This chapter is from the book
This chapter is from the book
This chapter is from the book
Custom Exceptions
Implement error handling in the UI: Create and implement custom error messages.
Implement error handling in the UI: Create and implement custom error handlers.
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Use ApplicationException As Base Class for Custom Exceptions Although you can derive directly from the Exception class, Microsoft recommends that you derive any custom exception class you create for your Windows application from the ApplicationException class.
In most cases, .NET's built-in exception classes, combined with custom messages that you create when instantiating a new exception, will suffice your exception handling requirements. However, in some cases you need exception types specific to the problem you are solving.
The .NET Framework allows you to define custom exception classes. To make your custom Exception class work well with the .NET exception handling framework, Microsoft recommends that you consider the following when you're designing a custom exception class:
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Create an exception class only if no existing exception class satisfies your requirement.
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Derive all programmer-defined exception classes from the System.ApplicationException class.
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End the name of custom exception class with the word Exception (for example, MyOwnCustomException).
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Implement three constructors with the signatures shown in the following code:
Public Class MyOwnCustomException Inherits ApplicationException ' Default constructor Public Sub New() End Sub ' Constructor accepting a single string message Public Sub New(ByVal message As String) MyBase.New(message) End Sub ' Constructor accepting a string message ' and an inner exception ' that will be wrapped by this custom exception class Public Sub New(ByVal message As String, _ ByVal inner As Exception) MyBase.new(message, inner) End Sub End Class
Step By Step 3.4 shows how to create a custom exception.
STEP BY STEP 3.4 Creating a Custom Exception
Add a new Windows Form to your Visual Basic .NET project.
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Place two Label controls, a TextBox control named txtDate, and a Button control named btnIsLeap on the form. Figure 3.6 shows a design for this form. Name the empty Label control lblResults.
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Figure 3.6 Leap Year Finder.
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Add a new class to your project. Add code to create a custom exception class:
Public Class MyOwnInvalidDateFormatException Inherits ApplicationException ' Default constructor Public Sub New() End Sub ' Constructor accepting a single string message Public Sub New(ByVal message As String) MyBase.New(message) Me.HelpLink = _ "file://MyOwnInvalidDateFormatExceptionHelp.htm" End Sub ' Constructor accepting a string ' message and an inner exception ' that will be wrapped by this custom exception class Public Sub New(ByVal message As String, _ ByVal inner As Exception) MyBase.new(message, inner) Me.HelpLink = _ "file://MyOwnInvalidDateFormatExceptionHelp.htm" End SubEnd Class
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Add a second new class to your project. Add code to create a class named LeapDate:
Public Class LeapDate ' This class does elementary date handling for the ' leap year form Private day, month, year As Integer Public Sub New(ByVal strDate As String) If strDate.Trim().Length = 10 Then ' Input data might be in invalid ' format; in that case ' the Convert.ToDateTime method will fail Try Dim dt As DateTime = _ Convert.ToDateTime(strDate) day = dt.Day month = dt.Month year = dt.Year ' Catch any exception, attach ' it to the custom exception and ' throw the custom exception Catch e As Exception Throw New MyOwnInvalidDateFormatException( _ "Custom Exception: Invalid Date Format", e) End Try Else ' bad input, throw a custom exception Throw New MyOwnInvalidDateFormatException( _ "The input does not match the required " & _ "format: MM/DD/YYYY") End If End Sub ' Find if the given date belongs to a leap year Public Function IsLeapYear() As Boolean IsLeapYear = (year Mod 4 = 0) And _ ((year Mod 100 <> 0) Or (year Mod 400 = 0)) End Function End Class -
Add the following event handler for the Click event of btnIsLeap:
Private Sub btnIsLeap_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles btnIsLeap.Click Try Dim dt As LeapDate = New LeapDate(txtDate.Text) If dt.IsLeapYear() Then lblResult.Text = "This date is in a leap year" Else lblResult.Text = _ "This date is NOT in a leap year" End If ' Catch the custom exception and ' display an appropriate message Catch dte As MyOwnInvalidDateFormatException Dim msg As String ' If some other exception was also ' attached with this exception If Not dte.InnerException Is Nothing Then msg = String.Format( _ "Message:" & vbCrLf & "{0}" & _ vbCrLf & vbCrLf & "Inner Exception:" & _ vbCrLf & "{1}", _ dte.Message, dte.InnerException.Message) Else msg = String.Format("Message:" & _ vbCrLf & "{0}" & _ vbCrLf & vbCrLf & _ "Help Link:" & vbCrLf & "{1}", _ dte.Message, dte.HelpLink) End If MessageBox.Show(msg, dte.GetType().ToString()) End Try End Sub -
Set the form as the startup object for the project.
