// Namespace Declaration
using System;
// helper class
class OutputClass
{
string myString;
// Constructor
public OutputClass(string inputString)
{
myString = inputString;
}
// Instance Method
public void printString()
{
Console.WriteLine("{0}", myString);
}
// Destructor
~OutputClass()
{
// Some resource cleanup routines
}
}
// Program start classclass ExampleClass
{
//Main begins program execution.
public static voidMain ()
{
// Instance of OutputClass
OutputClass outCl = new OutputClass("This is printed by the output class.");
// Call Output class' method
outCl.printString();
}
}
using System;
// helper class
class OutputClass
{
string myString;
// Constructor
public OutputClass(string inputString)
{
myString = inputString;
}
// Instance Method
public void printString()
{
Console.WriteLine("{0}", myString);
}
// Destructor
~OutputClass()
{
// Some resource cleanup routines
}
}
// Program start classclass ExampleClass
{
//
public static void
{
// Instance of OutputClass
OutputClass outCl = new OutputClass("This is printed by the output class.");
// Call Output class' method
outCl.printString();
}
}
Constructors are not mandatory, as indicated by the implementation of ExampleClass. In this case, a default constructor is provided. A default constructor is simply a constructor with no arguments. However, a constructor with no arguments is not always useful.
In C#, there are two types of class members, instance and static. Instance class members belong to a specific occurrence of a class. Every time you declare an object of a certain class, you create a new instance of that class. The ExampleClass Main() method creates an instance of the OutputClass named outCl. You can create multiple instances of OutputClass with different names. Each of these instances are separate and stand alone. For example, if you create two OutputClass instances as follows:
OutputClass oc1 = new OutputClass("OutputClass1");
OutputClass oc2 = new OutputClass("OutputClass2");
You create two separate instances of OutputClass with separate myString fields and separate printString() methods. On the other hand, if a class member is static, you can access it simply by using the syntax <classname>.<static class member>. The instance names are oc1 and oc2.
Suppose OutputClass had the following static method
Then you could call that function from Main () like this:
You must call static class members through their class name and not their instance name. This means that you don't need to instantiate a class to use its static members. There is only ever one copy of a static class member. A good use of static members is when there is a function to be performed and no intermediate state is required, such as math calculations. Matter of fact, the .NET Frameworks Base Class Library includes a Math class that makes extensive use of static members.
Another type of constructor is the static constructor. Use static constructor to initialize static fields in a class. You declare a static constructor by using the keyword static just in front of the constructor name. A static constructor is called before an instance of a class is created, before a static member is called, and before the static constructor of a derived class (covered in a later chapter). They are called only once.
OutputClass also has a destructor. Destructors look just like constructors, except they have a tilde, "~", in front of them. They don't take any parameters and do not return a value. Destructors are places where you could put code to release any resources your class was holding during its lifetime. They are normally called when the C# garbage collector decides to clean your object from memory.
So far, the only class members you've seen are Fields, Methods, Constructors, and Destructors. Here is a complete list of the types of members you can have in your classes:
l Constructors
l Destructors
l Fields
l Methods
l Properties
l Indexers
l Delegates
l Events
l Nested Classes
Class Inheritance
Inheritance is one of the primary concepts of object-oriented programming. It allows you to reuse existing code. Through effective employment of reuse, you can save time in your programming.
using System;
public class ParentClass
{
public ParentClass()
{
Console.WriteLine("Parent Constructor.");
}
public void print()
{
Console.WriteLine("I'm a Parent Class.");
}
}
public class ChildClass : ParentClass
{
public ChildClass()
{
Console.WriteLine("Child Constructor.");
}
public static void Main()
{
ChildClass child = new ChildClass();
child.print();
}
}
public class ParentClass
{
public ParentClass()
{
Console.WriteLine("Parent Constructor.");
}
public void print()
{
Console.WriteLine("I'm a Parent Class.");
}
}
public class ChildClass : ParentClass
{
public ChildClass()
{
Console.WriteLine("Child Constructor.");
}
public static void Main()
{
ChildClass child = new ChildClass();
child.print();
}
}
Output:
Parent Constructor.
Child Constructor.
I'm a Parent Class.
shows two classes. The top class is named ParentClass and the main class is called ChildClass. What we want to do is create a child class, using existing code from ParentClass.
First we must declare our intention to use ParentClass as the base class of ChildClass. This is accomplished through the ChildClass declaration public class ChildClass : ParentClass. The base class is specified by adding a colon, ":", after the derived class identifier and then specifying the base class name.
Note: C# supports single class inheritance only. Therefore, you can specify only one base class to inherit from. However, it does allow multiple interface inheritance.
ChildClass has exactly the same capabilities as ParentClass. Because of this, you can also say ChildClass "is" a ParentClass. This is shown in the Main () method of ChildClass when the print() method is called. ChildClass does not have its own print() method, so it uses the ParentClass print() method. You can see the results in the 3rd line of output.
