Questions

1. What is the difference between ref and out?

   • ref variables are “read-only”, their value cannot change inside a method.
   • ref is a keyword, out is not.
   • There isn’t any: they are both used to pass a reference to a method.
   • out variables may not be initialized going into the method, but have to receive a value inside the method.
   • There isn’t any: they are both used to pass a value to a method.

2. What would be displayed by the following?

   static void Main(){
       int x = 4, y = 3;
       Console.WriteLine($"x is {x}, y is {y}.");
       AddR(ref x, ref y);
       Console.WriteLine($"x is {x}, y is {y}.");
   }
   static void AddR(ref int a, ref int b){
       int tmp = a;
       a = a + b;
       b = tmp - b;
   }

   • “x is {x}, y is {y}.”, “x is {x}, y is {y}.”
   • “x is 4, y is 3.”, “x is 3, y is 4.”
   • “x is 4, y is 3.”, “x is 4, y is 3.”
   • “x is 4, y is 3.”, “x is 7, y is 1.”
   • This code would not compile.

3. The following method would not compile. Why?

   public static void Test(int a, out int b){
       if (a > 0) { b = 12; }
   }

   • A method cannot be static and have void as a return type.
   • Any out parameter must have its value set in the body of the method.
   • The else is missing.
   • The keyword out cannot be used in the header of a method.
   • An out parameter cannot be assigned a value.

Warm-up Exercises

1. Consider the following code:

   using System;
    
   class Program
   {
     static void Main()
     {
       int x = 1;
       int y = 2;
       int z;
       char c = Foo(x, ref y, out z);
       char d = Foo(x, ref y, out z, '%');
     }
    
     static char Foo(
       int x,
       ref int y,
       out int z,
       char symb = '*'
     )
     {
       x++;
       y--;
       z = x + y;
       return symb;
     }
   }

   1. What are the values of x, y and z

      1. Before the Foo method is called?
      2. Inside the Foo method?
      3. After the Foo method executed?

   2. What is the value of c?

   3. What is the value of d?

   Solution

   Before the Foo method is executed: 1, 2, and z is not set.

   Inside the Foo method: 2, 1 and 3.

   After the Foo method: 1, 0, and 2

   c holds '*', d holds %.

Problems

1. Write the AddRev method (header included) such that the following:

       int x0 = 4,
         y0 = 3;
       AddRev(ref x0, ref y0);
       Console.WriteLine($"x0 is {x0}, y0 is {y0}.");

   would display

   x0 is 7, y0 is 1.

   Solution

   using System;
    
   class Program
   {
     static void Main(string[] args)
     {
       // Example for AddRev
       int x0 = 4,
         y0 = 3;
       AddRev(ref x0, ref y0);
       Console.WriteLine($"x0 is {x0}, y0 is {y0}.");
       // Solution for AddRev
       void AddRev(ref int xP0, ref int yP0)
       {
         int temp = xP0;
         xP0 = xP0 + yP0;
         yP0 = temp - yP0;
       }
       // Example for AddLog
       string log;
       int x1 = 4,
         y1 = 3;
       int result = AddLog(x1, y1, out log);
       Console.WriteLine(log + "\n" + result);
       // Solution for AddLog
       int AddLog(int xP1, int yP1, out string logP)
       {
         logP = xP1 + " + " + yP1 + " = " + (xP1 + yP1) + ".";
         return xP1 + yP1;
       }
       // Example for AddReset
       int x2 = 2,
         y2 = 3,
         z2;
       AddReset(ref x2, ref y2, out z2);
       Console.WriteLine($"x2 = {x2}, y2 = {y2}, z2 = {z2}.");
       // Solution for AddReset
       void AddReset(ref int xP2, ref int yP2, out int zP2)
       {
         zP2 = xP2 + yP2;
         xP2 = 0;
         yP2 = 0;
       }
       // Done!
     }
   }

2. Write the AddLog method (header included) such that the following:

       string log;
       int x1 = 4,
         y1 = 3;
       int result = AddLog(x1, y1, out log);
       Console.WriteLine(log + "\n" + result);

   would display

   4 + 3 = 7.
   7

   Solution

       int AddLog(int xP1, int yP1, out string logP)
       {
         logP = xP1 + " + " + yP1 + " = " + (xP1 + yP1) + ".";
         return xP1 + yP1;
       }

3. Write the AddReset method (header included) such that the following:

       int x2 = 2,
         y2 = 3,
         z2;
       AddReset(ref x2, ref y2, out z2);
       Console.WriteLine($"x2 = {x2}, y2 = {y2}, z2 = {z2}.");

   would display

   x2 = 0, y2 = 0, z2 = 5.

