Respond to Errors and Unexpected Conditions

Transcription

1 Respond to Errors and Unexpected Conditions Callahan Chapter 7 Practice Time Artie s List Loose Ends ensure that frmrestaurant s module has Option Explicit modify tblrestaurant field sizes Restaurant - Text(50) StreetAddress - Text(40) City - Text(20) State - Text(2) Zip Text(10) Phone Text(12) modify bound controls for those fields frmrestaurant change ScrollBars property to None save frmrestaurant 1

2 Error Handling Concepts General Strategy trap the error (intercept it) determine which error occurred use VB statements to take appropriate corrective action or present a helpful message to user Error-handling should be present in each VBA function, sub, or event handler to handle VBA runtime errors each Form/Report Error event to handle ACE/Jet runtime errors Responding to the Error Event Error event a Form/Report-level event fires when a run-time error occurs when a form/report has the focus, before Access displays a default error message triggered by database engine errors not by VB run-time errors by executing an Form_Error procedure when an error event occurs, you can display a custom message and/or take corrective action DataErr argument an integer indicating which error occurred 3314 = field s Required property violated (we deleted FirstName) 3022 = duplicate Primary Key value 3200 = referential integrity violation child records not cascading delete 3316 = Validation Rule was violated 3058 = table's primary key left null 3201 = referential integrity violation foreign key violation Help Topic: Trappable Microsoft Jet and DAO Errors 2

3 Responding to the Error Event Response argument controls whether Access displays its default error message can be set to either of the following constants acdataerrcontinue ignore the error and continue without displaying the default error message acdataerrdisplay (Default) Display the default error message Private Sub Form_Error(DataErr As Integer, Response As Integer) ' if error is because of name, display custom error message End Sub Const conerrfieldrequired = 3314 If DataErr = conerrfieldrequired Then DisplayMessage "You must fill in both First Name and Last Name." Response = acdataerrcontinue Else Response = acdataerrdisplay 'display standard error message End If Practice Time Open Artie s List frmrestaurant try to delete a restaurant that has existing cuisines - error 3200 add VB code to present an alternate error message when error 3200 occurs Try to add a new cuisine that duplicates an existing cuisine for that restaurant the subform throws an error. which error number? build code to present an alternate message only when that error occurs test that it works correctly save frmrestaurant 3

4 Handling Any Possible Error Examples so far have only handled a single error should be able to handle any error condition explicitly handle it yourself via VB code let Access handle it for you Guided Tour Open Error Handling Demos.mdb Open frmwriter (no error handling) notice first record uses Writerid C200 start a new record enter your last name, try to save record, notice natural error message enter your first name, try to save record, notice natural error message enter Writerid C200, try to save record, notice natural error message enter Writerid C222, save record (success) edit your Freelancer to abc, press [Tab], notice error message, enter Yes edit your Amount to abc, press [Tab], notice error message, enter 100 delete your new record Handling Any Possible Error Select Case statement an ideal structure to handle multiple cases Case Else handles all cases you don t explicitly handle Guided Tour Open frmwriter (with error handling) start a new record enter your last name, try to save record, notice the custom message enter your first name, try to save record, notice the custom message enter Writerid C200, try to save record, notice the custom message enter Writerid C222, save record (success) edit your Freelancer to abc, press [Tab], notice custom message, enter Yes edit your Amount to abc, press [Tab], notice custom message, enter 100 delete your new record switch to Design View, examine the code that handles the form s Error event notice it contains nested SELECT CASE structures set a breakpoint, then re-do the steps above and watch it handle each error 4

5 Responding to a Combo Box Error Combo Box = List Box + Text Box list box makes it easy for a user to make a selection text box allows user to enter a value LimitToList property No (Default) Access accepts any text that conforms to the field s ValidationRule property Yes any entered text that doesn't match a listed item is rejected Access will either processes your NotInList event handler (if present) display its default error message (unless your NotInList event handler tells it not to) the user must either type an entry, select a listed item, press ESC, or Undo Responding to a Combo Box Error NotInList event occurs when combo box s Limit To List property is set to Yes and user enters a value not in the list NewData argument a string Access uses to pass the text the user entered to the event handler Response argument indicates how your code handled the NotInList event acdataerrdisplay (Default) Access displays the default message to the user. Use this when you don't want to allow the user to add a new value to the combo box list. acdataerradded Access doesn't display a message but enables your code to add the entry to the combo box. Access updates the list by requerying the combo box, than saves the entry in the field the combo box is bound to. acdataerrcontinue Access doesn't display the default message to the user. You can use this when you want to display a custom message to the user without adding the entry to the list. 5

