Understanding Inheritance in C#

Inspired from biological modeling, inheritance allows new classes to be constructed that inherit characteristics (fields and methods) from ancestor classes while typically introducing more specialized characteristics, new fields, or methods. A subclass is logically considered to be a specialized version or extension of its parent and by inference its ancestor classes.

In programming, you often create a model of something, and then need a more specialized version of that original model.

Fig 1. Shows the UML class diagrams that model the Employee and Manager classes.
Listing 1. A possible implementation of Employee class.
Listing 2. A possible implementation of Manager class.

These codes illustrate the duplication of attributes between Manager class and the Employee class. Additionally, there could be a number of methods applicable to both classes.
In object-oriented languages, special mechanisms are provided that enable you define a class in terms of a previously defined class.

One of its main mechanism is called Inheritance. Inheritance is a form of software reusability in which classes are created by absorbing an existing class’s data and behaviors and embellishing them with new capabilities.
The next figure shows the diagram in which the Manager is a derived class of Employee base class.

Fig 2. Class diagram using Inheritance.
Listing 3. The Employee class.
Listing 4. The Manager class that inherits from class Employee.

Single Inheritance

The C# programming language permits a class to extend one other class only. This restriction is called single inheritance. With single inheritance, a class is derived from one base class. C# does not support multiple inheritance.

Once created, each derived class can become the base class for future derived classes. Typically, the derived class contains the behaviors of its base class. Therefore, a derived class is more specific than its base class and represents a more specialized group of objects.

The next image shows the base class Employee and three derived classes: Engineer, Manager and Secretary. The Manager is also the base class from which the derived class Director explicitly inherits.

Fig 3. An example Inheritance tree.

The Employee class contains three attributes (Name, Salary, and BirthDate), as well as one method (GetDetails). The Manager class inherits all of these members and specifies an additional attribute, department, as well as the GetDetails method. The Director class inherits all of the members of Employee and Manager and specifies a CarAllowance attribute and a new method, IncreaseAllowance.

Download source code

The this Reference

Every object can access a reference to itself, called the this reference. The this reference can refer implicitly to the instance variable, properties and methods of an object.

Two uses of the this keyword are:

  1. To resolve ambiguity between instance variables and parameters
  2. To pass the current object as a parameter to another method

The following class demonstrates these uses.

Fig 1 The Use of the this Keyword

The first constructor receives three uint parameters which names are identical to the instance variables of the class. I did this to illustrate explicit use of the this reference.

Fig 2 This reference output

The explicit use of the this reference can increase program clarity in some contexts where this is optional.

Download the example source code

C# Networking Basics: Sockets

Socket is the name given to an abstraction through which an application may send and receive data. The term “Socket” refers to the Berkeley Sockets Interface, which was developed in 1978 for network programming with UNIX and was popularized by C programmers.

Because of the popularity of that abstraction, this term has been reused in many programming model including .NET technology.
The main types of sockets in TCP/IP today are: stream sockets and datagram sockets, both use TCP as the end-to-end protocol with IP underneath, the .NET framework provides a clear distinction between using TCP and UDP, defining a separate set of classes for both protocols.

Stream Sockets

Socket based communications in C# employ stream sockets, when processes communicate over a network .NET Framework technology uses the streams model to establish a connection between two o more processes. With stream sockets, a process (running program) establishes a connection to another process.

A socket can hold two streams: one input stream and one output stream when the sender sends data to another process through the network by writing to the output stream associated with the socket, the receiver reads data written by the sender by reading from the input stream associated with the socket. While the connection is in place, data flows between the processes in continuous stream.

Addressing the connection

When you make a network connection, you need to know the address or the name of the remote machine, IPv4 uses 32 bit binary addresses to identify communicating hosts a client must specify the IP address of the host running the server program when it initiates the communication.
In addition, a network connection requires a port number, after you connect to the proper host, you must identify a particular purpose for the connection. So, you can use a particular port number to communicate with the specific program.

