Exercise $6-1$: Review the Invoices by Due Date application

In this exercise, you$$ll review the Invoices by Due Date application that$$s presented in this

week$$s chapter. That will give you a better idea of how it works.

$1.$ Open the DisplayInvoicesDue application that$$s in your Chapter $6$ directory.

$2.$ In the Solution Explorer, notice that this solution includes two projects:

DisplayInvoicesDue and PayablesData. Then, expand the PayablesData project

to see that it contains the business and database classes that are used by this

application: Invoice, InvoiceDB, and PayablesDB.

$3.$ Double$-$click on any one of the classes in the PayablesData project to review its

code.

$4.$ Display the code for the Invoices by Due Date form $($frmInvoicesDue$)$ and note the

using statement at the top of the file. For this to work, the application must include a

reference to the PayablesData project.

$5.$ To see the required reference, expand the References folder for the project in the

Solution Explorer. If you don$$t see this folder, select the project and then click the

Show All Files button at the top of the Solution Explorer window.

$6.$ Run the application to see how it works. Then, when you$$re done experimenting,

close the application.

Exercise $6-2$: Enhance the Invoices by Due Date application

In this exercise, you$$ll enhance the Invoices by Due Date application. This will give you a

chance to work with commands that return scalar values. When you$$re done, the form

for the application should look like this:

Enhance the interface

$1.$ Open the DisplayInvoicesDue application in your Chapter $6$ directory.

$2.$ Add a ToolStrip control to the top of the Invoices by Date Due form and add the

buttons, separators, label, and text box shown above to it$.$ Then, set the properties

for these controls.

$3.$ Add a label and text box below the ListView control as shown above. Then, set the

properties for these controls.

$4.$ Modify the code for the form so it displays the unpaid invoices for all vendors when

the user clicks on the Get All Invoices button in the toolbar instead of when the

form is loaded. To do that, you just need to move the code from the Load event

handler to the Click event handler for the Get All Invoices button. Or$,$ if you prefer,

you can copy the code from the Load event handler to the Click event handler and

then comment out the code for the Load event handler.

$5.$ Test this modification to see how it works

Add the code for getting the invoices for a single vendor

$6.$ Add a method named GetVendorlnvoicesDue to the InvoiceDB class that

gets the unpaid invoices for a single vendor. This method should have one

parameter that accepts a vendor ID as an integer, and it should return a list

of Invoice objects that represent the unpaid invoices for that vendor ID$.$ The

easiest way to code this method is to copy and modify the code for the

GetlnvoicesDue method. For now, you can hardcode the parameter that

contains the vender ID into the Select statement. Then, in the next chapter,

you'll learn how to work with parameter objects.

$7.$ Add an event handler to the code for the form that handles the Click event

of the Get Vendor Invoices toolbar button. The easiest way to do that is to

copy and modify the code for the Click event of the Get All Invoices button.

$8.$ Test and debug this enhancement until it works right. Try vendor IDs $99$

and $102,$ and make sure the ListView control is cleared before any invoices

are added to it$.$ To do that, you can use this command:

lvinvoices.Items.Clear$()$;

Add the code for getting the total balances due

$9.$ Add a method named GetTotalBalanceDue to the InvoiceDB class. It should

use an aggregate query like the one in the first example in figure $6-10$ to get

the sum of all the unpaid invoices.

$10.$ Add a method named GetVendorBalanceDue to the lnvoiceDB class. It

should have one parameter that receives a vendor ID$,$ and it should get the

sum of all of the unpaid invoices for that vendor. Again, you can hardcode

the vendor ID parameter into the Select statement.

$11.$ Add the code to the form class that uses the methods that you created in

steps $9$ and $10$ to display the total balance due for all unpaid invoices or just

one vendor's unpaid invoices when the buttons in the toolbars are clicked.

$12.$ Test and debug this enhancement until it works right