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Exceptions When Things Go Wrong
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In the last section, you programmed events for things that you expect to happen to your object. Trains are expected to change location as they drive along, but sometimes things happen that aren t expected. In .NET programming, these exceptional situations are handled using exceptions, a programming construct for handling error conditions. A program is said to throw an exception when errors occur. You can write code to catch the exception, so that execution of your program doesn t stop completely. Additionally, you can create custom exceptions for your application to provide specific information to your program and your user about what has gone wrong. Generate an exception This small application demonstrates an exception being thrown. 1. Create a new Windows application and name it ThrowSystemException. 2. Add a Button control to the form. 3. Double-click the Button control to create the Click event handler, and add the following code, which attempts to access the tenth integer in an array of five integers. 4. 5. 6. 7. 8. 9. 10. 11. // Visual C# 12. private void button1_Click(object sender, System.EventArgs e) { 13. 14. int[] numbers = { 1, 2, 3, 4, 5}; MessageBox.Show(numbers[9].ToString()); Visual Basic Private Sub Button1_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles Button1.Click Dim numbers() As Integer = {1, 2, 3, 4, 5} MessageBox.Show(numbers(9)) End Sub
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} 15. Press F5 to run the application and click the Button control. The following message box is displayed. Click Break, and then click Stop Debugging from the Debug menu to stop the program.
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System.IndexOutOfRangeException is thrown because the index, 9, is out of range of the array, which is 0 through 4. You can prevent error messages from popping up like this by trapping the exceptions using exception handling. 5. Modify the code for the Click event as you see here: 6. Visual Basic 7. Private Sub Button1_Click(ByVal sender As System.Object, _ 8. ByVal e As System.EventArgs) Handles Button1.Click 9. Dim numbers() As Integer = {1, 2, 3, 4, 5} 10. Try 11. MessageBox.Show(numbers(9)) 12. Catch ex As Exception 13. MessageBox.Show("Something went wrong: " & ex.Messag e) 14. End Try 15. End Sub 16. 17. // Visual C# 18. private void button1_Click(object sender, System.EventArgs e) { 19. int[] numbers = { 1, 2, 3, 4, 5}; 20. try { 21. MessageBox.Show(numbers[9].ToString()); 22. } 23. catch (Exception ex) { 24. MessageBox.Show("Something went wrong: " + ex.Message ); 25. } } 26. Press F5 to run the program. In this case, no system error message appears on the screen, and you don t have the opportunity to choose whether to quit or continue. When you catch an exception this way, the code you write is responsible for that decision. Writing Your Own Exception Class You can generate exceptions using the throw keyword (Throw in Visual Basic, throw in Visual C#). The .NET convention is to throw objects that derive from the System.Exception class. More specifically, applications should throw objects that derive from System.ApplicationException, which itself derives from System.Exception. The following small application shows how you can derive an exception class, throw the exception under the right conditions, and catch the exception using a try block. This application uses a Person class with FirstName and LastName properties. The class provides one constructor that expects the name in First Last format. Should the constructor parameter not follow that format, the constructor will throw a NameFormatIncorrectException. Create the project 1. Create a new Windows Application and name it PersonList. 2. Drag a ListBox control onto the form. Set the Name property to personList and the Sorted property to True. 3. Drag a TextBox control onto the form. Set the Name property to personsName and the Text property to (blank ).
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Drag a Button control onto the form. Set the Name property to addPerson and the Text property to Add.
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Create the exception class 1. Right -click Form1 and click View Code on the shortcut menu. 2. Add the following code after the Form1 class to declare the exception class. 3. 4. 5. 6. 7. 8. 9. } The ApplicationException class has two properties of interest. The first is the Message property, which contains a string that describes the error that has occurred. The second is the Inner property. If you re throwing an exception because you caught an exception, you can pass on that exception in the Inner property. 10. Add this code for the overloaded constructors: 11. Visual Basic 12. Public Sub New() 13. 15. 16. Public Sub New(ByVal message As String) 17. 19. 20. Public Sub New(ByVal message As String,_ 21. ByVal innerException As Exception) 22. 24. 25. // Visual C# 26. public NameFormatIncorrectException() : base() { 27. } 28. 29. public NameFormatIncorrectException(string message) : base(m essage) { 30. } 31. 32. public NameFormatIncorrectException(string message, 33. Exception innerException) : MyBase.New(message, InnerException) 23. End Sub MyBase.New(Message) 18. End Sub MyBase.New() 14. End Sub // Visual C# public class NameFormatIncorrectException : System.Application Exception { Visual Basic Public Class NameFormatIncorrectException Inherits System.ApplicationException End Class
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34. 35. }
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base(message, innerException) {
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The exception classes in .NET have three constructors, all of which can call a matching base constructor. One is parameterless, and its message is blank. The second takes one exception, the message text. The third sets both the message text and the inner exception. The exception class is complete. Create the Person class 1. Add the following code to declare the Person class after the NameFormatIncorrectException class and add the FirstName and LastName properties. 2. Visual Basic 3. Public Class Person 4. Private m_first As String 5. Private m_last As String 6. 7. Public Property FirstName() As String 8. Get 9. Return m_first 10. End Get 11. Set(ByVal Value As String) 12. m_first = Value 13. End Set 14. End Property 15. 16. Public Property LastName() As String 17. Get 18. Return m_last 19. End Get 20. Set(ByVal Value As String) 21. m_last = Value 22. End Set 23. End Property 24. End Class 25. 26. // Visual C# 27. public class Person { 28. private string m_first; 29. private string m_last; 30. 31. public string FirstName { 32. get { return m_first; } 33. set { m_first = value; } 34. } 35. 36. public string LastName { 37. get { return m_last; } 38. set { m_last = value; } 39. } 40. } 41. Add the following code for the overridden ToString method. This property is used to display the Person class instance in the ListBox control. 42. 43. 44. Visual Basic Public Overrides Function ToString() As String Return m_last & ", " & m_first
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45. 46. 47. 48. 49.
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End Function // Visual C# public override string ToString() { return m_last + ", " + m_first;
} 50. Add the constructor that takes a name in First Last format and parses it into the FirstName and LastName properties: 51. Visual Basic 52. Public Sub New(ByVal firstlast As String) 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 65. 66. // Visual C# 67. public Person(string firstlast) { 68. 69. 70. ; 71. 72. 73. 74. 75. 76. 77. } Add the code for the user interface 1. Open the form in the designer and double-click the Add button to create the Click event. 2. Add the following code to add a new person to the list. You can have multiple catch blocks in a try block so that you can capture specific types of exceptions. 3. Visual Basic } } catch (Exception ex) { throw new NameFormatIncorrectException("Cannot find the first " + "name and last name in the string: " + firstlast, ex); m_first = names[0]; m_last = names[1]; try { string splitCharacters = " "; string[] names = firstlast.Split(splitCharacters.ToCharArray()) Try Dim splitCharacters As String = " " Dim names() As String = _ firstlast.Split(splitCharacters.ToCharArray()) m_first = names(0) m_last = names(1) Catch ex As Exception Throw New NameFormatIncorrectException( _ "Cannot find the first name and last name in the string: " _ & firstlast, ex) End Try
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