Port Numbers

In TCP/IP port numbers are 16-bit numbers and the values range from 0-65535. In practice, port numbers below 1024 are reserved for predefined services, and you should not use them unless communicating with one of those services, client port numbers are allocated by the operating system to something not in use, while server port numbers are specified by the programmer, and are used to identify a particular service.
Both client and server must agree in advance on which port to use. If the port numbers used by the two parts of the system do not agree, communication does not occur.
Ports are divided into the following ranges:

  • 0-1023: well-known ports
  • 1024-49151: registered ports
  • 49152-65535: dynamic or private ports
Minimal TCP/IP Server

The System.Net.Sockets.TcpListener class is used to create server applications. TcpListener provides a simple wrapper around the socket classes, here is a simple TCP Server that accepts a single client connection.

Example 1: Implementing a network streaming server

To compile this program using mono, type the following command at your command prompt:
$ mcs /t:exe -out:SimpleTCPServer.exe SimpleTCPServer.cs
And to run it, type this command line:
$mono SimpleTCPServer.exe
And you’ll see the following output on your screen:
Fig 1: Result of running the SimpleTCPServer program.
Minimal TCP/IP Client

The client is a simple console application, this application uses the TcpClient class to connect to the running server, and receives the returned message.

Example 2: Implementing a client

To compile this program using mono, type the following command at your command prompt:
mcs /t:exe -out:SimpleTCPClient.exe SimpleTCPClient.cs
And to run it, type this command line:
$mono SimpleTCPClient.exe
And you’ll see the following output on your screen:
Fig 2: Result of running the SimpleTCPClient program.

Download the SimpleTCPServer source code.

Download the SimpleTCPClient source code.

User Identification and Authentication with Transact-SQL

Doing user authentication in SQL Server can be customized; you can use all kinds of data from a database to authenticate users. Every application needs to deal with security, making sure that sensitive data cannot be accessed by the wrong users.
You can write your own custom logic to verify user names and passwords and make sure the information is stored.

Fig 1. The database diagram.

In the database under a secure account with a password that couldn’t easily be guessed by a user. The easiest way to accomplish this is to one-way encrypt user passwords on store procedure.

A simple but fully functional example is shown below.

Fig 2. The T-SQL code.

This code will insert one row, corresponding to the new user, in the users table.

The SQL Server way to store passwords is by wrapping them in a built-in encrypting function called HASHBYTES .

Download example source code.

Oracle Recipe #5 How to execute Oracle parameterized commands with ODP.NET

SQL statements can receive input-only parameters, output-only parameters, and bidirectional parameters.
You can use a OracleCommand object to execute parameterized SQL statements.
To execute a parameterized SQL statement use the following steps:

  1. Open a database connection,use OracleConnection.
  2. Create and initialize an OracleCommand object.
  3. Create a OracleParameter object, for each input parameter required by the SQL statement. Specify the name, type size, and value for each parameter, and add it to the parameters collection of the command object.
  4. Execute the command by calling the ExecuteScalar, ExecuteReader, ExecuteXmlReader, or ExecuteNonQuery method, as appropriate for the type of SQL statement.
  5. Use the return value obtained by executing the command.
  6. Dispose the command object.
  7. Close the database connection.

The following example shows how to execute a SQL statement that updates employee by employee id (please, check this post for further information).
The SQL statement requires the following parameters: prmFirstName , prmLastName, prmEmail, prmPhoneNumber, prmHireDate, prmSalary,prmCommission and prmEmployeeId.

Fig 1. The application code.

Fig 2. Running the program.

Download example source code.

Implementing a RESTFul service with Windows Communication Foundation (WCF) and Oracle HR Schema

What Are RESTful Web Services?

REST stands for Representational State Transfer is an architectural style rather than a prescribed way of building Web services, some of the most important aspects of the REST environment are:

  • HTTP or HTTPS may be used as the transfer protocol.
  • URLs including query strings are used to address resources.
  • Representation formats supported range from HTML and XML to JSON and ATOM.
  • A Simple and intuitive programming interface is achieved by using HTTP verbs and status codes.
  • Statelessness in the interaction between clients and services.

REST is not concerned with the definition of messages and the design of methods, the key point here is that REST describes a stateless, hierarchical scheme for representing resources and business objects over a network.
The main components of this model are: resources and actions.
The action of the resource is determined by four main HTTP verbs: GET, PUT, DELETE and POST, and the action which can affect those resources are mainly CRUD (Create, Read, Update and Delete) methods, the success of the action is found by the HTTP status code.

