1. Session 07: C# Design Patterns
Composite Pattern
Iterator Pattern
Interfaces in the libraries
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2. Example: Composite-pattern
Composite: Graphical editor, Word processor, Inventories etc..
(What will a Show()-method look like on Shape, Circle, Picture etc.)
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public abstract class Shape {
private int id;
private string color;
private int xpos;
private int ypos;
public void MoveTo(Position newPos){
// PRE none
// POST pos'=newPos }
public abstract void Show();
// POST the shape is displayed
public abstract void Hide();
// POST the shape is hidden
public abstract float Area();
// POST Computes the area with 4 digits.
}//end Shape
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public class Circle: Shape{
private int r; //radius
public void Show(){
//PRE none
//POST the circle is drawn
//Must be implemented using appropriate
//graphical routines }
public void Hide(){
//PRE none
//POST the circle is hidden
// More operations
}//end Circle
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5. Static type Shape – and Picture is a Shape itself!
public class Picture : Shape{
List<Shape> pictList;
// operations for adding, deleting etc. shapes
public void Show(){
//PRE none
//POST The composite shape is drawn
for(Shape s : pictList)
s.Show(); }
public float Area() {
float result=0;
for(int i=0;i<pictList.Count;i++) {
result= result+pictList[i].Area();
return result;
}//end Picture
Static type
Dynamic type definesshow()
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6. AK - IT: Softwarekonstruktion
Composite Pattern
The concept of patterns originates from architecture (Christopher Alexander, 1977):
“Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice”
(Christopher Alexander e. a.: “A Pattern Language”. Oxford University Press, New York, 1977.)
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(OO) Design Patterns
A well known and widely accepted concept in software engineering
Developed in the early 1990s and published by Gamma e.a. (“Gang of Four”, GoF) in 1995:
“(…) design patterns (…) are descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context.”
(Erich Gamma e.a.:”Design Patterns. Elements of Reusable Object-Oriented Software”. Addison-Wesley )
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8. The benefits of patterns
A pattern captures a proven good design:
A pattern is based on experience
A pattern is discovered – not invented
It introduces a new (and higher) level of abstraction, which makes it easier:
to talk and reason about design on a higher level
to document and communicate design
One doesn’t have to reinvent solutions over and over again
Patterns facilitate reuse not only of code fragments, but of ideas.
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9. Patterns as a learning tool
It is often said that good skills in software construction require experience and talent
…and neither can be taught or learned at school
Patterns capture experience (and talent) in a way that is communicable and comprehensible
…and hence experience can be taught (and learned)
So one should put a lot of effort in studying patterns
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More Examples
Organisational Structures
demos\RecursiveDirectoryTraverse
A directory is a collection of elements which are either files or directories themselves.
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Exercises
Exercise 4 on Session07.docx.
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Interfaces
An interface may be seen as a purely abstract class.
Interface: only method signatures, no implementation! (All methods are abstract)
An interface represents a design.
Example:
Designing an object that can be used for iterating over a data structure.
Also as generic
public interface IEnumerator {
void Reset(); // reset iterator to beginning
bool MoveNext(); // advance to next element
object Current { get; } // retrieve current element }
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13. Example – Iterator Pattern
This piece of client code iterates over any data structure that implements IEnumerator:
data
structure
iterator
IEnumerator iter;
iter = ...; // get ref to iterator object
iter.Reset();
while ( iter.MoveNext() )
MessageBox.Show( iter.Current.ToString() );
See it work!
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14. Different Collections
List:
Array-based (static)
Supports access by position
Uses extra memory
Expensive when re-allocating
LinkedList:
Linked list (dynamic, no extra memory)
Doesn’t support access by position.
HashSet (Dictionary):
Array-based (static)
Supports fast access (in average) by key – slow in worst case
Uses extra memory
Expensive when re-allocating
SortedSet (SortedDictionary):
Search tree (dynamic, no extra memory)
Supports fast access (logarithmic) in average and worst case.
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15. In .NET interfaces are used heavily.
IComparable
ICloneable
IDisposable
IEnumerable & IEnumerator
IList
ISerializable
IDBConnection, IDBCommand, IDataReader
etc.
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Typical Design in APIs
Specification in the interface.
Implementation in the concrete classes.
Operations with common implementation are implemented in the abstract class (no code duplication).
Operation specific for the different concrete classes are left abstract in the abstract class.
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8. Interfaces
January 2003
Example: Sorting
Goal:
To write a generic Sort() method as the one found in System.Array
Recall that System.Array class contains a static Sort() method that can sort any kind of array…
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Denmark .NET Workshop © 2003 Joe Hummel

18. Step 1: Define the interface
Sorting requires a way of comparison of objects:
returns < if this object < obj parameter
returns == if this object = obj parameter
returns > if this object > obj parameter
public interface IComparable<T> {
int CompareTo(T obj); }
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19. Step 2: Classes must implement the interface
8. Interfaces
Step 2: Classes must implement the interface
January 2003
Objects That are to be sorted must implement IComparable><>
Example:
sort Student objects on id
public class Student : Icomparable<Student> {
private int m_ID; .
int IComparable.CompareTo(Student other)
return this.m_ID – other.m_ID; }
interface
Student
type
Recall from earlier lecture that Student class inherited from Person class…
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Denmark .NET Workshop © 2003 Joe Hummel

20. Step 3: Clients program towards the interface
8. Interfaces
January 2003
Step 3: Clients program towards the interface
Sort assumes that the elements in the array a implement IComparable:
public class Array {
public static void Sort(Array a)
IComparable icobj;
for (int i = 0; i < a.Length-1; i++) {
for (int j = i+1; j < a.Length; j++) {
icobj = (IComparable) a.GetValue(i);
if (icobj.CompareTo(a.GetValue(j)) > 0)
swap(a, i, j);
}//for }
Not a very efficient sort, but you get the idea — this method will sort any array of objects, as long as those objects implement IComparable.
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Denmark .NET Workshop © 2003 Joe Hummel

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Step 4: test!
Example: sort an array of Student objects
View Code
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Exercises
Exercise 5 on Session07.docx.
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