Tài liệu Implement the Methods That Update the Database pptx

9.5 Implement the Methods That Update the Database Now that you have a class that can retrieve a specific row from the database, you will need to implement a method that will save change

9.5 Implement the Methods That Update the Database

Now that you have a class that can retrieve a specific row from the database, you will need to implement a method that will save changes to the database

You'll also need a method to insert new rows into-as well as delete existing rows from-the Customers table

Technique

In the case of updating and deleting rows in the database, you will need to implement the Save and Delete methods that are defined in the ICustomer interface

To insert new rows into the database, you'll need to add additional constructors to the class Having multiple methods with the same name is a new feature in Visual Basic .NET called overloading In this example, you will only overload constructors, but you can also overload functions and subs

Steps

The first task is to implement the Save and Delete methods that were defined in your interface in section 9.1 You have already set up all the database-access objects you will need, so it's just a matter of adding the relevant code

1 You will need two helper methods for the Save and Delete methods: a method whose sole responsibility is to call the update method of the data adapter, and a method that clears the properties of the object Both methods are defined in Listing 9.26

Listing 9.26 frmHowTo9_5.vb: The WriteChangesToDB and Clear Methods

Private Function WriteChangesToDB (ByVal pStateToHandle _

As DataRowState) As Boolean

Try

' Create a dataset of only those rows that have been

' changed (specifically, those changed in the manner

' specified by the DataRowState)

Dim dsChanges As DataSet = mdsCust.GetChanges(pStateToHandle)

' Pass this subset of the dataset to the data adapter

' to write the changes to the database The data adapter

' will handle all calls to the Delete, Insert, and Update

' commands

odaCustomers.Update(dsChanges)

Catch ex As Exception

' If the update fails, communicate this back to

' the calling function by returning False

mdsCust.RejectChanges()

Return False

End Try

' Update the customer's dataset to reflect the changes

' you have made

mdsCust.AcceptChanges()

Return True

End Function

Private Sub Clear()

' Consumers of this class should be savvy enough to dispose

' of the object after they call the Delete method, but

' to be sure, let's set all the pointers to null, so if a

' developer forgets NullPointerException

mdsCust = Nothing

mdrCust = Nothing

mstrCustomerID = Nothing

mstrCompanyName = Nothing

mstrContactName = Nothing

mstrContactTitle = Nothing

mstrAddress = Nothing

mstrCity = Nothing

mstrRegion = Nothing

mstrCountry = Nothing

mstrPostalCode = Nothing

mstrPhone = Nothing

mstrFax = Nothing

End Sub

Adding code to insert new rows into the database requires a bit more work The first problem is that you have only one way to create an instance of the CCustomer class: by passing a CustomerID value to the constructor that retrieves an existing

row from the database You will need to add a second constructor to your class that allows you to create new Customer objects

Having two methods with the same name is called overloading In Visual Basic 6, you had limited support for overloading with properties Think about it: Properties are pairs of methods with the same name, except that one is a sub and accepts a parameter, whereas the other is a parameterless function that returns a value Each independent Visual Basic 6 function or sub, however, must have a unique name

Visual Basic NET lifts this restriction by allowing you to overload any method,

be it function, sub, or constructor For constructors, all you need to do is add another New sub (For functions and subs, you need to preface the declaration with the Overloads keyword.)

You can add as many overloaded methods as you like, as long as the parameter list

is different Be aware that NET determines which overloaded method to call-constructor or otherwise-based on the order of the datatypes in the parameter list, and not the names of the parameters In other words, you can have two overloaded methods that take an integer and a string as their parameters, as long as one

method has a string as the first parameter, and the other has an integer as the first parameter-like the first two methods in Listing 9.27 It doesn't matter what you name the parameter: Two overloaded methods with a single integer parameter but with different names-like the second pair of methods in Listing 9.27-are not

allowed

The name of the method, the return type, and the order of its parameters make up what is called the method signature When it comes to overloading, however, Microsoft only takes into account the argument signature In other words, the return type doesn't matter

