CT862 Section 1 Introduction

Transcription

1 CT862 Section 1 Introduction VB.NET: VB introduced in 1991 Provided the first easily accessible means of writing Windows applications o The VB IDE was inherently graphical and mirrored the Windows OS itself Allegedly the world s most widely used language, c.3-5 million users Unrivalled as a RAD environment for business applications Simpler to use than other environments At the forefront of component-based development, acting as a glue language o OCX/ActiveX (COM Component Object Model) o DLLs o Windows API VB.NET is a total re-write of VB6 The VB.NET development environment.net programs execute on a virtual machine called the.net CLR (Common Language Runtime) This is so that a.net program can be run on any computer provided a version of CLR is implemented for it

2 CLR supports many programming languages not just Visual Basic CLR has a number of useful features for programmers Common Type System o A range of variable types that represent all of the common types of computer data o Numbers, dates, times, characters, text, currency etc. Since all.net languages use it, all are directly compatible with each other o Can use a.net class in a C# program etc. o.net provides genuine language independence: you can use your preferred language Data types are independent of the computer or operating system used VB.NET can be used to write: Windows console applications Standard windows applications Windows services Windows controls Web (ASP.NET) applications

3 Web services Web controls.net classes Applications that access application object models (e.g. MS Word, MS Excel) using COM Creating a VB.NET program Start Visual Studio and choose File > New > Project from the menu Then choose the VB language and Windows App. Template

4 The main components of the VB Integrated Development Environment (IDE): Run Program Solution Explorer Form Properties Toolbox Forms consist of visual objects (controls) and programmed behaviour (Basic Code), typically defining how the form and its control respond to events such as Click

5 A First VB.NET application (Temperature Converter) Form Labels Text boxes for entering and displaying text

6 1. Create the interface by selecting controls from the toolbox and dragging them out on the form (2 labels and 2 text boxes) 2. Set the properties of the controls, using the properties window: Resize the form and change its Text property to Converter Position the labels and change their Text properties to Celsius and Fahrenheit Position the text boxes and change their Name properties to txtcels and txtfahr 3. Write code to define the behaviour when events happen on the interface as the user does things Double-click the Celsius text box and the code view will automatically open The following line of Basic code, entered into the txtcels_textchanged event handler function, performs the conversion to Fahrenheit and updates the Fahrenheit text box Note the use of the Text property of the text boxes: this is a readable and writable property Public Class Form1 Private Sub txtcels_textchanged(byval sender As System.Object, ByVal e As System.EventArgs) Handles txtcels.textchanged txtfahr.text = (txtcels.text * 9 / 5) + 32 End Sub End Class Question: why is the TextChanged event a dangerous one to use in this case? consider what happens if we add a similar event handler for the txtfahr text box We should instead use the KeyPress event. Choose the Class (object) and the Method (event handler) from the drop-down lists at the top of the code window

7 Class List List of Methods (Event Handlers) For current object Using the KeyPress event, we can add an extra test so that the conversion happens only when the Return key (Ascii code 13) is pressed In fact, this is a very good idea since the KeyPress event happens before the latest keypress is added to the content of txtcels. Try it without the additional If statement and you ll see why! Private Sub txtcels_keypress(byval sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtcels.keypress If Asc(e.KeyChar) = 13 Then txtfahr.text = (txtcels.text * 9 / 5) + 32 End If End Sub Here s the corresponding code for the KeyPress event handler of the txtfahr text box:

8 Private Sub txtfahr_keypress(byval sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtfahr.keypress If Asc(e.KeyChar) = 13 Then txtcels.text = (txtfahr.text - 32) * 5 / 9 End If End Sub The concepts we have seen here include: Use of the toolbox Use of the properties window The Text property and Name property The TextChanged and KeyPress event handlers of the Text Box control The code window and its Class List and Methods List Console Applications versus Windows Applications Console programs follow an Input > Process > Output model easy to program although a lot of effort to make intuitive/user-friendly for the user: Input Process Output By contrast, a Windows program can have many different forms of input Programmer must orchestrate the various sources Each source of input generates events the program must react to This is event-driven programming It can take some time to learn the exact usefulness and timing of the various events

9 VB is a verbose language Software Event Handlers Each source of events can cause the execution of a block of code called an event handler An event handler is simply a Sub (equivalent to a C function that returns no value) that is called automatically by Windows in response to some external interaction o Typing into a text box control etc. o Clicking on a control o Selecting an item in a list Often, the event incorporates some additional information which key was pressed, the exact location of the mouse cursor etc. This information is passed in the form of parameters to the event handler (note the receipt of the actual key that was pressed in the KeyPress event handler we used above)