Base classes are automatically instantiated before derived classes. Notice the output from Listing. The ParentClass constructor executed before the ChildClass constructor.
public class Parent
{
string parentString;
public Parent()
{
Console.WriteLine("Parent Constructor.");
}
public Parent(string myString)
{
parentString = myString;
Console.WriteLine(parentString);
}
public void print()
{
Console.WriteLine("I'm a Parent Class.");
}
}
public class Child : Parent
{
public Child() : base("From Derived")
{
Console.WriteLine("Child Constructor.");
}
public new void print()
{
base.print();
Console.WriteLine("I'm a Child Class.");
}
public static void Main ()
{
Child child = new Child();
child.print();
((Parent)child).print();
}
}
Output:
From Derived
Child Constructor.
I'm a Parent Class.
I'm a Child Class.
I'm a Parent Class.
Derived classes can communicate with base classes during instantiation. Listing shows how this is done at the child constructor declaration. The colon, ":", and keyword base call the base class constructor with the matching parameter list. If the code had not appended base("From Derived") to the Derived constructor, the code would have automatically called Parent(). The first line of output shows the base class constructor being called with the string "From Derived".
Sometimes you may want to create your own implementation of a method that exists in a base class. The Child class does this by declaring its own print() method. The Child print() method hides the Parent print() method. The effect is the Parent print() method will not be called, unless we do something special to make sure it is called.
Inside the Child print() method, we explicitly call the Parent print() method. This is done by prefixing the method name with "base.". Using the base keyword, you can access any of a base class public or protected class members. The output from the Child print() method is on output lines 3 and 4.
Another way to access base class members is through an explicit cast. This is done in the last statement of the Child class Main () method. Remember that a derived class is a specialization of its base class. This fact allows us to perform a cast on the derived class, making it an instance of its base class. The last line of output from Listing shows the Parent print() method was indeed executed.
Notice the new modifier on the Child class print() method. This enables this method to hide the Parent class print() method and explicitly states your intention that you don't want polymorphism to occur. Without the new modifier, the compiler will produce a warning to draw your attention to this.
Polymorphism
Another primary concept of object-oriented programming is Polymorphism. It allows you to invoke derived class methods through a base class reference during run-time. This is handy when you need to assign a group of objects to an array and then invoke each of their methods. They won't necessarily have to be the same object type. However, if they're related by inheritance, you can add them to the array as the inherited type. Then if they all share the same method name, that method of each object can be invoked.
using System;
public class DrawingObject
{
public virtual void Draw()
{
Console.WriteLine("I'm just a generic drawing object.");
}
}
public class DrawingObject
{
public virtual void Draw()
{
Console.WriteLine("I'm just a generic drawing object.");
}
}
shows the DrawingObject class. This will be the base class for other objects to inherit from. It has a single method named Draw(). The Draw() method has a virtual modifier. The virtual modifier indicates to derived classes that they can override this method. The Draw() method of the DrawingObject class performs a single action of printing the statement, "I'm just a generic drawing object.", to the console.
using System;
public class Line : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I'm a Line.");
}
}
public class Circle : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I'm a Circle.");
}
}
public class Square : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I'm a Square.");
}
}
public class Line : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I'm a Line.");
}
}
public class Circle : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I'm a Circle.");
}
}
public class Square : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I'm a Square.");
}
}
These classes inherit the DrawingObject class. Each class has a Draw() method and each Draw() method has an override modifier. The override modifier allows a method to override the virtual method of its base class at run-time. The override will happen only if the class is referenced through a base class reference. Overriding methods must have the same signature, name and parameters, as the virtual base class method it is overriding.
using System;
public class DrawDemo
{
public static int Main( )
{
DrawingObject[] dObj = new DrawingObject[4];
dObj[0] = new Line();
dObj[1] = new Circle();
dObj[2] = new Square();
dObj[3] = new DrawingObject();
foreach (DrawingObject drawObj in dObj)
{
drawObj.Draw();
}
return 0;
}
}
public class DrawDemo
{
public static int Main( )
{
DrawingObject[] dObj = new DrawingObject[4];
dObj[0] = new Line();
dObj[1] = new Circle();
dObj[2] = new Square();
dObj[3] = new DrawingObject();
foreach (DrawingObject drawObj in dObj)
{
drawObj.Draw();
}
return 0;
}
}
shows a program that uses the classes defined in Listing 1 and Listing 2. This program implements polymorphism. In the Main () method of the DrawDemo class, there is an array being created. The type of object in this array is the DrawingObject class. The array is named dObj and is being initialized to hold four objects of type DrawingObject.
Next the dObj array is initialized. Because of their inheritance relationship with the DrawingObject class, the Line, Circle, and Square classes can be assigned to the dObj array. Without this capability, you would have to create an array for each type. Inheritance allows derived objects to act like their base class, which saves work.
After the array is initialized, there is a foreach loop that looks at each element of the array. Within the foreach loop the Draw() method is invoked on each element of the dObj array. Because of polymorphism, the run-time type of each object is invoked. The type of the reference object from the dObj array is a DrawingObject. However, that doesn't matter because the derived classes override the virtual Draw() method of the DrawingObject class. This makes the overriden Draw() methods of the derived classes execute when the Draw() method is called using the DrawingObject base class reference from the dObj array.. Here's what the output looks like:
Output:
I'm a Line.
