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

    // Passing by value.
    // The value of arg in Main is not changed.
    arg = 4;
    squareVal(arg);
    Console.WriteLine(arg);
    // Output: 4

    // Passing by reference.
    // The value of arg in Main is changed.
    arg = 4;
    squareRef(ref arg);
    Console.WriteLine(arg);
    // Output: 16 
  }

  static void squareVal(int valParameter)
  {
    valParameter *= valParameter;
  }

  // Passing by reference
  static void squareRef(ref int refParameter)
  {
    refParameter *= refParameter;
  }
}

class TheClass
{
  public string willIChange;
}

struct TheStruct
{
  public string willIChange;
}

class TestClassAndStruct
{
  static void ClassTaker(TheClass c)
  {
    c.willIChange = "Changed";
  }

  static void StructTaker(TheStruct s)
  {
    s.willIChange = "Changed";
  }

  static void Main()
  {
    TheClass testClass = new TheClass();
    TheStruct testStruct = new TheStruct();

    testClass.willIChange = "Not Changed";
    testStruct.willIChange = "Not Changed";

    ClassTaker(testClass);
    StructTaker(testStruct);

    Console.WriteLine("Class field = {0}", testClass.willIChange);
    Console.WriteLine("Struct field = {0}", testStruct.willIChange);

    // Keep the console window open in debug mode.
    Console.WriteLine("Press any key to exit.");
    Console.ReadKey();
  }
}

  Class field = Changed
  Struct field = Not Changed