Listing 9.27 Overloaded Methods: The Datatypes of the Parameters Are What Matter

'These two methods can be declared because the order

'of the datatypes differs If you do this, make sure

'your parameters have descriptive names Otherwise,

'your code can be confusing to other developers

Overloads Sub method1(ByVal IntegerValue As Integer,

ByVal StringValue As String)

Overloads Sub method1(ByVal StringValue As String,

ByVal IntegerValue As Integer)

'These two methods cannot be declared, even though the

'parameters have different names

Overloads Sub method2(ByVal IntegerValue As Integer)

Overloads Sub method2(ByVal AnotherIntegerValue As Integer)

Tip

You should always use Option Strict in applications in which you use overloaded methods Option Strict prevents you from

implicitly converting between datatypes when data might become lost as a result of the conversion

For example, if you have two overloaded methods-one that accepts a string and one that accepts an integer-without Option Strict, NET might get confused and convert the integer value to a string and call the wrong method With Option Strict, not only will this not happen, but you will not even be able to compile your code with an implicit type conversion

2 Now that you understand what an overloaded method is, add one more

constructor, as shown in Listing 9.28, that accepts the two required values for the Customers table: CustomerID and CompanyName You will also need to declare a private, class-level Boolean variable to track whether you have a new or existing record loaded into the class

Listing 9.28 frmHowTo9_5.vb: A New Constructor for Inserting New Rows into the Customers Table

Private mfNew As Boolean ' By default, this value will be false

Public Sub New(ByVal pCustomerID As String,

ByVal pCompanyName As String)

' This constructor is used to create new Customer records

' in the database

mdsCust = New dsCustomers()

mfNew = True

mstrCustomerID = pCustomerID

Me.CompanyName = pCompanyName

mstrContactName = ""

mstrContactTitle = ""

mstrAddress = ""

mstrCity = ""

mstrRegion = ""

mstrCountry = ""

mstrPostalCode = ""

mstrPhone = ""

mstrFax = ""

End Sub

Take a look at the first two lines of the constructor First, you need to make sure that an instance of the Customers dataset is created Second, you need some form

of flag to keep track of whether the instance is a new or existing row This

example will use a class-level Boolean variable called mfNew to fill this role

3 Next, implement a method that will take the values of your class' properties and write them to the data row, as shown in Listing 9.29

Listing 9.29 frmHowTo9_5.vb: Implementation of the Save Method

Private Sub WriteValuesToDataRow()

' This technique allows consumers to modify properties (class-

' level variables) but leaves the DataRow intact until the

' save method is called In essence, the DataRow holds the

' original state of the Customers row, while the class-level

' variables hold the current state This method synchronizes

' the two

With mdrCust

.CompanyName = mstrCompanyName

If mstrAddress.Length = 0 Then

SetAddressNull()

Else

Address = mstrAddress

End If

If mstrRegion.Length = 0 Then

Set_RegionNull()

Else

_Region = mstrRegion

End If

If mstrCountry.Length = 0 Then

SetCountryNull()

Else

Country = mstrCountry

End If

If mstrPostalCode.Length = 0 Then

SetPostalCodeNull()

Else

PostalCode = mstrPostalCode

End If

If mstrPhone.Length = 0 Then

SetPhoneNull()

Else

Phone = mstrPhone

End If

If mstrFax.Length = 0 Then

SetFaxNull()

Else

Fax = mstrFax

End If

If mstrContactTitle.Length = 0 Then

SetContactTitleNull()

Else

ContactTitle = mstrContactTitle

End If

If mstrContactName.Length = 0 Then

SetContactNameNull()

Else

ContactName = mstrContactName

End If

If mstrCity.Length = 0 Then

SetCityNull()

Else

City = mstrCity

End If

End With

End Sub

4 Now implement the Save method by pasting the code from Listing 9.30 into the class To make your code as easy to use as possible, have the Save method manage both new and existing ecords

