Why should use gc.collect () ?

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C#3.5

I had used destructor in program like this

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace destructors
{
    class Program
    {
        int i = 5;
        public Program()
        {
            Console.WriteLine("Object created");
        }
        ~Program()
        {
            Console.WriteLine("Object destroyed");
        }
        static void Main(string[] args)
        {
            Program p = new Program();
            Console.WriteLine(p.i);
            p = null;
            GC.Collect();
            Console.ReadLine();
        }
    }
}

in above program i had used GC.collect for destroy the object but whenever assign

p=null then p object memory deallocate why should use specifically GC.collect().what

is purpose GC.collect().is it mandatory if p=null use in program.

Thank u.

help me.

What I have tried:

in above program i had used GC.collect for destroy the object but whenever assign

p=null then p object memory deallocate why should use specifically GC.collect().what

is purpose GC.collect().is it mandatory if p=null use in program.