BindingContext in .NET

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BindingContext
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Text Binding Item("Cars") BindingManagerBase BackColor Binding DataBindings property ControlBindingCollection
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Figure 16-16 Data-binding objects
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Simple Data-Bound Form
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Let s see a complete example of a form with four controls, in which each control binds to one of the properties of the Employee class: FirstName, LastName, BirthDate, and PartTimeEmployee. (The code of the Employee class is omitted because of its simplic ity, but the code on the companion CD contains its complete definition.) The form is shown in Figure 16-17; it also contains all the buttons to navigate in the data source, which in this case is an array of Employee objects.
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Figure 16-17
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The Simple Data Binding demo form
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As you see in the following code, you create the individual Binding objects by using the DataBindings.Add method, which takes the name of the bound property, a refer ence to the data source (the Employees array in this case), and the name of the mem ber in the data source. The last argument is the name of a field when you re binding to a database table or the name of a property when you re binding to an object:
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This is the data source. Dim employees() As Employee = {New Employee( Joe", Doe", #1/3/1960#), _ New Employee( Robert", Smith", #11/23/1962#), _ New Employee( Ann", Ross", #2/5/1965#, True)} This is the BindingManagerBase object that manages all the bindings. Dim WithEvents bmb As BindingManagerBase Initialize the binding when the form loads. Private Sub SimpleBindingForm_Load(ByVal sender As Object, _ ByVal e As EventArgs) Handles MyBase.Load InitializeBinding() End Sub Create all the necessary bindings. Sub InitializeBinding() Save a reference to the BindingManagerBase object. bmb = Me.BindingContext(employees) Create the Binding object for each bound field. txtFirstName.DataBindings.Add( Text", employees, FirstName ) txtLastName.DataBindings.Add( Text", employees, LastName ) txtBirthDate.DataBindings.Add( Text", employees, BirthDate ) chkPartTime.DataBindings.Add( Checked", employees, PartTimeEmployee ) End Sub
Part IV:
Win32 Applications
The remainder of the code manages the actions performed when someone clicks on the navigational buttons. As you see, this code just changes the value of the Position property of the BindingManagerBase object and traps the PositionChanged event to display the correct record number:
Private Sub btnFirst_Click(ByVal sender As Object, _ ByVal e As EventArgs) Handles btnFirst.Click bmb.Position = 0 End Sub Private Sub btnPrevious_Click(ByVal sender As Object, _ ByVal e As EventArgs) Handles btnPrevious.Click bmb.Position -= 1 End Sub Private Sub btnNext_Click(ByVal sender As Object, _ ByVal e As EventArgs) Handles btnNext.Click bmb.Position += 1 End Sub Private Sub btnLast_Click(ByVal sender As Object, _ ByVal e As EventArgs) Handles btnLast.Click bmb.Position = bmb.Count - 1 End Sub Display current position in label. Private Sub bmb_PositionChanged(ByVal sender As Object, _ ByVal e As EventArgs) Handles bmb.PositionChanged lblRecord.Text = String.Format( {0} of {1}", bmb.Position + 1, bmb.Count) End Sub
When you bind to an IList object, you get two-way binding, and any new value the user enters in bound fields is assigned to the underlying data source. You can also support AddNew and Delete operations, even though you must implement them manually for example, by writing the code that adds or deletes elements in the source array.
Formatting Events
You can enforce much finer control over how values are transferred to and from the data source by using the two events that individual Binding objects fire. The Format event fires when data is transferred from the data source to the bound control, whereas the Parse event fires when data is moved back into the data source. The latter control fires only if the data in the control has been modified after the most recent Format event. Typically you take advantage of these events to change the display format of one or more values for example, to display data as uppercase but store it as lowercase. Or you might store currency values in a currency unit (such as U.S. dollars or Euros) and display the values in the unit selected by the end user. The demo application uses these two events to display the BirthDate value in dd-mm-yyyy format instead of the default long-date format. To trap the two events, you must modify the code in the InitializeBinding procedure:
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