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In this part of the tutorial, you will learn about C# methods.

In object oriented programming, we work with objects. Objects are the basic building blocks of a program. Objects consists of data and methods. Methods change the state of the objects created. They are the dynamic part of the objects; data is the static part.

A method is a code block containing a series of statements. Methods must be declared within a class or a structure. It is a good programming practice that methods do only one specific task. Methods bring modularity to programs. Proper use of methods bring the following advantages:

• Reducing duplication of code
• Decomposing complex problems into simpler pieces
• Improving clarity of the code
• Reuse of code
• Information hiding

Basic characteristics of methods are:

• Access level
• Return value type
• Method name
• Method parameters
• Parentheses
• Block of statements

Access level of methods is controlled with access modifiers. They set the visibility of methods. They determine who can call the method. Methods may return a value to the caller. In case our method returns a value, we provide its data type. If not, we use the void keyword to indicate that our method does not return values. Method parameters are surrounded by parentheses and separated by commas. Empty parentheses indicate that the method requires no parameters. The method block is surrounded with { } characters. The block contains one or more statements that are executed, when the method is invoked. It is legal to have an empty method block.

A method signature is a unique identification of a method for the C# compiler. The signature consists of a method name and the type and kind (value, reference, or output) of each of its formal parameters. Method signature does not include the return type.

Any legal character can be used in the name of a method. By convention, method names begin with an uppercase letter. The method names are verbs or verbs followed by adjectives or nouns. Each subsequent word starts with an uppercase character. The following are typical names of methods in C#:

• Execute
• FindId
• SetName
• GetName
• CheckIfValid
• TestValidity

Simple example

We start with a simple example.

using System;

public class Base
{
  public void ShowInfo()
  {
    Console.WriteLine("This is Base class");
  }
}

public class SimpleMethod
{
  static void Main()
  {
    Base bs = new Base();
    bs.ShowInfo();
  }
}

We have a ShowInfo() method that prints the name of its class.

public class Base
{
  public void ShowInfo()
  {
    Console.WriteLine("This is Base class");
  }
}

Each method must be defined inside a class or a structure. It must have a name. In our case the name is ShowInfo. The keywords that precede the name of the method are access specifier and the return type. Parentheses follow the name of the method. They may contain parameters of the method. Our method does not take any parameters.

static void Main()
{
   ...
}

This is the Main() method. It is the entry point to each console or GUI application. It must be declared static . We will see later why. The return type for a Main() method may be void or int . The access specifier for the Main() method is omitted. In such a case a default one is used, which is private . It is not recommended to use public access specifier for the Main() method. It is not supposed to be called by any other methods in the assemblies. It is only the CLR that should be able to call it when the application starts.

Base bs = new Base();
bs.ShowInfo();

We create an instance of the Base class. We call the ShowInfo() method upon the object. We say that the method is an instance method, because it needs an instance to be called. The method is called by specifying the object instance, followed by the member access operator — the dot, followed by the method name.

Method parameters

A parameter is a value passed to the method. Methods can take one or more parameters. If methods work with data, we must pass the data to the methods. We do it by specifying them inside the parentheses. In the method definition, we must provide a name and type for each parameter.

using System;

public class Addition
{
  public int AddTwoValues(int x, int y)
  {
    return x + y;
  }

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

public class MethodParameters
{
  static void Main()
  {
    Addition a = new Addition();
    int x = a.AddTwoValues(12, 13);
    int y = a.AddThreeValues(12, 13, 14);

    Console.WriteLine(x);
    Console.WriteLine(y);
  }
}

In the above example, we have two methods. One of them takes two parameters, the other one takes three parameters.

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

The AddTwoValues() method takes two parameters. These parameters have int type. The method also returns an integer to the caller. We use the return keyword to return a value from the method.

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

The AddThreeValues() is similar to the previous method. It takes three parameters.

int x = a.AddTwoValues(12, 13);

We call the AddTwoValues() method of the addition object. It takes two values. These values are passed to the method. The method returns a value which is assigned to the x variable.

