C# 2008 Programmer's Reference

In the following example, x and y are examples of parameters:

public int AddNumbers(int x, int y) {}

When you call the method, you pass in values/variables. In the following example, num1 and num2 are examples of arguments:

Console.WriteLine(AddNumbers(num1, num2));

Consider the method named AddNumbers() with two parameters, x and y:

public int AddNumbers(int x, int y) {
 x++;
 y++;
 return x + y;
}

When you call this method, you also need to pass two integer arguments (num1 and num2), as the following example shows:

int num1 = 4, num2 = 5;
//---prints out 11---
Console.WriteLine(AddNumbers(num1, num2));
Console.WriteLine(num1); //---4 ---
Console.WriteLine(num2); //---5---

In C#, all arguments are passed by value by default. In other words, the called method gets a copy of the value of the arguments passed into it. In the preceding example, for instance, even though the value of x and y are both incremented within the method, this does not affect the values of num1 and num2.

If you want to pass in arguments to methods by reference, you need to prefix the parameters with the ref keyword. Values of variables passed in by reference will be modified if there are changes made to them in the method. Consider the following rewrite of the AddNumbers() function:

public int AddNumbers(ref int x, ref int y) {
 x++;
 y++;
 return x + y;
}

Because C# functions can only return single values, passing arguments by reference is useful when you need a method to return multiple values.

In this case, the values of variables passed into this function will be modified, as the following example illustrates:

int num1 = 4, num2 = 5; //---prints out 11---
Console.WriteLine(AddNumbers(ref num1, ref num2));
Console.WriteLine(num1); //---5---
Console.WriteLine(num2); //---6---

After calling the AddNumbers() function, num1 becomes 5 and num2 becomes 6. Observe that you need to prefix the arguments with the ref keyword when calling the function. In addition, you cannot pass literal values as arguments into a method that requires parameters to be passed in by reference:

//---invalid arguments---
Console.WriteLine(AddNumbers(4, 5));

Also note that the ref keyword requires that all the variables be initialized first. Here's an example:

public void GetDate(ref int day, ref int month, ref int year) {
 day = DateTime.Now.Day;
 month = DateTime.Now.Month;
 year = DateTime.Now.Year;
}

The GetDate() method takes in three reference parameters and uses them to return the day, month, and year.

If you pass in the day, month and year reference variables without initializing them, an error will occur:

//---Error: day, month, and year not initialized---
int day, month, year;
GetDate(ref day, ref month, ref year);

If your intention is to use the variables solely to obtain some return values from the method, you can use the out keyword, which is identical to the ref keyword except that it does not require the variables passed in to be initialized first:

public void GetDate(out int day, out int month, out int year) {
 day = DateTime.Now.Day;
 month = DateTime.Now.Month;
 year = DateTime.Now.Year;
}

Also, the out parameter in a function must be assigned a value before the function returns. If it isn't, a compiler error results.

Like the ref keyword, you need to prefix the arguments with the out keyword when calling the function:

int day, month, year;
GetDate(out day, out month, out year);

The this Keyword

The this keyword refers to the current instance of an object (in a nonstatic class; discussed later in the section Static Classes). In the earlier section on methods, you saw the use of this:

Console.WriteLine('{0} {1}', this.FirstName, this.LastName);

While the FirstName and LastName variable could be referenced without using the this keyword, prefixing them with it makes your code more readable, indicating that you are referring to an instance member.

However, if instance members have the same names as your parameters, using this allows you to resolve the ambiguity:

public void SetName(string FirstName, string LastName) {
 this.FirstName = FirstName;
 this.LastName = LastName;
}

Another use of the this keyword is to pass the current object as a parameter to another method. For example:

public class AddressBook {
 public void AddContact(Contact c) {
  Console.WriteLine(c.ID);
  Console.WriteLine(c.FirstName);
  Console.WriteLine(c.LastName);
  Console.WriteLine(c.Email);
  //---other implementations here---