6 Responding to a Combo Box Error Let Users Add a Contact Type to the List Automatically Private Sub ContactTypeID_NotInList(NewData As String, Response As Integer) ' ask the user whether to add a value to the list Dim strmessage As String Dim dbscontacts As DATABASE Dim rsttypes As Recordset strmessage = "Are you sure you want to add '" & NewData & _ "' to the list of contact types?" If Confirm(strMessage) Then ' open the Contact Types table and add the NewData value Set dbscontacts = CurrentDb() Set rsttypes = dbscontacts.openrecordset("contact Types") rsttypes.addnew rsttypes!contacttype = NewData rsttypes.update Response = acdataerradded Else Response = acdataerrdisplay End If End Sub 'requery the list 'display the error this seemed a poor choice DAO (Data Access Objects) Enable you to programmatically manipulate database objects DBEngine object represents the Jet database engine contains and controls all other objects in the DAO hierarchy Database object represents an open database CurrentDB( )function a pointer to the current database 6

7 DAO (Data Access Objects) Recordset object an in-memory set of records from a table or from running a query used to programmatically add/edit/delete data in one or more tables AddNew method creates a new record in a recordset Update method saves change to the recordset s current record Types of Recordsets used here Dynaset a dynamic set of records you can use to add, change, or delete records Snapshot Set a static copy of a set of records, can't be updated used to have an object variable point to an actual object in memory Set rsttypes = dbscontacts.openrecordset("contact Types") Practice Time Modify frmrestaurant to provide a user the option to add a new price category rename the control cbopricecategoryid start an event procedure for NotInList event adjust any existing code references (none) launch another instance of Access, open Contacts.mdb, copy the code for Contacts form s ContactTypeID_NotInList paste the code into cbopricecategoryid_notinlist adapt the code to this situation (next slide) save frmrestaurant test the new code edit an existing restaurant s price category, change it to Unknown click No when asked whether you d like to add it as a new price category press [Tab] to leave, then click Yes when asked whether you d like to add it as a new price category open the table and verify that Unknown appears as a prioce category 7

8 Practice Time a better choice? Creating Error Handling Routines Errors can occur while VBA code executes General Structure of an error handling routine at top of the procedure use an On Error GoTo label to set the trap at bottom of the procedure include a block that determines which error occurred, then take corrective action or provide an informative message. Include Resume statement(s) to control execution flow. include an Exit statement to ensure the error handling block is not executed when no error occurs Notice this procedure has single point of exit single point of entry 8

9 Creating Error Handling Routines On Error Statement Indicates where to transfer control when an error occurs while VBA code is executing in the error handler, you can determine which error occurred and address it in the manner that you choose when a runtime error occurs while executing a procedure that does not have its own error handling, VB looks up the call stack for an error handler in the caller On Error Goto label labelindicates the location of the code that handles the error the label must be in the current procedure On Error Resume Next causes execution to continue with the statement following the statement that caused the run-time error On Error GoTo 0 disables the procedure s error handler If you don't have an enabled error handler, a run-time error is fatal execution stops and an error message is displayed Creating Error Handling Routines Resume Statements Used within your error handling code to indicate where execution should continue Resume execution resumes at the same statement that caused the error use if your code can determine and reliably fix the problem careful you run the risk of an endless loop! Resume Next execution resumes at the statement after the one that caused the error can be dangerous, but may make sense if the offending statement wasn t critical to the success of the remaining statements Resume label execution resumes at the specified label 9

10 Creating Error Handling Routines The Err Object Contains details about the most recent runtime error when a run-time error occurs, the Err object properties are filled with information that identifies the most recent error and its description Error object properties Err.Number Err.Description Error object methods Err.Clear clears the Err object after an error has been handled sufficiently VBA automatically calls the Clear method whenever any type of Resume statement (Resume, Resume Next, Resume label) Exit Sub, Exit Function, Exit Property any On Error statement Err.Raise number use to generate a specific error eg: to test your error handling code Handling Events that Occur When Your Application is Read-Only Forced an error by changing RecordsetType from Dynaset to Snapshot Private Sub Form_KeyDown(KeyCode As Integer, Shift As Integer) ' If key is Ctrl+1, enter the values for New York On Error GoTo Err_Form_Keydown Const conerrnotupdatable = 3326 If KeyCode = 49 And Shift = 2 Then ' Ctrl+1 was pressed City = "New York" StateOrProvince = "NY" Country = "USA" End If Exit_Form_KeyDown: Exit Sub Err_Form_Keydown: If Err.Number = conerrnotupdatable Then DisplayMessage "Can't edit record; data is read-only." Else DisplayMessage Err.Description End If Resume Exit_Form_KeyDown End Sub 10

11 Creating Error Handling Routines Error Handling Demos.mdb Doubler return type is Variant returns doubled value returns Null if an error occurs added some error handling Reciprocal argument is a Variant return type is Variant returns numeric reciprocal returns Null if an error occurs can handle a few errors Practice Time Artie s list frmrestaurant Control Wizards event handlers do have error handling modify Save Record_Click as illustrated add modest error handling to Form_AfterInsert copy/paste modest error handling to Form_AfterUpdate Form_Current Form_Undo cborestauranttodisplay_afterupdate frahaswebsite_afterupdate cmdeditrecord_click PermitChanges cbopricecategoryid_notinlist Form_Error (late addition, after webcast recorded) use Debug > Compile to compile modules test your form s functionality add/edit/delete restaurant records 11