Generics Base class constraints Doubts, C#

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//Title of this code
//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
	class A 
	{
        public object GetOb()
        {
            
            return this;
        }
        
        public int Add(int m,int n)
        {
            
            return(m+n);
        }
		public void Hello(object ob) 
		{

Console.WriteLine("Hello"+" "+this.GetType().Name.ToString());
            Console.WriteLine("Hello"+" "+ob.GetType().Name.ToString());

}
        public void Hello2(object ob) 
		{

Console.WriteLine("Hii"+" "+this.GetType().Name.ToString());
            Console.WriteLine("Hii"+" "+ob.GetType().Name.ToString());

}

// Class B inherits A.

class B : A 
    {
       public void Bmethod()
       {
           
       }
        
    }

// Class C does not inherit A.

class C { }

// Because of the base class constraint, all type arguments

// passed to Test must have A as a base class.

class Test<T> where T : A
	{

T obj;

public Test(T o) 
		{

obj = o;

}
		public void SayHello() 
        {

// OK to call Hello() because it’s declared

// by the base class A.
            
            obj.Hello2(obj);
			obj.Hello(this);
           // Hello();

}

}
	public class Program
	{
		public static void Main(string[] args)
		{
			A a = new A();

B b = new B();

C c = new C();

// The following is valid because A is the specified base class.

Test<A> t1 = new Test<A>(a);

t1.SayHello();
           // t1.Addnum(5,10);

// The following is valid because B inherits A.

Test<B> t2 = new Test<B>(b);
			t2.SayHello();

// The following is invalid because C does not inherit A.

// Test<C> t3 = new Test<C>(c); // Error!

// t3.SayHello(); // Error!
		}
	}
}

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