Variable number of arguments

A method can take variable number of arguments. For this we use the params keyword. No additional parameters are permitted after the params keyword. Only one params keyword is permitted in a method declaration.

using System;

public class SumOfValues
{
  static void Main()
  {
    Sum(1, 2, 3);
    Sum(1, 2, 3, 4, 5);
  }

  static void Sum(params int[] list)
  {
    Console.WriteLine("There are {0} items", list.Length);
    
    int sum = 0;
    foreach (int i in list) 
    {
      sum = sum + i;
    }

    Console.WriteLine("Their sum is {0}", sum);
  }
}

We create a Sum() method which can take variable number of arguments. The method will calculate the sum of values passed to the method.

Sum(1, 2, 3);
Sum(1, 2, 3, 4, 5);

We call the Sum() method twice. In one case, it takes 3 arguments, in the second case 5. We call the same method.

static void Sum(params int[] list)
{
...
}

The Sum() method can take variable number of integer values. All values are added to the list array.

Console.WriteLine("There are {0} items", list.Length);

We print the length of the list array.

int sum = 0;
foreach (int i in list) 
{
  sum = sum + i;
}

We compute the sum of the values in the list.

$ ./variableparams.exe 
There are 3 items
Their sum is 6
There are 5 items
Their sum is 15

This is the output of the example.

Anonymous methods

Anonymous methods are inline methods that do not have a name. Anonymous methods reduce the coding overhead by eliminating the need to create a separate method. Without anonymous methods developers often had to create a class just to call one method.

using System;
using System.Timers;

public class AnonymousMethod
{
  static void Main()
  {
    Timer timer = new Timer();

    timer.Elapsed += new ElapsedEventHandler(
      delegate(object source, ElapsedEventArgs e)
      {
        Console.WriteLine("Event triggered at {0}", e.SignalTime);
      }
    );

    timer.Interval = 2000;
    timer.Enabled = true;

    Console.ReadLine();
  }
}

We create a timer object and every 2 seconds we call an anonymous method.

Timer timer = new Timer();

A Timer class generates recurring events in an application.

timer.Elapsed += new ElapsedEventHandler(
  delegate(object source, ElapsedEventArgs e)
  {
    Console.WriteLine("Event triggered at {0}", e.SignalTime);
  }
);

Here we plug the anonymous method to the Elapsed event. The delegate keyword is used to denote an anonymous method.

Console.ReadLine();

At this moment, the program waits for an input from the user. The program ends when we hit the Return key. Otherwise, the program would finish immediately before the events could be generated.

Passing arguments by value, by reference

C# supports two ways of passing arguments to methods: by value and by reference. The default passing of arguments is by value. When we pass arguments by value, the method works only with the copies of the values. This may lead to performance overheads when we work with large amounts of data.

We use the ref keyword to pass a value by reference. When we pass values by reference, the method receives a reference to the actual values. The original values are affected when modified. This way of passing values is more time and space efficient. On the other hand, it is more error prone.

Which way of passing arguments should we use? It depends on the situation. Say we have a set of data, for example salaries of employees. If we want to compute some statistics of the data, we do not need to modify them. We can pass by values. If we work with large amounts of data and the speed of computation is critical, we pass by reference. If we want to modify the data, e.g. do some reductions or raises to the salaries, we might pass by reference.

The following example shows how we pass arguments by values.

using System;

public class PassingByValues
{
  static int a = 4;
  static int b = 7;

  static void Main()
  {
    Console.WriteLine("Outside Swap method");
    Console.WriteLine("a is {0}", a);
    Console.WriteLine("b is {0}", b);

    Swap(a, b);

    Console.WriteLine("Outside Swap method");
    Console.WriteLine("a is {0}", a);
    Console.WriteLine("b is {0}", b);
  }

  static void Swap(int a, int b)
  {
    int temp;
    temp = a;
    a = b;
    b = temp;
    Console.WriteLine("Inside Swap method");
    Console.WriteLine("a is {0}", a);
    Console.WriteLine("b is {0}", b);
  }
}

The Swap() method swaps the numbers between the a and b variables. The original variables are not affected.

static int a = 4;
static int b = 7;

At the beginning, these two variables are initiated. The variables must be declared static , because they are used from static methods.

Swap(a, b);

We call the Swap() method. The method takes a and b variables as arguments.

temp = a;
a = b;
b = temp;

Inside the Swap() method, we change the values. Note that the a and b variables are defined locally. They are valid only inside the Swap() method.

$ ./swapbyval.exe 
Outside Swap method
a is 4
b is 7
Inside Swap method
a is 7
b is 4
Outside Swap method
a is 4
b is 7

The output shows that the original variables were not affected.