I'm a Circle.
I'm a Square.
I'm just a generic drawing object.
The override Draw() method of each derived class executes as shown in the DrawDemo program. The last line is from the virtual Draw() method of the DrawingObject class. This is because the actual run-time type of the fourth array element was a DrawingObject object.
Properties provide the opportunity to protect a field in a class by reading and writing to it through the property. In other languages, this is often accomplished by programs implementing specialized getter and setter methods. C# properties enable this type of protection while also letting you access the property just like it was a field.
using System;
public class PropertyHolder
{
private int someProperty = 0;
public int getSomeProperty()
{
return someProperty;
}
public void setSomeProperty(int propValue)
{
someProperty = propValue;
}
}
public class PropertyTester
{
public static int Main(string[] args)
{
PropertyHolder propHold = new PropertyHolder();
propHold.setSomeProperty(5);
Console.WriteLine("Property Value: {0}", propHold.getSomeProperty());
return 0;
}
}
public class PropertyHolder
{
private int someProperty = 0;
public int getSomeProperty()
{
return someProperty;
}
public void setSomeProperty(int propValue)
{
someProperty = propValue;
}
}
public class PropertyTester
{
public static int Main(string[] args)
{
PropertyHolder propHold = new PropertyHolder();
propHold.setSomeProperty(5);
Console.WriteLine("Property Value: {0}", propHold.getSomeProperty());
return 0;
}
}
Listing shows the traditional method of accessing class fields. The Property Holder class has the field we're interested in accessing. It has two methods, getSomeProperty() and setSomeProperty(). The getSomeProperty() method returns the value of the someProperty field. The setSomeProperty() method sets the value of the someProperty field.
The PropertyTester class uses the methods of the PropertyHolder class to get the value of the someProperty field in the PropertyHolder class. The Main () method instantiates a new PropertyHolder object, propHold. Next it sets the someProperty field of propHold to the value 5 by using the setSomeProperty method. Then the program prints out the property value with a Console.WriteLine() method call. The argument used to obtain the value of the property is a call to the getSomeProperty() method of the propHold object. It prints out "Property Value: 5" to the console.
This method of accessing information in a field has been good because it supports the object-oriented concept of encapsulation. If the implementation of someProperty changed from an int type to a byte type, this would still work. Now the same thing can be accomplished much smoother with properties.
using System;
public class PropertyHolder
{
private int someProperty = 0;
public int SomeProperty
{
get
{
return someProperty;
}
set
{
someProperty = value;
}
}
}
public class PropertyTester
{
public static intMain (string[] args)
{
PropertyHolder propHold = new PropertyHolder();
propHold.SomeProperty = 5;
Console.WriteLine("Property Value: {0}", propHold.SomeProperty);
return 0;
}
}
public class PropertyHolder
{
private int someProperty = 0;
public int SomeProperty
{
get
{
return someProperty;
}
set
{
someProperty = value;
}
}
}
public class PropertyTester
{
public static int
{
PropertyHolder propHold = new PropertyHolder();
propHold.SomeProperty = 5;
Console.WriteLine("Property Value: {0}", propHold.SomeProperty);
return 0;
}
}
Listing shows how to create and use a property. The PropertyHolder class has the SomeProperty property implementation. Notice that the first letter of the first word is capitalized. That's the only difference between the names of the property SomeProperty and the field someProperty. The property has two accessors, get and set. The get accessor returns the value of the someProperty field to calling code. The set accessor sets the value of the someProperty field with the contents of value, which is the value being assigned by calling code. The value shown in the set accessor is a C# reserved word.
The PropertyTester class uses the SomeProperty property in the PropertyHolder class. The first line of the Main () method creates a PropertyHolder object named propHold. Next the value of the someProperty field of the propHold object is set to 5 by using the SomeProperty property. It is that simple -- just assign the value to the property as if it were a field.
After that, the Console.WriteLine() method prints the value of the someProperty field of the propHold object. It does this by using the SomeProperty property of the propHold object. Again, it is that simple -- just use the property as if it were a field itself.
Properties can be made read-only. This is accomplished by having only a get accessor in the property implementation.
using System;
public class PropertyHolder
{
private int someProperty = 0;
public PropertyHolder(int propVal)
{
someProperty = propVal;
}
public int SomeProperty
{
get
{
return someProperty;
}
}
}
public class PropertyTester
{
public static int Main(string[] args)
{
PropertyHolder propHold = new PropertyHolder(5);
Console.WriteLine("Property Value: {0}", propHold.SomeProperty);
return 0;
}
}
public class PropertyHolder
{
private int someProperty = 0;
public PropertyHolder(int propVal)
{
someProperty = propVal;
}
public int SomeProperty
{
get
{
return someProperty;
}
}
}
public class PropertyTester
{
public static int Main(string[] args)
{
PropertyHolder propHold = new PropertyHolder(5);
Console.WriteLine("Property Value: {0}", propHold.SomeProperty);
return 0;
}
}
Listing shows how to implement a read-only property. The PropertyHolder class has a SomeProperty property that only implements a get accessor. It leaves out the set accessor. This particular PropertyHolder class has a constructor which accepts an int parameter.
