Initializing Base Class: declaring a variable twice?

Question

I'm currently reading trough a C# tutorial. Now I came across this:

using System;

namespace RectangleApplication {
   class Rectangle {

      //member variables
      protected double length;
      protected double width;

      public Rectangle(double l, double w) {
         length = l;
         width = w;
      }
      public double GetArea() {
         return length * width;
      }
      public void Display() {
         Console.WriteLine("Length: {0}", length);
         Console.WriteLine("Width: {0}", width);
         Console.WriteLine("Area: {0}", GetArea());
      }
   }//end class Rectangle  
   class Tabletop : Rectangle {
      private double cost;
      public Tabletop(double l, double w) : base(l, w) { }

      public double GetCost() {
         double cost;
         cost = GetArea() * 70;
         return cost;
      }
      public void Display() {
         base.Display();
         Console.WriteLine("Cost: {0}", GetCost());
      }
   }
   class ExecuteRectangle {
      static void Main(string[] args) {
         Tabletop t = new Tabletop(4.5, 7.5);
         t.Display();
         Console.ReadLine();
      }
   }
}

In the class Tabletop there is cost declared twice. Once as private double cost; and 4 lines later as double cost;

Why is that so?

When removing double cost; the Code still works. When double cost is in the code I can hover over private double cost; and read the message: The field Tabletop.cost is never used". I pretty much can remove either of the cost and the code works fine.

1. Did they forget to remove one of the declareation or is there a reason behind?
2. Also, why don't I get an error message like "cost is already defined"?

Here is the Tutorial link

Answer 1

private double cost; is unused and can be removed.

You don't get an error because as John said in the comments, it's in different scopes; one is defined as a field of the class while the other is a local variable. When cost is used, the local variable is accessed. To access the field, this.cost can be used.

class A
{
  private int a = 1;

  void A()
  {
    int a = 2;

    Console.WriteLine(a); // 2
    Console.WriteLine(this.a); // 1
  }
}

Note you cannot have multiple local variables with the same name, even in different scopes:

void A()
{
  int a = 1;

  if(someCondition)
  {
    int b = 2; // Compiler error: A local variable named 'a' cannot be declared in this scope because it would give a different meaning to 'a', which is already used in a 'parent or current' scope to denote something else
  }
}