public void AddTypeDescriptorIsSingleton()
        {
            // Arrange
            var collection = new ServiceCollection();

            // Act
            collection.AddTypeActivator();

            // Assert
            var descriptor = Assert.Single(collection);

            Assert.Equal(typeof(ITypeActivator), descriptor.ServiceType);
            Assert.Null(descriptor.ImplementationInstance);
            Assert.Equal(LifecycleKind.Singleton, descriptor.Lifecycle);
        }