The Main () method of the PropertyTester class creates a new PropertyHolder object named propHold. The instantiation of the propHold object uses the constructor of the PropertyHolder that takes an int parameter. In this case, it is set to 5.This initializes the someProperty field of the propHold object to 5.
Since the SomeProperty property of the PropertyHolder class is read-only, there is no other way to set the value of the someProperty field. If you inserted propHold. SomeProperty = 7 into the listing, the program would not compile, because SomeProperty is read-only. When the SomeProperty property is used in the Console.WriteLine() method, it works fine. This is because it is a read operation which only invokes the get accessor of the SomeProperty property.
Using System;
public class PropertyHolder
{
private int someProperty = 0;
public int SomeProperty
{
set
{
someProperty = value;
Console.WriteLine("someProperty is equal to {0}", someProperty);
}
}
}
public class PropertyTester
{
public static intMain (string[] args)
{
PropertyHolder propHold = new PropertyHolder();
propHold.SomeProperty = 5;
return 0;
}
}
public class PropertyHolder
{
private int someProperty = 0;
public int SomeProperty
{
set
{
someProperty = value;
Console.WriteLine("someProperty is equal to {0}", someProperty);
}
}
}
public class PropertyTester
{
public static int
{
PropertyHolder propHold = new PropertyHolder();
propHold.SomeProperty = 5;
return 0;
}
}
Listing shows how to create and use a write-only property. This time the get accessor is removed from the SomeProperty property of the PropertyHolder class. The set accessor has been added, with a bit more logic. It prints out the value of the someProperty field after it has been modified.
The Main () method of the PropertyTester class instantiates the PropertyTester class with a default constructor. Then it uses the SomeProperty property of the propHold object to set the someProperty field of the propHold object to 5. This invokes the set accessor of the propHold object, which sets the value of its someProperty field to 5 and then prints "someProperty is equal to 5" to the console.
Indexers
Indexers are real easy. They allow your class to be used just like an array. On the inside of a class, you manage a collection of values any way you want. These objects could be a finite set of class members, another array, or some complex data structure. Regardless of the internal implementation of the class, its data can be obtained consistently through the use of indexers. Here's an example.
using System;
/// <summary>
/// A simple indexer example./// </summary>class IntIndexer
{
private string[] myData;
public IntIndexer(int size)
{
myData = new string[size];
for (int i=0; i < size; i++)
{
myData[i] = "empty";
}
}
public string this[int pos]
{
get
{
return myData[pos];
}
set
{
myData[pos] = value;
}
}
static void Main(string[] args)
{
int size = 10;
IntIndexer myInd = new IntIndexer(size);
myInd[9] = "Some Value";
myInd[3] = "Another Value";
myInd[5] = "Any Value";
Console.WriteLine("\nIndexer Output\n");
for (int i=0; i < size; i++)
{
Console.WriteLine("myInd[{0}]: {1}", i, myInd[i]);
}
}
}
/// <summary>
/// A simple indexer example./// </summary>class IntIndexer
{
private string[] myData;
public IntIndexer(int size)
{
myData = new string[size];
for (int i=0; i < size; i++)
{
myData[i] = "empty";
}
}
public string this[int pos]
{
get
{
return myData[pos];
}
set
{
myData[pos] = value;
}
}
static void Main(string[] args)
{
int size = 10;
IntIndexer myInd = new IntIndexer(size);
myInd[9] = "Some Value";
myInd[3] = "Another Value";
myInd[5] = "Any Value";
Console.WriteLine("\nIndexer Output\n");
for (int i=0; i < size; i++)
{
Console.WriteLine("myInd[{0}]: {1}", i, myInd[i]);
}
}
}
Listing shows how to implement an Indexer. The IntIndexer class has a string array named myData. This is a private array that external users can't see. this array is initialized in the constructor, which accepts an int size parameter, instantiates the myData array, and then fills each element with the word "empty".
The next class member is the Indexer, which is identified by the this keyword and square brackets, this[int pos]. It accepts a single position parameter, pos. As you may have already guessed, the implementation of an Indexer is the same as a Property. It has get and set accessors that are used exactly like those in a Property. This indexer returns a string, as indicated by the string return value in the Indexer declaration.
The Main () method simply instantiates a new IntIndexer object, adds some values, and prints the results. Here's the output:
Indexer Output
myInd[0]: empty
myInd[1]: empty
myInd[2]: empty
myInd[3]: Another Value
myInd[4]: empty
myInd[5]: Any Value
myInd[6]: empty
myInd[7]: empty
myInd[8]: empty
myInd[9]: Some Value
Using an integer is a common means of accessing arrays in many languages, but the C# Indexer goes beyond this. Indexers can be declared with multiple parameters and each parameter may be a different type. Additional parameters are separated by commas, the same as a method parameter list. Valid parameter types for Indexers include integers, enums, and strings. Additionally, Indexers can be overloaded. In next listing, we modify the previous program to accept overloaded Indexers that accept different types.
