.AppendLine($"Username: {Options.Value.Username}");
        builder.AppendLine($"Password: {Options.Value.Password}");
        builder.AppendLine("---------------------");
        builder.AppendLine($"From: {Options.Value.Sender}");
        builder.AppendLine($"To: {recipient}");
        builder.AppendLine("---------------------");
        builder.AppendLine($"Message: {message}");

        Logger.LogInformation(builder.ToString());

        return Task.FromResult(0);
    }
}

public static class ServiceCollectionExtensions
{
    public static IServiceCollection AddEmailService<TEmailService>(this IServiceCollection services, Action<EmailOptions> configure)
        where TEmailService : class, IEmailService
    {
        // Configure the EmailOptions and register it in the service collection, as IOptions<EmailOptions>.
        services.Configure(configure);

        // Add the service itself to the collection.
        return services.AddSingleton<IEmailService, TEmailService>();
    }
}

And here's the application running in the console:

As you can see, the application is pulling some information from the configured EmailOptions, and some information form the arguments passed in.

EDIT: These are the required packages:

"Microsoft.AspNet.Server.Kestrel": "1.0.0-rc1-final",
"Microsoft.Extensions.OptionsModel": "1.0.0-rc1-final",
"Microsoft.Extensions.Logging.Console": "1.0.0-rc1-final"