I have a class that impelemnts an interface. The interface is

public interface IRiskFactory
    {
       void StartService();
       void StopService();
    }

The class that implements the interface is

public class RiskFactoryService : IRiskFactory { }

Now I have a console application and one window service.

From the console application if I write the following code

static void Main(string[] args)
        {
            IRiskFactory objIRiskFactory = new RiskFactoryService();
            objIRiskFactory.StartService();
            Console.ReadLine();
            objIRiskFactory.StopService();
        }

It is working fine. However, when I mwrite the same piece of code in Window service

public partial class RiskFactoryService : ServiceBase
    {
        IRiskFactory objIRiskFactory = null;
        public RiskFactoryService()
        {
            InitializeComponent();
            objIRiskFactory = new RiskFactoryService(); <- ERROR
        }

        /// <summary>
        /// Starts the service
        /// </summary>
        /// <param name="args"></param>
        protected override void OnStart(string[] args)
        {
            objIRiskFactory.StartService();
        }

        /// <summary>
        /// Stops the service
        /// </summary>
        protected override void OnStop()
        {
            objIRiskFactory.StopService();
        }
    }

It throws error: Cannot implicitly convert type 'RiskFactoryService' to 'IRiskFactory'. An explicit conversion exists (are you missing a cast?)

When I type casted to the interface type, it started working

objIRiskFactory = (IRiskFactory)new RiskFactoryService();

My question is why so?

Thanks.(C#)