using System;
/// <summary>
/// Implements overloaded indexers./// </summary>class OvrIndexer
{
private string[] myData;
private int arrSize;
public OvrIndexer(int size)
{
arrSize = size;
myData = new string[size];
for (int i=0; i < size; i++)
{
myData[i] = "empty";
}
}
public string this[int pos]
{
get
{
return myData[pos];
}
set
{
myData[pos] = value;
}
}
public string this[string data]
{
get
{
int count = 0;
for (int i=0; i < arrSize; i++)
{
if (myData[i] == data)
{
count++;
}
}
return count.ToString();
}
set
{
for (int i=0; i < arrSize; i++)
{
if (myData[i] == data)
{
myData[i] = value;
}
}
}
}
static void Main(string[] args)
{
int size = 10;
OvrIndexer myInd = new OvrIndexer(size);
myInd[9] = "Some Value";
myInd[3] = "Another Value";
myInd[5] = "Any Value";
myInd["empty"] = "no value";
Console.WriteLine("\nIndexer Output\n");
for (int i=0; i < size; i++)
{
Console.WriteLine("myInd[{0}]: {1}", i, myInd[i]);
}
Console.WriteLine("\nNumber of \"no value\" entries: {0}", myInd["no value"]);
}
}
/// <summary>
/// Implements overloaded indexers./// </summary>class OvrIndexer
{
private string[] myData;
private int arrSize;
public OvrIndexer(int size)
{
arrSize = size;
myData = new string[size];
for (int i=0; i < size; i++)
{
myData[i] = "empty";
}
}
public string this[int pos]
{
get
{
return myData[pos];
}
set
{
myData[pos] = value;
}
}
public string this[string data]
{
get
{
int count = 0;
for (int i=0; i < arrSize; i++)
{
if (myData[i] == data)
{
count++;
}
}
return count.ToString();
}
set
{
for (int i=0; i < arrSize; i++)
{
if (myData[i] == data)
{
myData[i] = value;
}
}
}
}
static void Main(string[] args)
{
int size = 10;
OvrIndexer myInd = new OvrIndexer(size);
myInd[9] = "Some Value";
myInd[3] = "Another Value";
myInd[5] = "Any Value";
myInd["empty"] = "no value";
Console.WriteLine("\nIndexer Output\n");
for (int i=0; i < size; i++)
{
Console.WriteLine("myInd[{0}]: {1}", i, myInd[i]);
}
Console.WriteLine("\nNumber of \"no value\" entries: {0}", myInd["no value"]);
}
}
Listing shows how to overload Indexers. The first Indexer, with the int parameter, pos, is the same as in Listing first code, but there is a new Indexer that takes a string parameter. The get accessor of the new indexer returns a string representation of the number of items that match the parameter value, data. The set accessor changes each entry in the array that matches the data parameter to the value that is assigned to the Indexer.
The behavior of the overloaded Indexer that takes a string parameter is demonstrated in the Main () method of Listing . It invokes the set accessor, which assigns the value of "no value" to every member of the myInd class that has the value of "empty". It uses the following command: myInd["empty"] = "no value";. After each entry of the myInd class is printed, a final entry is printed to the console, indicating the number of entries with the "no value" string. This happens by invoking the get accessor with the following code: myInd["no value"]. Here's the output:
Indexer Output
myInd[0]: no value
myInd[1]: no value
myInd[2]: no value
myInd[3]: Another Value
myInd[4]: no value
myInd[5]: Any Value
myInd[6]: no value
myInd[7]: no value
myInd[8]: no value
myInd[9]: Some Value
Number of "no value" entries: 7 The reason both Indexers in Listing can coexist in the same class is because they have different signatures. An Indexer signature is specified by the number and type of parameters in an Indexers parameter list. The class will be smart enough to figure out which Indexer to invoke, based on the number and type of arguments in the Indexer call. An indexer with multiple parameters would be implemented something like this:
public object this[int param1, ..., int paramN]
{
get
{
// process and return some class data
}
set
{
// process and assign some class data
}
}
{
get
{
// process and return some class data
}
set
{
// process and assign some class data
}
}
Interfaces
An interface looks like a class, but has no implementation. The only thing it contains are definitions of events, indexers, methods and/or properties. The reason interfaces only provide definitions is because they are inherited by classes and structs, which must provide an implementation for each interface member defined.
So, what are interfaces good for if they don't implement functionality? They're great for putting together plug-n-play like architectures where components can be interchanged at will. Since all interchangeable components implement the same interface, they can be used without any extra programming. The interface forces each component to expose specific public members that will be used in a certain way.
