Decorator pattern in C#

DecoratorTests.cs

namespace 
{
    public class DecoratorTests
    {
        [Fact]
        public void should_decorate_service()
        {
            var services = new ServiceCollection();
            services.AddSingleton<MyService>();
            services.AddSingleton<IMyService, DecoratedMyService<MyService>>();

            var serviceProvider = services.BuildServiceProvider();
            var result = serviceProvider.GetService<IMyService>().DoTask();
            result.Should().Be("[Decorator] Task completed");
        }
    }

    public class DecoratedMyService<T> : IMyService where T : IMyService
    {
        private readonly T _origin;

        public DecoratedMyService(T origin)
        {
            _origin = origin;
        }
        public string DoTask()
        {
            return "[Decorator] " + _origin.DoTask();
        }
    }

    public class MyService : IMyService
    {
        public string DoTask()
        {
            return "Task completed";
        }
    }

    public interface IMyService
    {
        string DoTask();
    }
}