The next code example passes values to the method by reference. The original variables are changed inside the Swap() method. Both the method definition and the method call must use the ref keyword.

using System;

public class PassingByReference
{
  static int a = 4;
  static int b = 7;

  static void Main()
  {
    Console.WriteLine("Outside Swap method");
    Console.WriteLine("a is {0}", a);
    Console.WriteLine("b is {0}", b);

    Swap(ref a, ref b);

    Console.WriteLine("Outside Swap method");
    Console.WriteLine("a is {0}", a);
    Console.WriteLine("b is {0}", b);
  }

  static void Swap(ref int a, ref int b)
  {
    int temp;
    temp = a;
    a = b;
    b = temp;
    Console.WriteLine("Inside Swap method");
    Console.WriteLine("a is {0}", a);
    Console.WriteLine("b is {0}", b);
  }
}

In this example, calling the Swap() method will change the original values.

Swap(ref a, ref b);

We call the method with two arguments. They are preceded by the ref keyword to indicate that we are passing arguments by reference.

static void Swap(ref int a, ref int b)
{
...
}

Also in the method declaration, we use the ref keyword to inform the compiler that we accept references to the parameters and not the values.

$ ./swapbyref.exe 
Outside Swap method
a is 4
b is 7
Inside Swap method
a is 7
b is 4
Outside Swap method
a is 7
b is 4

Here we see that the Swap() method really changed the values of the variables.

The out keyword is similar to the ref keyword. The difference is that when using the ref keyword, the variable must be initialized before it is being passed. With the out keyword, it may not be initialized. Both the method definition and the method call must use the out keyword.

using System;

public class OutKeyword
{
  static void Main()
  {
    int val;
    SetValue(out val);

    Console.WriteLine(val);
  }

  static void SetValue(out int i)
  {
    i = 12;
  }
}

An example shows the usage of the out keyword.

int val;
SetValue(out val);

The val variable is declared, but not initialized. We pass the variable to the SetValue() method.

static void SetValue(out int i)
{
  i = 12;
}

Inside the SetValue() method it is assigned a value which is later printed to the console.

Method overloading

Method overloading allows the creation of several methods with the same name which differ from each other in the type of the input.

What is method overloading good for? The Qt4 library gives a nice example for the usage. The QPainter class has three methods to draw a rectangle. Their name is drawRect() and their parameters differ. One takes a reference to a floating point rectangle object, another takes a reference to an integer rectangle object, and the last one takes four parameters: x, y, width, height. If the C++ language, which is the language in which Qt is developed, didn't have method overloading, the creators of the library would have to name the methods like drawRectRectF() , drawRectRect() , drawRectXYWH() . The solution with method overloading is more elegant.

using System;

public class Sum
{
  public int GetSum()
  {
    return 0;
  }

  public int GetSum(int x)
  {
    return x;
  }

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

public class Overloading
{
  static void Main()
  {
    Sum s = new Sum();

    Console.WriteLine(s.GetSum());
    Console.WriteLine(s.GetSum(20));
    Console.WriteLine(s.GetSum(20, 30));
  }
}

We have three methods called GetSum() . They differ in input parameters.

public int GetSum(int x)
{
  return x;
}

This one takes one parameter.

Console.WriteLine(s.GetSum());
Console.WriteLine(s.GetSum(20));
Console.WriteLine(s.GetSum(20, 30));

We call all three methods.

$ ./overloading.exe 
0
20
50

And this is what we get when we run the example.

Recursion

Recursion, in mathematics and computer science, is a way of defining methods in which the method being defined is applied within its own definition. In other words, a recursive method calls itself to do its job. Recursion is a widely used approach to solve many programming tasks.

A typical example is the calculation of a factorial.

using System;

public class Recursion
{
  static void Main()
  {    
    Console.WriteLine(Factorial(6));
    Console.WriteLine(Factorial(10));
  }

  static int Factorial(int n)
  {
    if (n == 0)
    {
      return 1;
    } else
    {
      return n * Factorial(n-1);
    }
  }
}

In this code example, we calculate the factorial of two numbers.

return n * Factorial(n-1);

Inside the body of the factorial method, we call the factorial method with a modified argument. The function calls itself.

$ ./recursion.exe 
720
3628800

These are the results.