Because interfaces must be implemented by derived classes and structs, they define a contract. For instance, if class foo implements the IDisposable interface, it is making a statement that it guarantees it has the Dispose() method, which is the only member of the IDisposable interface. Any code that wishes to use class foo may check to see if class foo implements IDisposable. When the answer is true, then the code knows that it can call foo.Dispose(). Following Listing shows how to define an interface:
Listing defines an interface named IMyInterface. A common naming convention is to prefix all interface names with a capital "I". This interface has a single method named MethodToImplement(). This could have been any type of method declaration with different parameters and return types. Notice that this method does not have an implementation (instructions between curly braces - {}), but instead ends with a semi-colon, ";". This is because the interface only specifies the signature of methods that an inheriting class or struct must implement. Following Listing shows how this interface could be used.
class InterfaceImplementer : IMyInterface
{
static voidMain ()
{
InterfaceImplementer iImp = new InterfaceImplementer();
iImp.MethodToImplement();
}
public void MethodToImplement()
{
Console.WriteLine("MethodToImplement() called.");
}
}
{
static void
{
InterfaceImplementer iImp = new InterfaceImplementer();
iImp.MethodToImplement();
}
public void MethodToImplement()
{
Console.WriteLine("MethodToImplement() called.");
}
}
The InterfaceImplementer class in above Listing implements the IMyInterface interface. Indicating that a class inherits an interface is the same as inheriting a class. In this case, the following syntax is used:
Now that this class inherits the IMyInterface interface, it must implement its members. It does this by implementing the MethodToImplement() method. Notice that this method implementation has the exact same signature, parameters and method name, as defined in the IMyInterface interface. Any difference will cause a compiler error. Interfaces may also inherit other interfaces. Following Listing shows how inherited interfaces are implemented.
using System;
interface IParentInterface
{
void ParentInterfaceMethod();
}
interface IMyInterface : IParentInterface
{
void MethodToImplement();
}
class InterfaceImplementer : IMyInterface
{
static void Main()
{
InterfaceImplementer iImp = new InterfaceImplementer();
iImp.MethodToImplement();
iImp.ParentInterfaceMethod();
}
public void MethodToImplement()
{
Console.WriteLine("MethodToImplement() called.");
}
public void ParentInterfaceMethod()
{
Console.WriteLine("ParentInterfaceMethod() called.");
}
}
interface IParentInterface
{
void ParentInterfaceMethod();
}
interface IMyInterface : IParentInterface
{
void MethodToImplement();
}
class InterfaceImplementer : IMyInterface
{
static void Main()
{
InterfaceImplementer iImp = new InterfaceImplementer();
iImp.MethodToImplement();
iImp.ParentInterfaceMethod();
}
public void MethodToImplement()
{
Console.WriteLine("MethodToImplement() called.");
}
public void ParentInterfaceMethod()
{
Console.WriteLine("ParentInterfaceMethod() called.");
}
}
The code in listing contains two interfaces: IMyInterface and the interface it inherits, IParentInterface. When one interface inherits another, any implementing class or struct must implement every interface member in the entire inheritance chain. Since the InterfaceImplementer class in Listing inherits from IMyInterface, it also inherits IParentInterface. Therefore, the InterfaceImplementer class must implement the MethodToImplement() method specified in the IMyInterface interface and the ParentInterfaceMethod() method specified in the IParentInterface interface.
Introduction to Delegates and Events
Delegates
During previous lessons, you learned how to implement reference types using language constructs such as classes and interfaces. These reference types allowed you to create instances of objects and use them in special ways to accomplish your software development goals. Classes allow you to create objects that contained members with attributes or behavior. Interfaces allowed you to declare a set of attributes and behavior that all objects implementing them would publicly expose.
A delegate is a C# language element that allows you to reference a method. If you were a C or C++ programmer, this would sound familiar because a delegate is basically a function pointer. However, developers who have used other languages are probably wondering, "Why do I need a reference to a method?". The answer boils down to giving you maximum flexibility to implement any functionality you want at runtime.
Think about how you use methods right now. You write an algorithm that does its thing by manipulating the values of variables and calling methods directly by name. What if you wanted an algorithm that was very flexible, reusable, and allowed you to implement different functionality as the need arises? Furthermore, let's say that this was an algorithm that supported some type of data structure that you wanted to have sorted, but you also want to enable this data structure to hold different types. If you don't know what the types are, how could you decide an appropriate comparison routine? Perhaps you could implement an if/then/else or switch statement to handle well-known types, but this would still be limiting and require overhead to determine the type. Another alternative would be for all the types to implement an interface that declared a common method your algorithm would call, which is actually a nice solution. However, since this lesson is about delegates, we'll apply a delegate solution, which is quite elegant.
You could solve this problem by passing a delegate to your algorithm and letting the contained method, which the delegate refers to, perform the comparison operation. Such an operation is performed in following Listing.
using System;
// this is the delegate declarationpublic delegate int Comparer(object obj1, object obj2);
public class Name
{
public string FirstName = null;
public string LastName = null;
public Name(string first, string last)
{
FirstName = first;
LastName = last;
}
// this is the delegate method handler
public static int CompareFirstNames(object name1, object name2)
{
string n1 = ((Name)name1).FirstName;
string n2 = ((Name)name2).FirstName;
if (String.Compare(n1, n2) > 0) //n1 is greater than n2.
{
return 1;
}
else if (String.Compare(n1, n2) < 0) //n1 is less than n2.