   Solution

       void AddReset(ref int xP2, ref int yP2, out int zP2)
       {
         zP2 = xP2 + yP2;
         xP2 = 0;
         yP2 = 0;
       }

4. Consider the “regular” implementation of the Rectangle class:

   using System;
   class Rectangle
   {
       private int length;
       public int Length
       {
           get { return length; }
           set { if (value < 0) { throw new ArgumentNullException(); } else length = value; }
       }
    
       private int width;
       public int Width
       {
           get { return width; }
           set { if (value < 0) { throw new ArgumentNullException(); } else width = value; }
       }
    
       public Rectangle(int wP, int lP)
       {
           Width = wP;
           Length = lP;
       }
    
       public override string ToString()
       {
           return $"Width: {Width}\nLength: {Length}";
       }
   }

   And try to answer the following questions.

   Solution

   A possible solution to those questions is available.

   1. Write a Draw method that takes one optional char parameter and draw a rectangle of the calling object’s width and length using that character if provided, * otherwise. If your method is correctly implemented, then

      Rectangle r0 = new Rectangle(3, 2);
       
      r0.Draw();
      r0.Draw('-');

      should display

      ***
      ***
       
      ---
      ---

      Solution

      A possible solution is: public void Draw(char symb = '*') { string drawing = ""; for (int i = 0; i < Length; i++) { for (int j = 0; j < Width; j++) { drawing += symb; } drawing += "\n"; } Console.WriteLine(drawing); }

   2. Write a Copy method that does not take arguments, and return a copy of the calling object. If your method is correctly implemented, then

      Rectangle original = new Rectangle(5, 10);
      Rectangle copy = original.Copy();
      Console.WriteLine("Original:\n" + original + "\nCopy:\n"+ copy + "\n");
      copy.Length = 12;
      Console.WriteLine("\nOriginal:\n" + original + "\nCopy:\n" + copy + "\n");

      should display

      Original:
      Width: 5
      Length: 10
      Copy:
      Width: 5
      Length: 10
       
       
      Original:
      Width: 5
      Length: 10
      Copy:
      Width: 5
      Length: 12

      If the length of the original object changed after copy.Length = 12; was executed, then your method makes a shallow copy instead of a “deep” copy.

      Solution

      A possible solution is: public Rectangle Copy() { return new Rectangle(Width, Length); }

   3. Write an Equals method that return true if the calling object and the argument are both non-null rectangles with the same length and width, false otherwise. If your method is correctly implemented, then

      Rectangle r1 = new Rectangle(5, 10);
      Rectangle r2 = new Rectangle(5, 10);
      Rectangle r3 = null;
      Rectangle r4 = r1;
      Rectangle r5 = new Rectangle(10, 5);
       
      Console.WriteLine(
          "r1 and r2 identical: " + r1?.Equals(r2)
          + "\nr1 and r3 identical: " + r1?.Equals(r3)
          + "\nr3 and r1 identical: " + r3?.Equals(r1)
          + "\nr3 and r3 identical: " + r3?.Equals(r3)
          + "\nr1 and r4 identical: " + r1?.Equals(r4)
          + "\nr1 and r5 identical: " + r1?.Equals(r5)
          );

      should display

      r1 and r2 identical: True
      r1 and r3 identical: False
      r3 and r1 identical: 
      r3 and r3 identical: 
      r1 and r4 identical: True
      r1 and r5 identical: False

      Solution

      A possible solution is: public bool Equals(Rectangle rP) { if (rP == null) return false; return rP.Length == Length && rP.Width == Width; }