DT265-2
Object Oriented Software
Development 2
Lecture 4 : C# language features &
Databases
Lecturer Pat Browne patrick.browne@dit.ie

1. Generics
2. Delegates
3. Covariance & Contravariance
4. Attributes
5. Database connectivity

• Generics are the most powerful feature of C# 2.0.
Generics allow you to define type-safe data structures, without committing to actual data types. This results in a significant performance boost and higher quality code, because you get to reuse data processing algorithms without duplicating type-specific code. The benefits of the programming model, and unique innovations, such as constrains, generic methods and delegates, and generic inheritance. Generics are utilized in other areas of the .NET Framework such as reflection, arrays, collections, serialization, and remoting, and how to improve on the basic offering.

using System.Collections.Generic; class Program { static void Main() {
List<int> list = new List<int>(); list.Add(2); list.Add(3); list.Add(5); list.Add(7); } }

Generic Method OutputThing < T >
// Define generic method static void OutputThing < T > (T thing)
{
Console.WriteLine(“Thing: {0}”, thing);
}
// Run method for string and Int.
OutputThing < string > (“A string”);
OutputThing < int > (42);

public class Stack<T> { int position;
T[] data = new T[100]; public void Push (T obj) { data[position++] = obj; } public T Pop() { return data[--position]; }}
-- Stack<T> can be used as follows:
Stack<int> stack = new Stack<int>(); stack.Push(5); stack.Push(10); int x = stack.Pop(); // x is 10 int y = stack.Pop(); // y is 5

First we will look at the non-generic
IEnumerable and IEnumerator .
http://msdn.microsoft.com/en-us/library/system.collections.ienumerable.aspx
http://www.dotnetperls.com/ienumerable

•
•
• using System;
• using System.Collections.Generic;
• using System.Linq;
• class Program
• { static void Main()
• IEnumerable<int> result = from value in Enumerable.Range(0, 2) select value;
•
• // Loop.
foreach (int value in result){
•
•
•
• Console.WriteLine(value); }
• // We can use extension methods on IEnumerable<int> double average = result.Average();
// Extension methods can convert IEnumerable<int>
List<int> list = result.ToList(); int[] array = result.ToArray();
}}

using System; using System.Collections.Generic;
{ class Program
{ static void Main()
Display(new List<bool> { true, false, true });
}
}
}
{ static void Display(IEnumerable<bool> argument) foreach (bool value in argument)
Console.WriteLine(value);

• A delegate is a type that references a method.
Once a delegate is assigned a method, it behaves exactly like that method. The delegate method can be used like any other method, with parameters and a return value.
– Tasks:
– Type in examples from hand-out.
– Change the Square function to Double that doubles its argument.

• Covariance Example: Manager and Employee types.
Because there’s an inheritance relationship between these classes, there’s an implicit reference conversion from Manager to Employee:
• Manager → Employee
• And now, because of the annotation of T in
IEnumerable<out T>, there’s also an implicit reference conversion from IEnumerable<Manager> to
IEnumerable<Employee>.
• IEnumerable<Manager> → IEnumerable<Employee>

• Covariance Example: The arrow are reversed:
•
• Manager → Employee
IComparable<Manager> ← IComparable<Employee>

• Attributes add metadata to certain parts of the source code (classes, properties, fields, methods, parameters, etc.) . The compiler takes the information in the attribute and places it in the C# Intermediate Language (IL).
• Attributes by themselves don't do anything unless they are consumed. They remain in the IL until they are found and acted upon, which is usually done is using reflection .
• Do C# Attribute Tutorial
• http://msdn.microsoft.com/en-us/library/aa288454%28v=vs.71%29.aspx

• Attributes represent declarative information.
Programmers can attach attributes to various program entities, and retrieve attribute information in a run-time environment. For instance, a framework might define a
HelpAttribute attribute that can be placed on certain program elements (such as classes and methods) to provide a mapping from those program elements to their documentation.

• Find where csc.exe is on your machine
• You can add this location you the PATH system variable.
• csc.exe file.cs

• A database connection is a facility that allows client software to communicate with database server software, whether on the same machine or not. A connection is required to send commands and receive answers.
• A database driver is a piece of software for accessing a database. There are various DB and language specific database drivers.
• A JDBC driver is a software component enabling a Java application to interact with a database. JDBC drivers are analogous to ODBC drivers, ADO.NET data providers, and OLE DB providers.

• For C# there are alternative ways to connect to a database:
– ODBC
– OLE
– ADO.NET

Connecting C# program SQL Server
2008R2 Database
• Download and load Northwind database as instructed at:
• http://www.codeproject.com/Articles/42837/Ho wTo-Install-the-Northwind-and-Pubs-Sample-
Databa
• Do a sample queries:
SELECT LastName
FROM Employees
ORDER BY LastName;

Connecting C# program SQL Server
2008R2 Database
• Run queries in C# based on:
• http://www.codeproject.com/Articles/4416/B eginners-guide-to-accessing-SQL-Serverthrough-C
• Run code in notes section (below) using your own user name and server name:
• “Id=YOUR-USER-NAME;" +
• "Server=YOUR-SERVER-NAME;" +

Adding Data Source in Visual Studio
• Visual Studio provides tools to connect applications to data from many different sources, such as databases, Web services, and objects. In
Visual Studio connection objects can be created as a result of completing one of the data wizards or of dragging data objects onto your form. To connect an application to data in a database,
Web service, or object, use the Data Source
Configuration Wizard. From the main menu choose, Add New Data Source from the Data
Sources Window.

Connect SQL server 2008 databases to you Visual Studio 2010 application.
• To connect an Application from Visual Studio 2010 to
SQL Server by using wizard:
• a) Click on Tools menu from menu
• b) click on "Connect to Database" it will open "Choose
Data Source" dialog
• c) select your SQL server 2008 as "Microsoft SQL
Server" out of all installed Data Source on your System
• e) press Continue button, after that it will give you option to put your Server name, user ID and password.
You should not need a password on your own machine.

Adding a Data Source to Visual Studio

• The program in the notes section uses the
Employee Table from the Northwind database.
• SQL Server should be running.
• SqlDataAdapter interacts with the
DataTable type. It fills the DataTable with the query results from the Employees table and displays data using DataGridView .
• See
• http://www.dotnetperls.com/sqldataadapter

Connecting C# program to PostgreSQL
• A particular database driver that can be used with C# and PostgreSQL is called Npgsql.
• Npgsql is a .Net Data Provider for PostgreSQL. It allows any program developed for .Net
framework to access a PostgreSQL database server. It is implemented in C#.
• Npgsql allows a .Net client application (Console,
WinForms, ASP.NET, Web Services...) to send and receive data to & from a PostgreSQL server.

• Download from:
• http://pgfoundry.org/frs/?group_id=1000140
• Unzip to say:
• C:\Npgsql2.0.12.0-bin-ms.net4.0
• You might need a different .NET version
• In Project add Reference. Click the ‘Browse’ tab and select the Npqsql dll only.
• Then you should include using Npgsql; in applications that access PostgreSQL.

• Run this program:
• The area of each county is held in a column called km2 . Add this column to the SQL query in the above form.

• IP address for http://cork.ict.ad.dit.ie
• 147.252.234.32
• PostgreSQL Port: 5432

• Database1: SQL Client
• Database2: Data source
• Database3: PostgreSQL
• More detailed example at:
• http://www.codeproject.com/Articles/8477/U sing-ADO-NET-for-beginners

• Run this program