Method scope

A variable declared inside a method has a method scope. The scope of a name is the region of program text within which it is possible to refer to the entity declared by the name without the qualification of the name. A variable which is declared inside a method has a method scope. It is also called a local scope. The variable is valid only in this particular method.

using System;

public class Test
{
  int x = 1;

  public void exec1() 
  {    
    Console.WriteLine(this.x);
    Console.WriteLine(x);
  }
  
  public void exec2() 
  {    
    int z = 5;
    
    Console.WriteLine(x);
    Console.WriteLine(z);
  }   
}

public class MethodScope
{
  static void Main()
  {
    Test ts = new Test();
    ts.exec1();
    ts.exec2();
  }
}

In the preceding example, we have the x variable defined outside the exec1() and exec2() methods. The variable has a class scope. It is valid everywhere inside the definition of the Test class, e.g. between its curly brackets.

public void exec1() 
{    
  Console.WriteLine(this.x);
  Console.WriteLine(x);
}

The x variable, also called the x field, is an instance variable. And so it is accessible through the this keyword. It is also valid inside the exec1() method and can be referred by its bare name. Both statements refer to the same variable.

public void exec2() 
{    
  int z = 5;
  
  Console.WriteLine(x);
  Console.WriteLine(z);
}   

The x variable can be accessed also in the exec2() method. The z variable is defined in the exec2() method. It has a method scope. It is valid only in this method.

$ ./methodscope.exe 
1
1
1
5

This is the output of the methodscope.exe program.

A variable defined inside a method has a local/method scope. If a local variable has the same name as an instance variable, it shadows the instance variable. The class variable is still accessible inside the method by using the this keyword.

using System;

public class Test
{
  int x = 1;

  public void exec()
  {
    int x = 3;

    Console.WriteLine(this.x);
    Console.WriteLine(x);
  }
}

public class Shadowing
{
  static void Main()
  {
    Test ts = new Test();
    ts.exec();
  }
}

In the preceding example, we declare the x variable outside the exec() method and inside the exec() method. Both variables have the same name, but they are not in conflict because they live in different scopes.

Console.WriteLine(this.x);
Console.WriteLine(x);

The variables are accessed differently. The x variable defined inside the method, also called the local variable, is simply accessed by its name. The instance variable can be referred by using the this keyword.

$ ./shadowing.exe 
1
3

This is the output of the shadowing.exe program.

Static methods

Static methods are called without an instance of the object. To call a static method, we use the name of the class and the dot operator. Static methods can only work with static member variables. Static methods are often used to represent data or calculations that do not change in response to object state. An example is a math library which contains static methods for various calculations. We use the static keyword to declare a static method. When no static modifier is present, the method is said to be an instance method. We cannot use the this keyword in static methods. It can be used in instance methods only.

The Main() method is an entry point to the C# console and GUI application. In C#, the Main() method is required to be static. Before the application starts, no object is created yet. To invoke non-static methods, we need to have an object instance. Static methods exist before a class is instantiated so static is applied to the main entry point.

using System;

public class Basic
{
  static int Id = 2321;

  public static void ShowInfo()
  {
    Console.WriteLine("This is Basic class");
    Console.WriteLine("The Id is: {0}", Id);
  }
}

public class StaticMethod
{
  static void Main()
  {
    Basic.ShowInfo();
  }
}

In our code example, we define a static ShowInfo() method.

static int Id = 2321;

A static method can only work with static variables.

public static void ShowInfo()
{
  Console.WriteLine("This is Basic class");
  Console.WriteLine("The Id is: {0}", Id);
}

This is our static ShowInfo() method. It works with a static Id member.

Basic.ShowInfo();

To invoke a static method, we do not need an object instance. We call the method by using the name of the class and the dot operator.

$ ./staticmethod.exe 
This is Basic class
The Id is: 2321

This is the output of the example.

Hiding methods

When a derived class inherits from a base class, it can define methods that are already present in the base class. We say that we hide the method of the class that we have derived from. To explicitly inform the compiler about our intention to hide a method, we use the new keyword. Without this keyword, the compiler issues a warning.

using System;

public class Base
{
  public void Info()
  {
    Console.WriteLine("This is Base class");
  }
}

public class Derived : Base
{
  public new void Info()
  {  
    base.Info();
    Console.WriteLine("This is Derived class");
  }
}

public class HidingMethods
{
  static void Main()
  {
    Derived d = new Derived();
    d.Info();
  }
}

We have two classes: the Derived and the Base class. The Derived class inherits from the Base class. Both have a method called Info() .

public class Derived : Base
{
...
}

The (:) character is used to inherit from a class.

public new void Info()
{  
  base.Info();
  Console.WriteLine("This is Derived class");
}

This is an implementation of the Info() method in the Derived class. We use the new keyword to inform the compiler that we are hiding a method from the base class. Note that we can still reach the original Info() method. With the help of the base keyword, we invoke the Info() method of the Base class too.

