Interface

They have a similar to structures and classes. Methods in Interfaces do not have a declaration (they are only defined similar to Abstract Methods)
Access Modifiers are also not specified for the methods in a Interface
Interfaces are used to design loosely coupled applications
The methods and Properties that are declared in a interface when implemented in a class should be public (Fields cannot be declared in Interface)

C# Interface

using System;
using System.Collections;
using System.Collections.Generic;
 
namespace WorkflowEngine
{
		public class CallWebService : IActivity
		{
			public void Execute()
			{
				Console.WriteLine(@"Call a web service provided by a third-party video encoding 
				service to tell them you have a video ready for encoding");
			}
		}
 
		public class UploadVideo : IActivity
		{
			public void Execute()
			{
				Console.WriteLine("Upload a video to a cloud storage");
			}
		}
 
		public interface IActivity
		{
			void Execute();
		}
 
		public class Workflow
		{
			private readonly IList<IActivity> _workflow;
 
			public Workflow()
			{
				_workflow = new List<IActivity>();
			}
 
			public void Add(IActivity activity)
			{
				_workflow.Add(activity);
			}
 
			public void Remove(IActivity activity)
			{
				_workflow.Remove(activity);
			}
 
			public IEnumerable<IActivity> GetActivities()
			{
				return _workflow;
			}
		}
 
		public class WorkflowEngine
		{
			public void Run(Workflow workflow)
			{
				foreach (IActivity activity in workflow.GetActivities())
				{
					activity.Execute();
				}
			}
		}
 
		class Program
		{
			static void Main(string[] args)
			{
				var workflow1 = new Workflow();
				workflow1.Add(new UploadVideo());
				workflow1.Add(new CallWebService());
 
				var engine = new WorkflowEngine();
				engine.Run(workflow1);
			}
		}
}