Hello, the essence of the question is – I do not understand why the result of multiplication is 0:

public void multiply ()
{
  Double [] Numbers = New Double [9999999];
  Double Res = 1.0;
  for (int i = 0;; i ++)
  {
    Console.WriteLine ("Please Enter A Number:");
    String A = Console.ReadLine ();
    Double.tryParse (A, Out Numbers [i]);
    res * = numbers [i];
    if (a == "quit")
    {
      Console.WriteLine ($ "Result is {res}");
      Break;
    }
  }
}

I made the same code to add, only in the RES variable I recorded 0.0, but it worked correctly.
Please explain what error is: D

Answer 1, Authority 100%

Because if a is not a number, then in numbers [i] turns out to be zero.

if (double.tryparse (a, out numbers [i]))
    res * = numbers [i];

Answer 2, Authority 100%

Answer from @igor is unconditionally correct, I will only add that if you still remove all the damnings without changing the behavior of the method, the code will turn out:

public void multiply ()
{
  Double Res = 1.0;
  While (True)
  {
    Console.Write ("Please Enter A Number:");
    String A = Console.ReadLine ();
    If (Double.tryParse (A, Out Double Number))
    {
      Res * = Number;
    }
    ELSE if (a == "quit")
    {
      Console.WriteLine ($ "Result is {res}");
      Break;
    }
  }
}

This code will consume about 9999999 times less memory.