McGraw-Hill © 2010 The McGraw-Hill Companies, Inc. All rights reserved.

Chapter 5

Windows Database Updates

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Objectives

• Update a database table in a grid and in individual controls

• Use the BindingSource properties for navigation and for determining the current record number

• Update the original data source by saving the values from a dataset

• Validate user input in an update program

• Update related tables

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A Simple Update in a Grid - 1

• A DataGridView object includes features that allow the user to update the dataset

• Records can be added or deleted

• The user can modify data in any row in the grid

• All changes are made in the dataset (the in-memory copy of the data)

• When the user clicks the Save button, the BindingNavigator’s SaveItem event is fired and an attempt is made to save the data back to the original data source

• The update code is automatically generated by the designer when a BindingNavigator is added to a form

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A Simple Update in a Grid - 2

• A dataset is a temporary set of data in memory

• Changes can be made to the rows of data in the

dataset

– Changes are not automatically sent back to the

original data source

• TableAdapter serves as a go-between for the

data source and the dataset

– Execute the Update method of the TableAdapter or

the UpdateAll method of the TableAdapterManager to

send changes to the original data source

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A Simple Update in a Grid - 3

• TableAdapter retrieves data from the data source to create the dataset and sends back changes to the data source

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Database Handling in

the Visual Studio IDE

• First time a program is run in the debugger, the database file is copied into the bin\Debug folder and is the file used by the program

• By default, the file’s Copy to Output Directory property is set to Copy always

• For database updates to show up from one run to the next, select the filename in the Solution Explorer – Change the setting for Copy to Output Directory to

Copy if newer

– The debugger checks the file version and copies the file when it is newer or did not exist

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The Data Objects,

Methods, and Properties - 1

• To write more sophisticated update

programs, the various methods and

properties of data objects must be

understood

See Table 5.1 page 191 Overview of Useful Data Methods and Properties

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The Data Objects,

Methods, and Properties - 2

• A DataSet object can consist of multiple

tables

• Each table can consist of multiple data

rows

– Data values are stored in data rows

– A row’s RowState property indicates whether

changes have been made to the row

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The Data Objects,

Methods, and Properties - 3

DataRowState Enumeration Values

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The HasChanges Method

• Call the HasChanges method to determine if any changes have been made to a dataset, returns a Boolean value

If PubsDataSet.HasChanges() Then

‘ Ask the user to save the changes. End If

• Specific types of changes can be checked for using an overloaded version of the HasChanges method

If PubsDataSet.HasChanges(DataRowState.Deleted) Then ‘ Code to handle the deletion(s). End If

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The GetChanges Method

• Use to retrieve the rows that have changes

• Use an empty argument to retrieve all changed

rows OR specify type of changes using

enumeration values Dim EmployeeChangesDataSet As DataSet

EmployeeChangesDataSet = PubsDataSet.GetChanges()

• Create a dataset to hold all rows marked for

deletion Dim EmployeeDeletesDataSet As DataSet

EmployeeDeletesDataSet = PubsDataSet.GetChanges(DataRowState.Deleted)

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The Edit Methods

• To modify a row of data, the row must be in edit

mode

• Edit methods are called automatically for bound

controls

• BeginEdit method executes when an edit begins

• EndEdit method executes when an edit

terminates

• CancelEdit method returns field values to their

original values

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DataRow Versions

• The DataRow object maintains several versions of its data

– Current, Original, Default, and Proposed

• If no changes have been made, the Current and Original versions are the same

• When EndEdit executes, the Current version is replaced by the Proposed version

• The EndEdit method confirms the changes

• Changes are made when the AcceptChanges method executes

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The AcceptChanges Method - 1

• Removes all rows marked for deletion

• Makes the adds and edits indicated for the table

• Sets the Original version of each changed row to the Current version

• Sets the RowState of each row to Unchanged

• Clears any RowError information and sets the HasErrors property to False

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The AcceptChanges Method - 2

• The AcceptChanges method commits all changes to the dataset

• The RejectChanges method rolls back all changes that have been made by replacing Current versions with the Original versions

• After AcceptChanges or RejectChanges executes, all RowState properties are reset to Unchanged

• The dataset is disconnected so changes are made to the dataset and not to the original data source – Execute the TableAdapter’s Update method or the