Listing 9.30 frmHowTo9_5.vb: Implementation of the Save Method

Public Function Save() As Boolean Implements ICustomer.Save

Dim nState As DataRowState

If mfNew Then

' If this is a new Customer, you need to add a data row

' to the dataset

mdrCust = mdsCust.Customers.AddCustomersRow(mstrCustomerID, _ mstrCompanyName, mstrContactName, mstrContactTitle, _

mstrAddress, mstrCity,mstrRegion, mstrPostalCode, _

mstrCountry, mstrPhone, mstrFax)

nState = DataRowState.Added

mfNew = False

Else

nState = DataRowState.Modified

End If

' Begin editing the data row

mdrCust.BeginEdit()

' If you have a problem writing values to the data row, you want to

' catch the exception, cancel editing, reject the changes, and

' immediately dump out of the method with a return value of false

Try

writeValuesToDataRow()

Catch ex As Exception

mdrCust.CancelEdit()

mdrCust.RejectChanges()

Return False

End Try

' If you succeed in writing the new values to the data row,

' end the editing block, and then write the changes to the

' database If you have a problem writing to the DB, then

' reject the changes to the row and return false

mdrCust.EndEdit()

If WriteChangesToDB (nState) Then

Return True

Else

mdrCust.RejectChanges()

Return False

End If

End Function

5 Finally, implement the Delete method The Delete method flags the data row for deletion and then calls the WriteChangesToDB method with a DataRowState of Deleted Paste the Delete method from Listing 9.31 into your class

Listing 9.31 frmHowTo9_5.vb: Implementation of the Delete Method

Public Function Delete() As Boolean Implements ICustomer.Delete

If mfNew Then

' If you're working with a new record, you don't need to

' communicate with the database, so clear the properties

' of the object and return true to indicate that the delete

' succeeded

Clear()

Return True

Else

' If this is an existing record, flag the data row as

' deleted, then write the changes to the DB

mdrCust.Delete()

If WriteChangesToDB (DataRowState.Deleted) Then

Clear()

Return True

Else

mdrCust.RejectChanges()

Return False

End If

End If

End Function

6 To test this code, try creating a new CCustomer instance, set its properties, and save it to the database Then instantiate a new CCustomer instance, loading the new row into the object Make a change to that instance, and save the changes

back to the database Last, load that new row into a third CCustomer instance and delete it Add the code in Listing 9.32 to the click events of the Delete, Save, and New buttons of frmHowTo9_5 to test the new code in your class

Listing 9.32 frmHowTo9_5.vb: Testing the New Constructor and the Delete and Save Methods

Private Sub btnNew_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles btnNew.Click

Try

' These two properties are the minimum required to insert a new row

If txtCustomerID.Text.Length > 0 Then

If txtCompanyName.Text.Length > 0 Then

mCustomer = New CCustomer(txtCustomerID.Text,

txtCompanyName.Text)

Else

MsgBox("A company name is required for a new Customer.")

End If

Else

MsgBox("A CustomerID is required for a new Customer.")

Exit Sub

End If

GetProperties()

Me.rtbToString.Text = mCustomer.ToString

Catch ex As Exception

MsgBox(ex.Message)

End Try

End Sub

Private Sub btnSave_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles btnSave.Click

Try

If mCustomer.Save() Then

MsgBox("Save succeeded.")

Else

MsgBox("Save failed.")

End If

Catch ex As Exception

MsgBox(ex.Message)

End Try

End Sub

Private Sub btnDelete_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles btnDelete.Click

Try

If mCustomer.Delete() Then

MsgBox("Delete succeeded.")

mCustomer = Nothing

ClearAllTextBoxes()

Else

MsgBox("Delete failed.")

End If

Catch ex As Exception

MsgBox(ex.Message)

End Try

End Sub

How It Works

When a consumer of the CCustomer class instantiates an object using the new

constructor, the required properties of the object (the CustomerID and CompanyName) are set using the parameters of the constructor, whereas the optional properties are all set

to zero-length strings This constructor also sets an internal flag saying that the current instance is a new record When the Save method is called, the internal flag tells the class that a new data row should be added to the dsCustomer dataset Finally, the

WriteChangesToDB is called and a new row is inserted into the Customers table

If the Save method is called with an existing record, the internal flag lets the class know that the data row already exists in the dsCustomer dataset, so the WriteChangesToDB method is called to update that row in the Customers table