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Run the project. Enter a date and click the button. If the date was in the correct format, you'll see a result displayed in the Results label. Otherwise, you'll get a message box showing the custom error message thrown by the custom exception, as shown in Figure 3.7.
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Figure 3.7 Custom error message thrown by the custom exception.
In Guided Practice Exercise 3.1 you'll create a custom defined Exception class that helps implement custom error messages and custom error handling in your programs.
GUIDED PRACTICE EXERCISE 3.1
Your goal is to create a keyword-searching application. The application should ask for a filename and a keyword and search for the keyword in the file. The application should then display the number of lines that contain the keyword. The application assumes that the entered keyword will be a single word. If not, you must create and throw a custom exception for that case.
Try this on your own first. If you get stuck or would like to see one possible solution, follow these steps:
Add a new form to your Visual Basic .NET Project.
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Place three Label controls, two TextBox controls, and two Button controls on the form, arranged as shown in Figure 3.8. Name the TextBox for accepting filenames txtFileName and the Browse button btnBrowse. Name the TextBox for accepting keywords txtKeyword and the search button btnSearch. Name the results label lblResult.
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Figure 3.8 Form allowing a keyword search in a file.
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Add an OpenFileDialog control to the form and change its name to dlgOpenFile.
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Create a new class named BadKeywordFormatException that derives from ApplicationException and place the following code in it:
Public Class BadKeywordFormatException Inherits ApplicationException ' Default constructor Public Sub New() End Sub ' Constructor accepting a single string message Public Sub New(ByVal message As String) MyBase.New(message) End Sub ' Constructor accepting a string ' message and an inner exception ' that will be wrapped by this custom exception class Public Sub New(ByVal message As String, _ ByVal inner As Exception) MyBase.new(message, inner) End Sub End Class -
Create a method named GetKeywordFrequency in the form's module. This class accepts a filename and returns the number of lines containing the keyword. Add the following code to the method:
Private Function GetKeywordFrequency( _ ByVal strPath As String) As Integer If Me.txtKeyword.Text.Trim().IndexOf(" ") >= 0 Then Throw New BadKeywordFormatException( _ "The keyword must only have a single word") End If Dim count As Integer = 0 If File.Exists(strPath) Then Dim sr As StreamReader = _ New StreamReader(txtFileName.Text) Do While (sr.Peek() > -1) If sr.ReadLine().IndexOf(txtKeyword. _ Text) >= 0 Then count += 1 End If Loop End If GetKeywordFrequency = count End Function -
Add the following code to the Click event handler of the btnBrowse button:
Private Sub btnBrowse_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles btnBrowse.Click If dlgOpenFile.ShowDialog() = DialogResult.OK Then txtFileName.Text = dlgOpenFile.FileName End If End Sub -
Add the following code to the Click event handler of the btnSearch Button:
Private Sub btnSearch_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles btnSearch.Click If txtKeyword.Text.Trim().Length = 0 Then MessageBox.Show( _ "Please enter a keyword to search", _ "Missing Keyword") Exit Sub End If Try lblResult.Text = String.Format("The keyword: '{0}'" & _ "was found in {1} lines", _ txtKeyword.Text, _ GetKeywordFrequency(txtFileName.Text)) Catch bkfe As BadKeywordFormatException Dim msg As String = _ String.Format("Message:" & vbCrLf & _ "{0}" & vbCrLf & vbCrLf & "StackTrace:" & _ vbCrLf & "{1}", _ bkfe.Message, bkfe.StackTrace) MessageBox.Show(msg, bkfe.GetType().ToString()) End Try End Sub -
Set the form as the startup object for the project.
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Run the project. Click the Browse button and select an existing file. Enter a keyword to search for in the file and press the Search button. If the keyword entered is in wrong format (contains a space) then the custom exception will be raised.