TableAdapterManager’s UpdateAll method before calling the AcceptChanges method

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The TableAdapter and

TableAdapterManager - 1

• Updates can be performed after every change or upon program termination

– Combine the two techniques by providing a Save option

– Prompt for unsaved changes when program terminates

• TableAdapterManager manages updates to multiple related tables

• TableAdapter updates single tables

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The TableAdapter and

TableAdapterManager - 2

• Update considerations

– Where does the application and data reside

– How many users can make changes

– Does the data source need to be up-to-date

at all times

• To prompt for unsaved changes, place

update method in the form’s FormClosing

event handler

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The BindingSource Object

• The BindingSource object manages table

data for bound controls

– Controls the record position within a table

– Assures all bound controls on a form display

data from the same record

– Uses properties to determine current record

and for navigation

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Binding Source

Properties and Methods

• Displaying the record number

– The Position property holds the current row

number

• Add 1 to the Position property, since it is zero

based

– Count property indicates number of records in

a table

– Current property retrieves the current row

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Navigating Using

BindingSource Methods

• MoveFirst moves to the first row

• MoveNext moves to the next row

• MoveLast moves to the last row

• MovePrevious moves to the previous row

– Do not try to move to a record beyond the last

record or before the first record

• Last record is at a position of Count minus one

• First record is at position zero

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Binding Source Update Methods

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Binding Source Events

• Two useful events for the BindingSource

class

– CurrentChanged event occurs when a bound

value is changed

– PositionChanged event occurs when a user

navigates to another record

• The PostionChanged event handler is a

good place to display the current record

number in a label or the status bar

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DataSet Updating

• ADO.NET handles the complicated process of

updating the original data source

• Each row in a table has a RowState property

– Can be set to Unchanged, Modified, Added, or

Deleted

• Execute the Update method

– All of the indicated changes from the dataset are

made in the original data source for all rows that have

a RowState other than Unchanged

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SQL Statements for Updates

• When a data source is added, several

SQL statements are generated

– SELECT, INSERT, DELETE, and UPDATE

statements are created by default

• Examine the CommandText properties of

the DeleteCommand, InsertCommand,

and UpdateCommand properties of the

TableAdapter

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Concurrency

• Concurrency problems can occur when more than one user can update a file at the same time

• Concurrency control is the process of handling conflicts in updates by multiple users

• Three types of concurrency controls in ADO .NET – Pessimistic—A row is unavailable from the time the

record is retrieved until the update is complete

– Optimistic—A row is unavailable only while an update is in progress (the default)

– “Last in wins”—A row is unavailable only when the update is being made

• No checks are made for multiple changes to the same record

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Testing Update Programs

• Many types of errors may be encountered when testing an Add or Update in an update program

• Must have proper rights to the database to allow writing to the data source

• Be aware of constraints – Fields that contain nulls, required fields, specific

values in fields

• Include exception handling for all statements that access the database

• Display the exception message to help determine the cause of any problems

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Updating a DataSet in

Bound Controls

• Using bound individual controls is more common than using a grid

• Display dataset fields in bound text boxes so users can type in changes

– Keep text boxes set to Read Only unless an Add or Edit is in progress

• All of the techniques for updating a dataset apply equally to an Access database and a SQL Server database

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The Logic of an Update Program

• Update program needs procedures to

modify existing records (editing records),

delete records, and add new records

– Call the Update method after every change so

that the data source is up-to-date

– Enclose all statements that access the

dataset in Try/Catch blocks

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User Options during an Update

• Limit the options available to users during

an update

– While Add or Edit is in progress, the only

options should be Save or Cancel

– While navigating from one record to another,

do not allow changes to data

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Adding a Record

• Logic for an Add operation is more

complicated than other operations

• User clicks an Add button

– Program must clear and unlock text boxes

and allow the user to enter data for the new

record

– Record navigation must be disabled

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Pseudocode for an

Add Operation

• Call the BindingSource’s AddNew method, which begins the Add and clears the text boxes

• Set AddingBoolean to True

• Set the focus to the first text box

• Disable navigation

• Set the text boxes’ ReadOnly Property to False

• Set up the buttons for an Add: – Set up the Save and Cancel buttons

– Disable the Edit button

– Display “Adding” in the status bar

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The Edit Logic

• Display data fields in bound text boxes

• Set the ReadOnly property of each text box to

true, locking the text box

• For bound check boxes, lists, and grids set the

Enabled property to false

• Disable navigation so users cannot move off the

record and automatically save the changes

• The only choices a user should have during an

Edit are Save or Cancel

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Pseudocode to Begin an Edit

• Set EditingBoolean to True

• Disable navigation

• Set the text boxes’ ReadOnly property to

False

• Set up the buttons for an Edit:

– Set up the Save and Cancel buttons

– Disable the Edit button

– Display “Editing” in the status bar

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Navigating from a

Combo Box Selection - 1

• During navigation using a combo box, the

text box of the combo box must not allow

changes to the displayed value

– Combo box text binding should be set to

DataSourceUpdateMode.Never

– Change the binding at run time using bang

notation

NameComboBox.DataBindings!text.DataSourceUpdateMode = _

DataSourceUpdateMode.OnValidation

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Navigating from a

Combo Box Selection - 2

• During an Add or Edit a user must not be allowed to make a new selection from the combo box

– Must allow text to be typed in the text portion of the combo box

– Set the DropDownStyle to Simple during an Add or Edit

– After a Save or Cancel change the combo box’s DropDownStyle back to DropDownList

– Handle the Escape key in code

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Validating User Input Data

• As users enter data, some fields need to be validated – Constraints, such as required fields or the data type

• There may be business rules for validating data

• Validating data before sending it back to the database can reduce the number of round trips between the application and the database

• Perform field-level or record-level validation in the code for the form or the dataset – Recommended practice is to place it in the dataset

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Checking for Nulls

• A common problem for programming database updates relates to null fields

• If the user does not enter data in a required field and the record is sent to the database, an exception occurs if that field does not permit nulls

• Check to see which fields allow nulls in two ways – In the DataSet Designer click on each individual field and view

the AllowDBNull property

– View settings for an entire table by using the Server Explorer

• Select View/Server Explorer to see the window

• Expand the connection node and find the name of the table

• Right-click on table name and select Open Table Definition

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Adding Validation to a

Details View Program

• Use the Validating event of bound controls to validate user data – Set Cancel button’s CausesValidation property to

false • User can cancel an edit

– In a control’s Validating event handler, set e.Cancel = True for bad data

– Display an error message to the user using the ErrorProvider control

• Allow the user to close an application, even when a validation error holds the focus in a control

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Adding Validation to the DataSet

for a DataGridView Program

• Write validation code inside the dataset

class (preferred approach)

– Keeps the validation logic separate from the

form’s logic

• Write event handlers for events of the data

table

– ColumnChanged and TableNewRow events

– Handle the Escape key

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Handling Data Exceptions

• Never good for a program to cancel with a data

exception

– Prevent data input errors with validation

– Catch other errors with exception handling

• BindingSource for each table has a DataError

event

– Fires when an attempt is made to save bad data

• DataGridView has a DataError event

– Helps identify errors in the data in the grid

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The DataGridView

DataError Event

• Catch an error in the data for a

DataGridView

– Display a message and icon in the current

row

– RowIndex property of the EventArgs for the

DataError event handler identifies the row in

error

• Set e.Cancel = True to hold the user on the current

row

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The BindingSource

DataError Event

• Catch an error in data displayed in details

view

– Write code for the BindingSource’s DataError

event

– Specific information not given regarding

location of the error

• Write a generalized message in a message box or

in an error provider

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Updating Related Tables

• The TableAdapter.Update method issues the proper INSERT, DELETE, and UPDATE SQL commands for a single table – If updating multiple tables with parent and child

relationships, the programmer must make sure the commands are executed in the correct sequence

• TableAdapterManager component greatly simplifies updating related tables – Generated automatically when a table or field is

dragged to a form and a new BindingSource is created

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Parent and Child Relationships

• To add a new child and parent record, add the parent first

– Only way to set relationship for child record

– Cannot add child records for a parent record that has not yet been created

• To delete a parent record, all child records must be deleted first

– A parent record cannot be deleted if there are associated child records in existence

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Cascading Deletes and Updates

• Help to maintain

referential

integrity

• Set up the

relationship for

cascading

updates and

cascading deletes

in the Relation

dialog box

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Hierarchical Updates

• The TableAdapterManager.UpdateAll

method issues the correct updates to the

original data source in the correct order

– Sends all inserts (parent, then child) first

– Sends all updates (parent, then child) next

– Sends all deletes (child, then parent) last

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A Related-Table Update Program –

Step-by-Step - 1

• User can add, delete, and edit store

records (the parents) and add, delete and

edit sales records (the child records)

• Data source is updated when Save or

FormClosing event handlers are executed

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A Related-Table Update Program –

Step-by-Step - 2

• Modify the tables’ relationships

• Create the data-bound controls

• Modify the generated code

• Commit added parent records before

adding child records

• Add the save query for exit

• Run the application

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A Related-Table Update Program –

Step-by-Step - 3

• Help the user enter valid data

• Add exception handling

• Add a table lookup column for the titles

• Add validation to the dataset

• Run the application

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Security Considerations

• Do not provide a user with information that could

be used to violate database security

– Do not use actual field names in error messages

– Use descriptive, friendly names for the data items

• The practice of displaying ex.Message in a

Catch clause is useful for testing and debugging

a program

– It should not appear in a production program because

it may contain actual field names