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21.9 Creating a Master/Detail View Application

The previous examples demonstrated using LINQ to SQL to combine data from different tables and displaying data from a database in a GUI. It is often necessary to combine the two, as the example in this section does. Figure 21.28 demonstrates a master/detail view—one part of the interface (the master) allows you to select an entry, and another part (the details) displays detailed information about that entry. In this example, you select either a book title or an author, and the details displayed are the co-authors of the book or the titles the author has written, respectively.

Fig. 21.28 Using a DataGridView to display details based on a selection.

 1  // Fig. 21.28: MasterDetailForm.cs
 2  // Using a DataGridView to display details based on a selection.
 3  using System;
 4  using System.Linq;
 5  using System.Windows.Forms;
 7  namespace MasterDetail
 8  {
 9     public partial class MasterDetailForm : Form
10     {
11        public MasterDetailForm()
12        {
13           InitializeComponent();
14        } // end constructor
16        // connection to database
17        private BooksDataContext database = new BooksDataContext();
19        // this class helps us display each author's first
20        // and last name in the authors drop-down list
21        private class AuthorBinding
22        {
23           public Author Author { get; set; } // contained Author object
24           public string Name { get; set; } // author's full name
25        } // end class AuthorBinding
27        // initialize data sources when the Form is loaded
28        private void MasterDetailForm_Load( object sender, EventArgs e )
29        {
30           // display AuthorBinding.Name
31           authorComboBox.DisplayMember = "Name";
33           // set authorComboBox's DataSource to the list of authors
34           authorComboBox.DataSource =
35              from author in database.Authors
36              orderby author.LastName, author.FirstName
37              let name = author.FirstName + " " + author.LastName
38              select new AuthorBinding { Author = author, Name = name };
40           // display Title.BookTitle
41           titleComboBox.DisplayMember = "BookTitle";
43           // set titleComboBox's DataSource to the list of titles
44           titleComboBox.DataSource =
45              from title in database.Titles
46              orderby title.BookTitle
47              select title;
49           // initially, display no "detail" data
50           booksBindingSource.DataSource = null;
52           // set the DataSource of the DataGridView to the BindingSource
53           booksDataGridView.DataSource = booksBindingSource;
54        } // end method MasterDetailForm_Load
56        // display titles that were co-authored by the selected author
57        private void authorComboBox_SelectedIndexChanged(
58           object sender, EventArgs e )
59        {
60           // get the selected Author object from the ComboBox
61           Author currentAuthor =
62              ( ( AuthorBinding ) authorComboBox.SelectedItem ).Author;
64           // set booksBindingSource's DataSource to the
65           // list of titles written by the selected author
66           booksBindingSource.DataSource =
67              from book in currentAuthor.AuthorISBNs
68              select book.Title;
69        } // end method authorComboBox_SelectedIndexChanged
71        // display the authors of the selected title
72        private void titleComboBox_SelectedIndexChanged(
73           object sender, EventArgs e )
74        {
75           // get the selected Title object from the ComboBox
76           Title currentTitle = ( Title ) titleComboBox.SelectedItem;
78           // set booksBindingSource's DataSource to the
79           // list of authors for the selected title
80           booksBindingSource.DataSource =
81              from book in currentTitle.AuthorISBNs
82              select book.Author;
83        } // end method titleComboBox_SelectedIndexChanged
84     } // end class MasterDetailForm
85  } // end namespace MasterDetail

In the previous examples, the IDE automatically generated the BindingSource, BindingNavigator and GUI elements when you dragged a data source onto the Form. While this works for simple applications, those with more complex operations involve writing more substantial amounts of code. Before explaining the code, we list the steps required to create the interface the code manipulates.

Step 1: Creating the Project

Create a new Windows Forms Application called MasterDetail. Name the source file and class as indicated in Fig. 21.28, and set the Form’s Text property to Master/Detail.

Step 2: Creating LINQ to SQL Classes

Follow the instructions in Section 21.6.1 to add the Books database and create the LINQ to SQL classes to interact with the database.