The REST model relies on the application that accesses the data sending the appropriate HTTP verb as part of the request used to access the data.

  • GET is used exclusively to retrieve data and, therefore, the result can also be buffered.
  • POST is used to add new records.
  • PUT is used to add or change a resource.
  • DELETE is used for delete resources.

The data can be returned in a number of formats, but for portability the most common formats include XML (POX) and JSON.

WCF and REST

The REST architecture is becoming increasingly common, and WCF provides attributes, methods, and types with which you can build and access REST Web Services quickly and easily.

  • WebHttpBinding: An binding that uses the HTTP transport and text message encoder.
  • WebBehavior: This is an endpoint behavior that will modify the dispatch layer on all operations on a contract. The modifications cause messages to be dispatched to methods on your service based on URIs and HTTP verbs.
  • WebServiceHost: This is a ServiceHost-derived class that simplifies the configuration of a web-based service.
  • WebOperationContext: This is a new context object, which contains the state of the incoming request and ongoing response, and simplifies coding against HTTP using WCF.
  • WebGetAttribute/WebInvokeAttribute: Operation behaviors that are applied as attributes on a ServiceContract’s methods. WebGetAttribute is for GET verb and WebInvokeAttribute is for all the other verbs. It also tells the dispatcher how to match the methods to URIs and how to parse the URI into method parameters.

The following table shows the properties of both WebGetAttribute and WebInvokeAttribute.

Method The HTTP verb the method should respond to.
UriTemplate The definition of the URI the CLR method should respond to.
RequestFormat Enumeration that specifies the format for deserializing the request (Xml or Json).
ResponseFormat Enumeration that specifies the format for serializing the response (Xml or Json).
BodyStyle Enumeration that specifies whether the request and the response data should be wrapped in an element with the same name as the CLR method name. Bare is typically used with RESTful services.

The essential components to construct a REST Service with WFC can be found in System.ServiceModel.Web assembly.
However, the most important part of the process is designing the schema that you will use to provide access to the resources exposed by the service.
So the main idea behind REST is to design your URIs in a way that makes logical sense based on your resource set. The URIs should, if possible, make sense to the application that consumes the data.

Depending on the volume of data in the database, a query might retrieve a large number of items, therefore, it makes sense to provide additional query parameters that a user can specify to limit the number of items returned.

Implementing a simple RESTful Service Example with WCF and Oracle.

In this example, we will develop a WCF RESTful service by using Oracle HR Sample Schema, ODP.NET, ADO.NET and Visual Studio 2015.
You can learn about the HR Schema in this post, I have written to introduce you to this schema.
The following illustration shows the components in the Employee service that I have written for this post.

Fig 1. Components of our Employee RESTFul service.
Step 1:Write the POCO object.
Fig 2. Employee entity class.
Step 2: Write the following utility class.
Fig 3. Utility class.
Step 3: Write the following helper class adding the Oracle Data Provider for .NET.
Fig 4. DAC data access class.
Step 4:Define and write the service contract.
Fig 5. Service contract interface.
Step 5: Write the service implementation class.
Fig 6. Service implementation class.
Step 6: Write the following Employee.svc file. Use the WebServiceHost class, the WebServiceHost class inherits from ServiceHost and automatically assigns the correct binding and behavior to your endpoint. You no longer need to be concerned about the content of your configuration file.
Fig 7. EmployeeService.svc File.
Step 7: Write the configuration file and store the connection string for the Oracle HR schema.
Fig 8. Configuration File.

In this example a GET at http://localhost/WcfRest/EmployeeService.svc/Employees shows all the employees in the HR database.
Fig 9. Running the example, querying all the employees.

Here a GET at http://localhost/WcfRest/EmployeeService.svc/102 show only one employee with the ID 102.

Fig 10. Querying only one employee.
Also, a GET at http://localhost/WcfRest/EmployeeService.svc/110 show the employee with the ID 110.
Fig 11. Querying another employee.

Download example source code.