object class

public class Object
{
  public Object();
  
  public extern Type GetType();
  
  public virtual bool Equals (object obj);
  public static bool Equals  (object objA, object objB);
  public static bool ReferenceEquals (object objA, object objB);
  
  public virtual int GetHashCode();
  
  public virtual string ToString();
  
  protected override void Finalize();
  protected extern object MemberwiseClone();
}

using System;

class Rectangle{
   
}

class Program
{
    static void Main(string[] args)
    {

        object o = new Rectangle();
    }
}