Step 3: Creating GUI Elements

Add two Labels and two ComboBoxes to the top of the Form. Position them as shown in Fig. 21.29. The Label and ComboBox on the left should be named authorLabel and authorComboBox, respectively. The Label and ComboBox on the right should be named titleLabel and titleComboBox. Set the Text property of the Labels as indicated in the screenshot. Also change the DropDownStyle properties of the ComboBoxes from DropDown to DropDownList.

Figure 21.29

Fig. 21.29 Finished design of MasterDetail application.

Create a DataGridView called booksDataGridView to hold the details that are displayed. Unlike previous examples, do not automatically create it by dragging a data source from the Data Sources window, because this example sets the data source programmatically. Instead, manually add it from the Toolbox. Resize the DataGridView so that it fills the remainder of the Form. Because this control is only for viewing data, set its ReadOnly property to True using the Properties window.

Finally, we need to add a BindingSource named booksBindingSource from the Data section of the Toolbox, to act as an intermediary between booksDataGridView and booksDataContext. As in the previous examples, it does not appear as a visible component on the Form, but appears in the component tray. The BindingSource will be used as a data source for booksDataGridView—we will fill booksBindingSource with data from the Authors or Titles table, depending on which drop-down is selected. With the GUI creation complete, we can now write the code to provide the functionality we require.

The Master/Detail Application

When the code has been written, the Master/Detail application allows the user to select an author or book title from one of two drop-down lists, and view the details of the titles that author worked on or the authors of that book, respectively.

Class MasterDetailForm contains the nested class AuthorBinding (lines 21–25). Class definitions may be nested inside other classes. AuthorBinding is private because it should be accessed only from its containing class. We created the AuthorBinding class to allow us to associate a full Author object—the Author class is automatically generated by LINQ to SQL—with each row in the drop-down list, but have it display the author’s full name. Recall that the author’s name is stored as two separate fields in the database, so the Author class does not have single property that retrieves the full name. The Name property of the AuthorBinding class stores the author’s full name, and the Author property stores the Author object that contains the author’s information from the database.

The ComboBox’s DisplayMember property is set to the String "Name" (line 31), which tells the ComboBox to use the Name property of AuthorBinding to determine what text to display for each AuthorBinding object. Lines 34–38 create an AuthorBinding object for each author and assign it to the authorComboBox’s DataSource property. This makes authorComboBox create one entry per author. Recall from Section 10.17 that object initializers (e.g., line 38) can be used to initialize an object without calling a constructor.

There is no need to create a custom class to wrap the Title class, because it already has a property for what we want to display—the BookTitle property. The text in the ComboBox is set to be retrieved from the BookTitle property (line 41). Lines 44–47 create and assign the DataSource for titleComboBox—a sorted list of Title objects.

Initially, we don’t want to display any data in the DataGridView. However, when the ComboBoxes are created and initialized with their DataSources, their SelectedIndexChanged event handlers are called, setting a DataSource for booksBindingSource (connected to booksDataGridView on line 53). To prevent this, we explicitly set the DataSource property to null (line 50)—this is done before connecting the DataGridView to avoid loading data that will just be thrown away when the DataSource is reset.

Simple GUI elements like ComboBoxes can be populated from a data source—in this case, the result of a LINQ to SQL query. However, the DataGridView is more complex and requires an intermediary class (a BindingSource) as its DataSource. We can change the columns and data displayed in booksDataGridView merely by changing the DataSource property of booksBindingSourcebooksDataGridView will update the data and even column names automatically.

The authorComboBox_SelectedIndexChanged event handler (lines 57–69) performs two distinct operations. First, it retrieves the selected Author (lines 61–62). The ComboBox’s SelectedItem property returns an object, so the SelectedItem property’s value must be cast to the AuthorBinding class we used as the data source. The event handler then accesses the AuthorBinding’s Author property to retrieve the wrapped Author object. Second, the event handler uses LINQ to retrieve the Title objects representing books the Author worked on (lines 67–68)—the mechanism for this was explained in the preceding example. The results of the LINQ query are assigned to the DataSource property of booksBindingSource (line 66)—note that the event handler for authorComboBox sets the DataSource to a data from the Titles table because we want to display Title rows associated with that Author.

The event handler for titleComboBox is structured identically to the authorComboBox one. The primary difference is that the author and title names are switched. Also, the first step is slightly simpler, because the Title class did not need to be wrapped to display correctly in the ComboBox.

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