{
return -1;
}
else // n1 equals n2. {
return 0;
}
}
public override string ToString()
{
return FirstName + " " + LastName;
}
}
class SimpleDelegate
{
Name[] names = new Name[5];
public SimpleDelegate()
{
names[0] = new Name("Joe", "Mayo");
names[1] = new Name("John", "Hancock");
names[2] = new Name("Jane", "Doe");
names[3] = new Name("John", "Doe");
names[4] = new Name("Jack", "Smith");
}
static void Main(string[] args)
{
SimpleDelegate sd = new SimpleDelegate();
// this is the delegate instantiation
Comparer cmp = new Comparer(Name.CompareFirstNames);
Console.WriteLine("\nBefore Sort: \n");
sd.PrintNames();
// observe the delegate argument sd.Sort(cmp);
Console.WriteLine("\nAfter Sort: \n");
sd.PrintNames();
}
// observe the delegate parameter
public void Sort(Comparer compare)
{
object temp;
for (int i=0; i < names.Length; i++)
{
for (int j=i; j < names.Length; j++)
{
// using delegate "compare" just like a normal method if ( compare(names[i], names[j]) > 0 )
{
temp = names[i];
names[i] = names[j];
names[j] = (Name)temp;
}
}
}
}
public void PrintNames()
{
Console.WriteLine("Names: \n");
foreach (Name name in names)
{
Console.WriteLine(name.ToString());
}
}
}
// this is the delegate declarationpublic delegate int Comparer(object obj1, object obj2);
public class Name
{
public string FirstName = null;
public string LastName = null;
public Name(string first, string last)
{
FirstName = first;
LastName = last;
}
// this is the delegate method handler
public static int CompareFirstNames(object name1, object name2)
{
string n1 = ((Name)name1).FirstName;
string n2 = ((Name)name2).FirstName;
if (String.Compare(n1, n2) > 0) //n1 is greater than n2.
{
return 1;
}
else if (String.Compare(n1, n2) < 0) //n1 is less than n2.
{
return -1;
}
else // n1 equals n2. {
return 0;
}
}
public override string ToString()
{
return FirstName + " " + LastName;
}
}
class SimpleDelegate
{
Name[] names = new Name[5];
public SimpleDelegate()
{
names[0] = new Name("Joe", "Mayo");
names[1] = new Name("John", "Hancock");
names[2] = new Name("Jane", "Doe");
names[3] = new Name("John", "Doe");
names[4] = new Name("Jack", "Smith");
}
static void Main(string[] args)
{
SimpleDelegate sd = new SimpleDelegate();
// this is the delegate instantiation
Comparer cmp = new Comparer(Name.CompareFirstNames);
Console.WriteLine("\nBefore Sort: \n");
sd.PrintNames();
// observe the delegate argument sd.Sort(cmp);
Console.WriteLine("\nAfter Sort: \n");
sd.PrintNames();
}
// observe the delegate parameter
public void Sort(Comparer compare)
{
object temp;
for (int i=0; i < names.Length; i++)
{
for (int j=i; j < names.Length; j++)
{
// using delegate "compare" just like a normal method if ( compare(names[i], names[j]) > 0 )
{
temp = names[i];
names[i] = names[j];
names[j] = (Name)temp;
}
}
}
}
public void PrintNames()
{
Console.WriteLine("Names: \n");
foreach (Name name in names)
{
Console.WriteLine(name.ToString());
}
}
}
The first thing the program in Listing does is declare a delegate. Delegate declarations look somewhat like methods, except they have the delegate modifier, are terminated with a semi-colon (;), and have no implementation. Below, is the delegate declaration from Listing .
This delegate declaration defines the signature of a delegate handler method that this delegate can refer to. The delegate handler method, for the Comparer delegate, can have any name, but must have a first parameter of type object, a second parameter of type object, and return an int type. The following method from Listing shows a delegate handler method that conforms to the signature of the Comparer delegate
To use a delegate, you must create an instance of it. The instance is created, similar to a class instance, with a single parameter identifying the appropriate delegate handler method, as shown below.
Comparer cmp = new Comparer(Name.CompareFirstNames);
The delegate, cmp, is then used as a parameter to the Sort() method, which uses it just like a normal method. Observe the way the delegate is passed to the Sort() method as a parameter in the code below.
sd.Sort(cmp);
Using this technique, any delegate handler method may be passed to the Sort() method at run-time. i.e. You could define a method handler named CompareLastNames(), instantiate a new Comparer delegate instance with it, and pass the new delegate to the Sort() method.
Events
Traditional Console applications operate by waiting for a user to press a key or type a command and press the Enter key. Then they perform some pre-defined operation and either quit or return to the original prompt that they started from. This works, but is inflexible in that everything is hard-wired and follows a rigid path of execution. In stark contrast, modern GUI programs operate on an event-based model. That is, some event in the system occurs and interested modules are notified so they can react appropriately. With Windows Forms, there is not a polling mechanism taking up resources and you don't have to code a loop that sits waiting for input. It is all built into the system with events.
A C# event is a class member that is activated whenever the event it was designed for occurs. I like to use the term "fires" when the event is activated. Anyone interested in the event can register and be notified as soon as the event fires. At the time an event fires, registered methods will be invoked.
