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Chapter Three

Using Methods

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Objectives

• Learn how to write methods with no arguments and no return value

• Learn about implementation hiding and how to use multiple files

• Learn how to write methods that require a single argument

• Learn how to write methods that require multiple arguments

• Learn how to write methods that return a value

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Objective

• Learn how to use reference and output parameters with methods

• Learn how to overload methods• Learn how to avoid ambiguous methods

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Writing Methods with No Arguments and No Return Value

• A method is a series of statements that carry out a task• A method can be invoked or called by other methods • Method names are always followed by a set of

parentheses• Programmers can create custom methods• Example:

Console.WriteLine(“HELLO WORLD”);

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Writing Methods with No Arguments and No Return Value

• In C#, a method must include:– A method declaration– An opening curly brace– A method body– A closing brace

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Writing Methods with No Arguments and No Return Value

• The optional access modifiers for a method include: public, protected internal, protected, internal, and private

• If an access modifier for a method is not declared, the method becomes private by default

• The static keyword indicates that a method can be called without referring to an object

• Every method has a return type

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Writing Methods with No Arguments and No Return Value

• This is an example of a method calling another method

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Hiding Implementation by Using Multiple Files

• An important principle of object-oriented programming is the notion of implementation hiding

• The users of methods should only be aware of the interface

• The following implementation, with two newline escape sequences, could have been used instead of the previous code—neither implementation effects the user

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Hiding Implementation by Using Multiple Files

• Two different implementations of the WelcomeMessage() method

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Hiding Implementation by Using Multiple Files

• A multifile assembly is a program that is composed of many different files and classes

• A netmodule file is a file that contains modules to be used as part of another program

• Compiling multifile assemblies and netmodules requires a slightly different command

csc /t:module LogoNamespace.cscsc DemoLogo2.cs /addmodule:LogoNameSpace.netmodule

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Writing Methods That Require a Single Argument

• Arguments or parameters are used to communicate and send additional information to methods

• The following items are included in the method declaration parentheses when declaring a method that can receive an argument:– The type of the argument– A local name for the argument

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Writing Methods That Require a Single Argument

• The identifier saleAmount is simply the name the value “goes by” while it is used in the method, regardless of the name the value “goes by” in the calling program

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Writing Methods That Require a Single Argument

• A formal parameter is a parameter in the method declaration

• An actual parameter refers to an argument within a method call

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Writing Methods That Require a Single Argument

• Complete program using the ComputeSevenPercentSalesTax() method

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Writing Methods That Require a Single Argument

• Output of UseSevenPercentSalesTax program

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Writing Methods That Require Multiple Arguments

• You can pass multiple arguments to a method by listing the arguments within the call to the method and separating them with commas

• You can write a method to take any number of arguments in any order

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Writing Methods That Return Values

• The return type for a method can be any type used in the C# programming language

• A method can also return nothing, in which case the return type is void

• A method’s return type is also known as a method’s type

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Writing Methods That Return Values

• The return type in the above example is double• The return statement causes the value stored in gross

to be sent back to any method that calls the CalcPay() method

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Writing Methods That Return Values

• Program using the CalcPay() method

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Using ref and out Parameters Within Methods

• In C#, you can write methods with four kinds of formal parameters listed within the parentheses in the method header– Value parameters– Reference parameters– Output parameters– Parameter arrays

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Using ref and out Parameters Within Methods

• When you use a value parameter in a method header, you indicate the parameter’s type and name, and the method receives a copy of the value passed to it

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Using ref and out Parameters Within Methods

• Both the reference and output parameters have memory addresses that are passed to a method, allowing it to alter the original variables

• When you use a reference parameter to a method, the parameter must be assigned a value before you use it in the method call

• When you use an output parameter, it does not contain an original value

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Using ref and out Parameters Within Methods

• Both reference and output parameters act as aliases, or other names, for the same memory location

• The keyword ref is used to indicate a reference parameter

• The keyword out is used to indicate an output parameter

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Using ref and out Parameters Within Methods

• Program calling method with a reference parameter

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Using ref and out Parameters Within Methods

• In the preceding code:– The modifier ref precedes the variable in both the

method call and the method header– The passed and received variables occupy the same

memory location– The passed variable was assigned a value

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Using ref and out Parameters Within Methods

• Unlike the reference parameter, the output parameter does not need a value assigned to it (before it is used in a method call)

• The output parameter is convenient when the passed variable doesn’t have a value yet

• The output parameter gets its value from the method, and these values persist back to the calling program

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Overloading Methods

• Overloading involves using one term to indicate diverse meanings

• When you overload a C# method, you write multiple methods with a shared name

• The compiler understands the meaning based on the arguments you use with the method

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Avoiding Ambiguous Methods

• By overloading methods, you run the risk of creating an ambiguous situation

• An ambiguous situation is one in which the compiler cannot determine which method to use

• The compiler usually distinguishes between overloaded methods by the argument lists, but this is NOT always possible

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Avoiding Ambiguous Methods

• If only one method exists, there is no chance of an ambiguous situation• This code would work fine even if your arguments were both of type int

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Avoiding Ambiguous Methods

• What happens when another version of the same method is added?• An ambiguous situation does not exist because the compiler can

determine the appropriate method

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Avoiding Ambiguous Methods

• A more complicated and potentially ambiguous situation arises when the compiler cannot determine which of several versions of a method to use

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Avoiding Ambiguous Methods

• An ambiguous situation arises because there is no exact match for the method call

• An overloaded method is not ambiguous on its own—it become ambiguous only if you create an ambiguous situation

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Chapter Summary

• A method is a series of reusable statements that carry out a task

• Invoking programs must know the interface to methods but need not understand the hidden implementation

• Methods receive data in the form of parameters or arguments

• You can pass multiple arguments to a method by listing the arguments within the call to the method and separating them with commas

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Chapter Summary

• The return type for a method can be any type in the C# programming language

• In C#, you can write methods with four kinds of formal parameters listed within the parentheses in the method header

• Overloading a method involves writing multiple methods with the same name, but with different arguments

• The compiler determines which of several versions of a method to call based on argument lists