いつもなんとなくDIしてるので、基本をメモ

    public interface IGetWeather
    {
        public string GetWeather();
    }

    internal class GetSunny: IGetWeather
    {
        public string GetWeather()
        {
            return "晴れ";
        }
    }

    internal class GetRain : IGetWeather
    {
        public string GetWeather()
        {
            return "雨";
        }
    }

    internal class GetMessage
    {
        private IGetWeather getWeather;

        public GetMessage(IGetWeather getWeather)
        {
            this.getWeather = getWeather;
        }

        public void ShowMessage()
        {
            Console.WriteLine(getWeather.GetWeather());
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var services = new ServiceCollection();
            services.AddTransient<IGetWeather, GetSunny>();
            services.AddTransient<GetMessage>();

            var provider = services.BuildServiceProvider();
            var message = provider.GetRequiredService<GetMessage>();
            message.ShowMessage();
        }
    }