$ ./hidingmethods.exe 
This is Base class
This is Derived class

We have invoked both methods.

Overriding methods

Now we will introduce two new keywords: the virtual keyword and the override keyword. They are both method modifiers. They are used to implement polymorphic behaviour of objects. The virtual keyword creates a virtual method. Virtual methods can be redefined in derived classes. Later in the derived class we use the override keyword to redefine the method in question. If the method in the derived class is preceded with the override keyword, objects of the derived class will call that method rather than the base class method.

using System;

public class Base
{
  public virtual void Info()
  {
    Console.WriteLine("This is Base class");
  }
}

public class Derived : Base
{
  public override void Info()
  {
    Console.WriteLine("This is Derived class");
  }
}

public class CSharpApp
{
  static void Main()
  {
    Base[] objs = {new Base(), new Derived(), new Base(),
            new Base(), new Base(), new Derived() };

    foreach (Base obj in objs)
    {
      obj.Info();
    }
  }
}

We create an array of the Base and Derived objects. We go through the array and invoke the Info() method upon all of them.

public virtual void Info()
{
  Console.WriteLine("This is Base class");
}

This is the virtual method of the Base class. It is expected to be overridden in the derived classes.

public override void Info()
{
  Console.WriteLine("This is Derived class");
}

We override the base Info() method in the Derived class. We use the override keyword.

Base[] objs = {new Base(), new Derived(), new Base(),
        new Base(), new Base(), new Derived() };

Here we create an array of Base and Derived objects. Note that we used the Base type in our array declaration. This is because a Derived class can be converted to the Base class because it inherits from it. The opposite is not true. The only way to have both objects in one array is to use a type which is top most in the inheritance hierarchy for all possible objects.

foreach (Base obj in objs)
{
  obj.Info();
}

We traverse the array and call Info() on all objects in the array.

$ ./virtualmethods.exe 
This is Base class
This is Derived class
This is Base class
This is Base class
This is Base class
This is Derived class

This is the output.

Now change the override keyword for new keyword. Compile the example again and run it.

$ ./virtualmethods.exe 
This is Base class
This is Base class
This is Base class
This is Base class
This is Base class
This is Base class

This time we have a different output.

Sealed methods

A sealed method overrides an inherited virtual method with the same signature. A sealed method shall also be marked with the override modifier. Use of the sealed modifier prevents a derived class from further overriding the method. The word further is important. First, a method must be virtual. It must be later overridden. And at this point, it can be sealed.

using System;

public class A
{
  public virtual void F()
  {
    Console.WriteLine("A.F");
  }

  public virtual void G()
  {
    Console.WriteLine("A.G");
  }
}

public class B : A
{
  public override void F()
  {
    Console.WriteLine("B.F");
  }

  public sealed override void G()
  {
    Console.WriteLine("B.G");
  }
}

public class C : B
{
  public override void F()
  {
    Console.WriteLine("C.F");
  }

  /*public override void G()
  {
    Console.WriteLine("C.G");
  }*/
}


public class SealedMethods
{
  static void Main()
  {
    B b = new B();
    b.F();
    b.G();

    C c = new C();
    c.F();
    c.G();
  }
}

In the preceding example, we seal the method G() in class B.

public sealed override void G()
{
  Console.WriteLine("B.G");
}

The method G() overrides a method with the same name in the ancestor of the B class. It is also sealed to prevent from further overriding the method.

/*public override void G()
{
  Console.WriteLine("C.G");
}*/

These lines are commented because otherwise the code example would not compile. The Mono compiler would give the following error: sealedmethods.cs(36,26): error CS0239: `C.G()': cannot override inherited member `B.G()' because it is sealed.

c.G();

This line prints "B.G()" to the console.

$ ./sealedmethods.exe 
B.F
B.G
C.F
B.G

This is the output.

In this part of the C# tutorial, we covered methods.