IMPLEMENTING OBJECT COLLABORATION in Font

Generating Data Matrix in Font IMPLEMENTING OBJECT COLLABORATION

Public Class User ... Public Sub Logon(ByVal UserName As String, _ ByVal UserPassword As String) 'Code to check logon credentials 'if successful . . . UserLog.IncrementUserCount() End Sub . . . End Class Although you may not use shared properties and methods often when creating the classes in your applications, they are useful when creating base class libraries and are used throughout the .NET Framework system classes. The following code demonstrates the use of the Compare method of the System.String class. This is a shared method that compares two strings alphabetically. It returns a positive value if the first string is greater, a negative value if the second string is greater, or zero if the strings are equal: Public Function CheckStringOrderAscending(ByVal String1 As String, _ ByVal String2 As String) As Boolean If String.Compare(String1, String2, True) >= 0 Then Return True End If End Function

In this activity, you will become familiar with the following: Creating and calling shared methods of a class Using structured exception handling in VB

To create the shared methods, follow these steps: 1. Start VS. Select File New Project. 2. Choose a Windows Application project. Name the project Act8_3. 3. A default form is included in the project. Add controls to the form and change the property values, as listed in Table 8-3. Your completed form should look similar to Figure 8-4.

4. Select Project Add Class. Name the class Logger. 5. Because you will be using the System.IO class within the Logger class, add an Imports statement to the file: Imports System.IO Public Class Logger . . . End Class 6. Add a shared LogWrite function to the class. This function will write information to a log file. To open the file, you will create a FileStream object. You will create a StreamWriter object to write the information to the file. Public Shared Function LogWrite(ByVal LogFilePath As String, _ ByVal LogInfo As String) As String Dim oFileStream As FileStream = _ New FileStream(LogFilePath, FileMode.Open, FileAccess.Write) Dim oStreamWriter As StreamWriter = _ New StreamWriter(oFileStream) oFileStream.Seek(0, SeekOrigin.End) oStreamWriter.WriteLine(Now) oStreamWriter.WriteLine(LogInfo) oStreamWriter.WriteLine()

oStreamWriter.Close() Return "Info logged" End Function 7. Open frmLogger in the code editor. In the right object drop-down list, choose btnLogInfo. In the left drop-down list, choose the click event. Add the following code, which runs the LogWrite method of the Logger class and displays the results in the form s text property. Note that because you designated the LogWrite method as shared (in step 6), the client does not need to create an object instance of the Logger class. Shared methods are accessed directly through a class reference. Private Sub btnLogInfo_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles btnLogInfo.Click Me.Text = Logger.LogWrite(txtLogPath.Text, txtLogInfo.Text) End Sub 8. In the Solution Explorer, select the project node. Right-click the project node and select Properties. In the Property Pages dialog box, change the startup object to frmLogger. 9. Select Build Build Solution. Make sure there are no build errors in the Error List window. If there are, fix them, and then rebuild. 10. Select Debug Run. When the form launches, click the Log Info button. You should get an unhandled exception message of type System.IO.FileNotFoundException. Stop the debugger.

To create the structured exception handler, follow these steps: 1. Open the Logger class code in the code editor. 2. Locate the LogWrite method and add a Try-Catch block around the current code. In the Catch block, return a string stating the logging failed: Try Dim oFileStream As FileStream = New FileStream _ (strLogFilePath, FileMode.Open, FileAccess.Write) . . . rest of code Catch Return "Logging failed!" End Try 3. Select Build Build Solution. Make sure there are no build errors in the Error List window. If there are, fix them, and then rebuild. 4. Select Debug Run. When the form launches, click the Log Info button. This time, you should not get the exception message because it was handled by the LogWrite method. You should see the message Login Failed! in the form s caption. Close the form.