Abstract Classes : Net frame work: compare with PHP
Abstract class is an incomplete class, as expected to be changed down the road and must be inherited by a class and a method override method will do the job 
    //csc interface_abstract.cs
using System;

namespace soccer
{
	interface team
	{
		string method_1(string str);
		string method_2(string str);
	}
//abstract class A : team
//note the error when a method is abstract and class in not an abstract
 class A : team
	{
		public abstract string method_1(string str);
		public abstract string method_2(string str);
	}
	class B : A
	{
         public override string method_1(string str){ Console.WriteLine(" Hello : " + str); return str;}
	 public override string method_2(string str){ Console.WriteLine(" Hello : " + str); return str ;}
	} 
	class test: B
	{
		public static void Main(string[] args)
		{
			Console.WriteLine("Player for team 1 : ");
			string s1 = Console.ReadLine();
			Console.WriteLine("Player for team 2 : ");
			string s2 = Console.ReadLine();
			B b = new B();
			b.method_1(s1);
			b.method_2(s2);
			
		}
	}
}

 
You must override the method

 
 
 
  
    //csc interface_abstract_1.cs
using System;

namespace soccer
{
	interface team
	{
		string method_1(string str);
		string method_2(string str);
	}
//abstract class A : team
//note the error when a method is abstract and class in not an abstract
 abstract class A : team
	{
		public abstract string method_1(string str);
		public abstract string method_2(string str);
	}
	class B : A
	{
	//public override string method_1(string str){ Console.WriteLine(" Hello : " + str); return str;}
	public string method_1(string str){ Console.WriteLine(" Hello : " + str); return str;}
	public override string method_2(string str){ Console.WriteLine(" Hello : " + str); return str ;}
	} 
	class test: B
	{
		public static void Main(string[] args)
		{
			Console.WriteLine("Player for team 1 : ");
			string s1 = Console.ReadLine();
			Console.WriteLine("Player for team 2 : ");
			string s2 = Console.ReadLine();
			B b = new B();
			b.method_1(s1);
			b.method_2(s2);
			
		}
	}
}

 
now let us correct all the code and compile

    //csc interface_abstract.cs
using System;

namespace soccer
{
	interface team
	{
		string method_1(string str);
		string method_2(string str);
	}
abstract class A : team
	{
		public abstract string method_1(string str);
		public abstract string method_2(string str);
	}
	class B : A
	{
	public override string method_1(string str){ Console.WriteLine(" Hello : " + str); return str;}
	public override string method_2(string str){ Console.WriteLine(" Hello : " + str); return str ;}
	} 
	class test: B
	{
		public static void Main(string[] args)
		{
			Console.WriteLine("Player for team 1 : ");
			string s1 = Console.ReadLine();
			Console.WriteLine("Player for team 2 : ");
			string s2 = Console.ReadLine();
			B b = new B();
			b.method_1(s1);
			b.method_2(s2);
			
		}
	}
}
Now let us retrun the outp put from the method to the caller

 

    //csc interface_abstract_return.cs
using System;

namespace soccer
{
	interface team
	{
		string method_1(string str);
		string method_2(string str);
	}
abstract class A : team
	{
		public abstract string method_1(string str);
		public abstract string method_2(string str);
	}
	class B : A
	{
			public override string method_1(string str){  return str;}
			public override string method_2(string str){ return str ;}
	} 
	class test: B
	{
		public static void Main(string[] args)
		{
			Console.WriteLine("Player for team 1 : ");
			string s1 = Console.ReadLine();
			Console.WriteLine("Player for team 2 : ");
			string s2 = Console.ReadLine();
			B b = new B();
			Console.WriteLine(" Hello : " + b.method_1(s1));
			Console.WriteLine(" Hello : " + b.method_2(s2));
			
		}
	}
}