Difference between Abstract Class and Interface with example in c#

1. Multiple inheritance

 A class may inherit several interfaces.
 class abc : ICrud,IDisposal

 A class may inherit only one abstract class.
 class abc : parentClass

2. Default implementation

Interface: An interface cannot provide any code, just the signature.
Example:

interface IPerson
    {
        void Add(string name, string email);
    }
   
Abstract class: An abstract class can provide complete, default code and/or just the details that have to be overridden.
Example:

abstract class APerson
    {
        //
        string Name;
        string Email;

       // like interface
       public abstract void Add(string name, string email);
      
       public void Update()
       {

       }

    }

3. Access Modifiers

Interface: An interface cannot have access modifiers for the subs, functions, properties etc everything is assumed as public

    interface IPerson
    {
        void Add(string name, string email);
    }

    class iPerson : IPerson
    {
        string Name;
        string Email;

        public void Add(string name, string email)
        {
            Email = email;
            Name = name;
        }
    }

Abstract Class: An abstract class can contain access modifiers for the subs, functions, properties

abstract class APerson
    {
        //
        protected string Name;
        protected string Email;

        // like interface
        protected abstract void Add(string name, string email);

        protected void Update()
        {

        }

    }

    class person : APerson
    {
        protected override void Add(string name, string email)
        {
            Email = email;
            Name = name;
        }
    }

4. Where to Use

Interface: If various implementations only share method signatures then it is better to use Interfaces.

Abstract Class: If various implementations are of the same kind and use common behaviour or status then abstract class is better to use.

5. Speed

Interface: Requires more time to find the actual method in the corresponding classes.

Abstract Class: Faster then Interface