Interfaces and Abstract Class Between Differences

Feature	Interface	Abstract class
Multiple inheritance	A class may inherit several interfaces.	A class may inherit only one abstract class.
Default implementation	An interface cannot provide any code, just the signature.	An abstract class can provide complete, default code and/or just the details that have to be overridden.
Access Modifiers	An interface cannot have access modifiers for the subs, functions, properties etc everything is assumed as public	An abstract class can contain access modifiers for the subs, functions, properties
Core VS Peripheral	Interfaces are used to define the peripheral abilities of a class. In other words both Human and Vehicle can inherit from a IMovable interface.	An abstract class defines the core identity of a class and there it is used for objects of the same type.
Speed	Requires more time to find the actual method in the corresponding classes.	Fast
Adding functionality (Versioning)	If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method.	If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.
Fields and Constants	No fields can be defined in interfaces	An abstract class can have fields and constrants defined

Also Understand using code

using System;

namespace Abstracts
{
    
    
    public abstract class Employee
    {
        //we can have fields and properties 


        //in the Abstract class

        protected String id;
        protected String LastName;
        protected String FirstName;

        //properties


        public abstract String ID
        {
            get;
            set;
        }

        public abstract String FirstName
        {
            get;
            set;
        }
        
        public abstract String LastName
        {
            get;
            set;
        }
        //completed methods


        public String Update1()
        {
            return "Employee " + id + " " + 
                      lname + " " + fname + 
                      " updated";
        }
        //completed methods


        public String Add()
        {
            return "Employee " + id + " " + 
                      lname + " " + fname + 
                      " added";
        }
        //completed methods


        public String Delete1()
        {
            return "Employee " + id + " " + 
                      lname + " " + fname + 
                      " deleted";
        }
        //completed methods


        public String Search1()
        {
            return "Employee " + id + " " + 
                      lname + " " + fname + 
                      " found";
        }

        //abstract method that is different 


        //from Fulltime and Contractor

        //therefore i keep it uncompleted and 

        //let each implementation 

        //complete it the way they calculate the wage.


        public abstract String CalculateWage();
        
    }
}