Events and delegates work hand-in-hand to provide a program's functionality. It starts with a class that declares an event. Any class, including the same class that the event is declared in, may register one of its methods for the event. This occurs through a delegate, which specifies the signature of the method that is registered for the event. The delegate may be one of the pre-defined .NET delegates or one you declare yourself. Whichever is appropriate, you assign the delegate to the event, which effectively registers the method that will be called when the event fires. Listing shows a couple different ways to implement events.
using System;
using System.Drawing;
using System.Windows.Forms;
// custom delegatepublic delegate void Startdelegate();
class Eventdemo : Form
{
// custom event public event Startdelegate StartEvent;
public Eventdemo()
{
Button clickMe = new Button();
clickMe.Parent = this;
clickMe.Text = "Click Me";
clickMe.Location = new Point(
(ClientSize.Width - clickMe.Width) /2,
(ClientSize.Height - clickMe.Height)/2);
// an EventHandler delegate is assigned to the button's Click event clickMe.Click += new EventHandler(OnClickMeClicked);
// our custom "Startdelegate" delegate is assigned
// to our custom "StartEvent" event. StartEvent += new Startdelegate(OnStartEvent);
// fire our custom event
StartEvent();
}
// this method is called when the "clickMe" button is pressed public void OnClickMeClicked(object sender, EventArgs ea)
{
MessageBox.Show("You Clicked My Button!");
}
// this method is called when the "StartEvent" Event is fired public void OnStartEvent()
{
MessageBox.Show("I Just Started!");
}
static void Main(string[] args)
{
Application.Run(new Eventdemo());
}
}
using System.Drawing;
using System.Windows.Forms;
// custom delegatepublic delegate void Startdelegate();
class Eventdemo : Form
{
// custom event public event Startdelegate StartEvent;
public Eventdemo()
{
Button clickMe = new Button();
clickMe.Parent = this;
clickMe.Text = "Click Me";
clickMe.Location = new Point(
(ClientSize.Width - clickMe.Width) /2,
(ClientSize.Height - clickMe.Height)/2);
// an EventHandler delegate is assigned to the button's Click event clickMe.Click += new EventHandler(OnClickMeClicked);
// our custom "Startdelegate" delegate is assigned
// to our custom "StartEvent" event. StartEvent += new Startdelegate(OnStartEvent);
// fire our custom event
StartEvent();
}
// this method is called when the "clickMe" button is pressed public void OnClickMeClicked(object sender, EventArgs ea)
{
MessageBox.Show("You Clicked My Button!");
}
// this method is called when the "StartEvent" Event is fired public void OnStartEvent()
{
MessageBox.Show("I Just Started!");
}
static void Main(string[] args)
{
Application.Run(new Eventdemo());
}
}
Note: If you're using Visual Studio or another IDE, remember to add references to System.Drawing.dll and System.Windows.Forms.dll before compiling Listing or just add the code to a Windows Forms project. Teaching the operation of Visual Studio or other IDE's is out-of-scope for this tutorial.
You may have noticed that Listing is a Windows Forms program. Although I haven't covered Windows Forms in this tutorial, you should know enough about C# programming in general that you won't be lost. To help out, I'll give a brief explanation of some of the parts that you may not be familiar with.
The Eventdemo class inherits Form, which essentially makes it a Windows Form. This automatically gives you all the functionality of a Windows Form, including Title Bar, Minimize/Maximize/Close buttons, System Menu, and Borders. A lot of power, that inheritance thing, eh?
The way a Windows Form's application is started is by calling the Run() method of the static Application object with a reference to the form object as its parameter. This starts up all the underlying Windows plumbing, displays the GUI, and ensures that events are fired as appropriate.
Let's look at the custom event first. Below is the event declaration, which is a member of the Eventdemo class. It is declared with the event keyword, a delegate type, and an event name.
public event Startdelegate StartEvent;
Anyone interested in an event can register by hooking up a delegate for that event. On the next line, we have a delegate of type Startdelegate, which the event was declared to accept, hooked up to the StartEvent event. The += syntax registers a delegate with an event. To unregister with an event, use the -= with the same syntax.
StartEvent += new Startdelegate(OnStartEvent);
Firing an event looks just like a method call, as shown below:
StartEvent();
This was how to implement events from scratch, declaring the event and delegate yourself. However, much of the event programming you'll do will be with pre-defined events and delegates. This leads us to the other event code you see in Listing 14-2, where we hook up an EventHandler delegate to a Button Click event.
clickMe.Click += new EventHandler(OnClickMeClicked);
The Click event already belongs to the Button class and all we have to do is reference it when registering a delegate. Similarly, the EventHandler delegate already exists in the System namespace of the .NET Frameworks Class Library. All you really need to do is define your callback method (delegate handler method) that is invoked when someone presses the clickMe button. The OnClickMeClicked() method, shown below, conforms to the signature of the EventHander delegate, which you can look up in the .NET Framework Class Library reference.
public void OnClickMeClicked(object sender, EventArgs ea)
{
MessageBox.Show("You Clicked My Button!");
}
{
MessageBox.Show("You Clicked My Button!");
}
Any time the clickMe button is pressed with a mouse, it will fire the Click event, which will invoke the OnClickMeClicked() method. The Button class takes care of firing the Click event and there's nothing more you have to do. Because it is so easy to use pre-defined events and delegates, it would be a good idea to check if some exist already that will do what you need, before creating your own.
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