Chapter 31: Value type vs Reference type

Section 31.1: Passing by reference using ref keyword

From the documentation :

In C#, arguments can be passed to parameters either by value or by reference. Passing by reference enables function members, methods, properties, indexers, operators, and constructors to change the value of the parameters and have that change persist in the calling environment. To pass a parameter by reference, use the ref or out keyword.

The di erence between ref and out is that out means that the passed parameter has to be assigned before the function ends.in contrast parameters passed with ref can be changed or left unchanged.

using System;

class Program

{

static void Main(string[] args)

{

int a = 20;

Console.WriteLine("Inside Main - Before Callee: a = {0}", a); Callee(a);

Console.WriteLine("Inside Main - After Callee: a = {0}", a);

Console.WriteLine("Inside Main - Before CalleeRef: a = {0}", a);

CalleeRef(ref a);

Console.WriteLine("Inside Main - After CalleeRef: a = {0}", a);

Console.WriteLine("Inside Main - Before CalleeOut: a = {0}", a); CalleeOut(out a);

Console.WriteLine("Inside Main - After CalleeOut: a = {0}", a);

Console.ReadLine();

}

static void Callee(int a)

{

a = 5;

Console.WriteLine("Inside Callee a : {0}", a);

}

static void CalleeRef(ref int a)

{

a = 6;

Console.WriteLine("Inside CalleeRef a : {0}", a);

}

static void CalleeOut(out int a)

{

a = 7;

Console.WriteLine("Inside CalleeOut a : {0}", a);

}

}

Output :

Inside Main - Before Callee: a = 20

Inside Callee a : 5

Inside Main - After Callee: a = 20

Inside Main - Before CalleeRef: a = 20

Inside CalleeRef a : 6

Inside Main - After CalleeRef: a = 6

Inside Main - Before CalleeOut: a = 6

Inside CalleeOut a : 7

Inside Main - After CalleeOut: a = 7

Section 31.2: Changing values elsewhere

public static void Main(string[] args)

{

var studentList = new List<Student>(); studentList.Add(new Student("Scott", "Nuke")); studentList.Add(new Student("Vincent", "King")); studentList.Add(new Student("Craig", "Bertt"));

// make a separate list to print out later

var printingList = studentList; // this is a new list object, but holding the same student objects inside it

//oops, we've noticed typos in the names, so we fix those studentList[0].LastName = "Duke"; studentList[1].LastName = "Kong"; studentList[2].LastName = "Brett";

//okay, we now print the list

PrintPrintingList(printingList);

}

private static void PrintPrintingList(List<Student> students)

{

foreach (Student student in students)

{

Console.WriteLine(string.Format("{0} {1}", student.FirstName, student.LastName));

}

}

You'll notice that even though the printingList list was made before the corrections to student names after the typos, the PrintPrintingList method still prints out the corrected names:

Scott Duke

Vincent Kong

Craig Brett

This is because both lists hold a list of references to the same students. SO changing the underlying student object propogates to usages by either list.

Here's what the student class would look like.

public class Student

{

public string FirstName { get; set; } public string LastName { get; set; }

public Student(string firstName, string lastName)

{

this.FirstName = firstName; this.LastName = lastName;

}

}

Section 31.3: ref vs out parameters

Code

class Program

{

static void Main(string[] args)

{

int a = 20;

Console.WriteLine("Inside Main - Before Callee: a = {0}", a); Callee(a);

Console.WriteLine("Inside Main - After Callee: a = {0}", a); Console.WriteLine();

Console.WriteLine("Inside Main - Before CalleeRef: a = {0}", a);

CalleeRef(ref a);

Console.WriteLine("Inside Main - After CalleeRef: a = {0}", a);

Console.WriteLine();

Console.WriteLine("Inside Main - Before CalleeOut: a = {0}", a); CalleeOut(out a);

Console.WriteLine("Inside Main - After CalleeOut: a = {0}", a); Console.ReadLine();

}

static void Callee(int a)

{

a += 5;

Console.WriteLine("Inside Callee a : {0}", a);

}

static void CalleeRef(ref int a)

{

a += 10;

Console.WriteLine("Inside CalleeRef a : {0}", a);

}

static void CalleeOut(out int a)

{

// can't use a+=15 since for this method 'a' is not intialized only declared in the method declaration

a = 25; //has to be initialized

Console.WriteLine("Inside CalleeOut a : {0}", a);

}

}

Output

Inside Main - Before Callee: a = 20

Inside Callee a : 25

Inside Main - After Callee: a = 20

Inside Main - Before CalleeRef: a = 20

Inside CalleeRef a : 30

Inside Main - After CalleeRef: a = 30