Binding Data to Individual Controls on a Form in .NET

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Another kind of data binding that you will commonly want to do is to bind properties on data items in a collection of data to properties on individual controls on a form As mentioned earlier, this form of data binding is called simple data binding To perform simple data binding, you use the DataBindings collection that every control inherits from theControl base class implementation of IBindableComponent The DataBindings property exposes a collection of Binding objects, each of which represents an individual association between a data member within a data source and a property on the control itself A straightforward example of using simple data binding is to bind individual columns within a table to the Text property of TextBox or Label controls on a form, so the presented text in those controls shows the values from the current row of data in the table If we extend the example shown in Figure 31 and add a set of text boxes to the form below the grid using the designer, we can add data binding so those text boxes will show the contents of some of the columns of the current row in the Customers table (see Listing 34)
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Listing 34 TextBox Data Bindings
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private void AddTextBoxDataBindings(CustomersDataSet nwData) { DataTable table = nwDataCustomers; m_CustomerIDTextBoxDataBindingsAdd("Text", table, "CustomerID", true); m_CompanyNameTextBoxDataBindingsAdd("Text", table, "CompanyName", true); m_ContactNameTextBoxDataBindingsAdd("Text", table, "ContactName", true); m_ContactPhoneTextBoxDataBindingsAdd("Text", table, "Phone", true); }
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This code adds a new entry in theDataBindings collection for each of a series of text boxes on the form The DataBinding property exposes a reference to an object of type ControlBindingsCollection This collection class exposes an overloadedAdd method that allows you to set up each data binding in a single line of code by providing the four parameters shown in Listing 34 This method constructs a Binding object under the covers and passes the parameters to its constructor, and then adds the Binding object to the DataBindings collection for the control
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The first parameter to the Add method is the name of the property on the control that you want to bind to; it can be any of the public properties exposed on the control In Listing 34, all of the bindings are created against the Text property of the TextBox controls The second parameter is the data source containing the data you are binding to, and could be one of the many forms of data collections discussed in the Data Sources section of 1 It can also be a reference to a single data object In this code example, the data source is the same for each of the bindings The code first stores the table reference in a local variable of type DataTable, and then passes that local variable as the data source argument to the Add method
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This document was created by an unregistered ChmMagic, please go to http://wwwbisentercom to register it Thanks The third parameter to the Add method in Listing 34 is the name of the data member within the data source This parameter can take on a number of forms, depending on what the data source is and where the data member can be found within that data source This code example just specifies the column name, because the data source is the table that contains that column The last parameter, which is set to true for each of the bindings, is a flag to indicate whether to automatically format the displayed values The next chapter discusses automatic and custom formatting in detail, but in general this should always be turned on
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If you run the SimpleNorthwindClient sample application from the download code, you will see the application shown in Figure 31 with the text boxes and a combo box added The data is bound to the grid, combo box, and text boxes using code similar to that shown in Listing 32, 33, and 34 If you click on different rows in the grid, you'll see that the content of the text boxes also automatically updates to reflect the contents of the bound fields in the data set based on the row that was selected If you change the contents of one of the fields displayed in the text boxes and click on a different row in the grid, you'll see that the changes you entered in the text box automatically change the contents of the underlying data source and update the grid display of that same data Wow! How the heck did that happen We didn't write any code to keep the data in the grid and the data in the text boxes synchronized, or to propagate changes from the UI controls back into the underlying data source Well, that is part of the "magic" of the two-way complex data-binding mechanisms in Windows Forms As mentioned earlier, when you set up a simple data binding between a control property and a data member within a data source, the control property would be set to the current item's bound property value Any changes made to the bound control property (Text in the case of the text boxes) will also be automatically pushed into the underlying data source when focus leaves the control Things are a little more complex for a grid, but the same basic process happens if you think of each cell within the grid being a control itself You might be wondering how the controls determine what the current item is within the collection This is discussed later in this chapter, but the short answer is that something called a CurrencyManager for each list-based data source keeps track of what the current item is within the collection The bound controls know about their own currency manager and can subscribe to an event raised by the currency manager when the current item changes When that notification occurs, the bound controls can update their presentation of the data based on the contents of the new current item
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