Using Constructors with Object Oriented Database Development with VB.NET 2005
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This is the fourth article in a series on developing object oriented database applications using Visual Basic.NET 2005. In this article, I shall deal with the concept of “constructors.”
A downloadable file for this article is available
here.
For this article, I assume that you understand enough of the basics of working with VB.NET controls, ADO.NET and so forth using Visual Basic.NET 2005. Even though this article only gives you the basics of OOP together with database development, I shall extend it in the form of a series to cover the most advanced topics in Visual Basic.NET 2005. If you are very new to OOP in VB.NET, I request that you go through my first article in this series.
The entire source code for this article is available in the form of a downloadable zip. The solution was developed using Microsoft Visual Studio 2005 Professional Edition with Microsoft SQL Server 2005 Developer Edition on Microsoft Windows Server 2003 Enterprise Edition. Even though I believe that the source code available with this contribution can work with Microsoft Visual Studio.NET 2003/2002, I didn't really test it in any other environment. I request that you post in the discussion area if you have any problems in execution.
To make this article simple, I created a sample database named "sample," with a table "emp" containing the columns empno (string), ename (string), sal (double) and deptno (integer) and a few rows.
Defining constructors in a class
I already introduced fields/methods/properties in my previous articles. Now, we are about to deal with a new concept called "constructors." A constructor is a special type of method that has the name "new." A constructor is executed automatically when an object of the respective class is created.
Let us go through the following code:
Public Class Emp
Private m_empno As String
Private m_ename As String
Private m_sal As Double
Private m_deptno As Integer
Private m_errMsg As String
Public Sub New()
m_empno = ""
m_ename = ""
m_sal = 0
m_deptno = 0
m_errMsg = ""
End Sub
...
End Class
The above class contains a method named "New," which is called a constructor. It is automatically executed for every object created from that class. A constructor generally contains all the statements necessary for initialization. In this scenario, it may not be essential.
Some developers use constructors to read connection strings from XML files. Some use them to open files or streams. It all depends on the needs of the application.
Next: Defining constructors with parameters in a class >>
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