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10 Data Binding
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Note How can a data-bound control figure out which actual object it is bound to Will it be a collection, a data reader, or perhaps a DataTable
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All standard data-bound controls are designed to work only through the IEnumerable interface. For this reason, any object bound to DataSource is normalized to an object that implements IEnumerable. In some cases, the normalization is as easy (and fast) as casting the object to the IEnumerable interface. In other cases specifically, when DataTable and DataSet are involved an extra step is performed to locate a particular named collection of data that corresponds to the value assigned to the DataMember property. There s no public function to do all this work, although a similar helper class exists in the ASP.NET framework but is flagged as internal. What this helper class does, though, can be easily replicated by custom code: it just combines an array of if statements to check types and does casting and conversion as appropriate.
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The DataSourceID Property
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The DataSourceID property gets or sets the ID of the data source component from which the data-bound control retrieves its data. This property is the point of contact between databound controls and a special family of controls the data source controls that includes SqlDataSource and ObjectDataSource. (I ll cover these controls in more detail later in the chapter.)
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public virtual string DataSourceID {get; set;}
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By setting DataSourceID, you tell the control to turn to the associated data source control for any needs regarding data retrieval, paging, sorting, counting, or updating. Like DataSource, DataSourceID is available on all data-bound controls. The two properties are mutually exclusive. If both are set, you get an invalid operation exception at run time. Note, though, that you also get an exception if DataSourceID is set to a string that doesn t correspond to an existing data source control.
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The DataMember Property
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The DataMember property gets or sets the name of the data collection to extract when data binding to a data source:
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public virtual string DataMember {get; set;}
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You use the property to specify the name of the DataTable to use when the DataSource property is bound to a DataSet object:
var data = new DataSet(); var adapter = new SqlDataAdapter(commandText, connectionString); adapter.Fill(data);
Part II
ASP.NET Pages and Server Controls
// Table is the default name of the first table in a // DataSet filled by an adapter grid.DataMember = "Table"; grid.DataSource = data; grid.DataBind();
DataMember and DataSource can be set in any order, provided that both are set before DataBind is invoked. DataMember has no relevance if you bind to data using DataSourceID with standard data source components.
The DataTextField Property
Typically used by list controls, the DataTextField property specifies which property of a data-bound item should be used to define the display text of the nth element in a list control:
public virtual string DataTextField {get; set;}
For example, for a drop-down list control the property feeds the displayed text of each item in the list. The following code creates the control shown in Figure 10-2:
CountryList.DataSource = data; CountryList.DataTextField = "country"; CountryList.DataBind();
FIGURE 10-2 A drop-down list control filled with the country column of a database table.
An analogous behavior can be observed for other list controls, such as ListBox and CheckBoxList. Note List controls can automatically format the content of the field bound through the
DataTextField property. The format expression is indicated via the DataTextFormatString property.
10 Data Binding
The DataValueField Property
Similar to DataTextField, the DataValueField property specifies which property of a data-bound item should be used to identify the nth element in a list control:
public virtual string DataValueField {get; set;}
To understand the role of this property, consider the markup generated for a drop-down list, set as in the code snippet shown previously:
<select name="CountryList" id="CountryList"> <option selected="selected" value="[All]">[All]</option> <option value="Argentina">Argentina</option> <option value="Austria">Austria</option> ... </select>
The text of each <option> tag is determined by the field specified through DataTextField; the value of the value attribute is determined by DataValueField. Consider the following code that fills a ListBox with customer names:
CustomerList.DataMember = "Table"; CustomerList.DataTextField = "companyname"; CustomerList.DataValueField = "customerid"; CustomerList.DataSource = data; CustomerList.DataBind();
If DataValueField is left blank, the value of the DataTextField property is used instead. Here s the corresponding markup:
<select size="4" name="CustomerList" id="CustomerList"> <option value="BOTTM">Bottom-Dollar Markets</option> <option value="LAUGB">Laughing Bacchus Wine Cellars</option> ... </select>
As you can see, the value attribute now is set to the customer ID the unique, invisible value determined by the customerid field. The content of the value attribute for the currently selected item is returned by the SelectedValue property of the list control. If you want to access programmatically the displayed text of the current selection, use the SelectedItem.Text expression.
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