ADONET Programming in Visual Studio .NET

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You accomplish this synchronization of versions and changing of states by calling the DataRow class s AcceptChanges method The DataSet and DataTable classes also have AcceptChanges methods, which operate by cascading the AcceptChanges call down to the individual rows Never call the AcceptChanges method just prior to updating the database The class used for transferring data from a data table to the database, the data adapter class, examines the rows status first Data in rows that have their status marked as Unchanged do not cause updates to the database (See The SqlCeDataAdapter Class subsection later in this chapter) Having mentioned the Current, Original, and Default versions of a data row, we need to say a word about the fourth version, the Proposed version The Proposed version is used to provide transaction commit and rollback capabilities and exists only during the life of a transaction
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6212 Working with Data Rows
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As mentioned earlier, data rows contain the actual data, which can be accessed or modified by specifying the row and the column within the row Columns can be specified by either name or index Data rows have methods for accessing other rows based on data relations that have been defined in the data set Two of the most common methods are GetChildRows and GetParentRow For example, the code in Listing 62 creates a relationship between the Categories and Products tables and then retrieves an array of all rows containing all products in the Widgets category, assuming that the dsetDB data set already contains the Categories and Products tables
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Listing 62: Creating and Using a DataRelation Object
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// // Define the relationship between Products and Categories data tables
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dsetDBRelationsAdd( "FKProdCat", Tables["Categories"]Columns["CategoryID"], Tables["Products"]Columns["CategoryID"], true); // // // Select the Widgets row in the Categories data table Use it to retrieve all Widgets rows from the Products data table
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dsetDBTables["Categories"]
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62 Working with Data Sets
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Select("CategoryName = 'Widgets'")[0] GetChildRows("FKProdCat"); End With
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6213 Introducing Data Views
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A data view is always a view of a single data table, and it provides only two capabilities: sorting and filtering rows A DataView object cannot be used to do joins, evaluate aggregate functions, or filter out columns It is not intended to behave like a SQL Server view Its purpose, instead, has to do with data binding Every data table has a DefaultView property When this property is first accessed, the DataView object is created, and its Filter and Sort properties are set to Stringempty So, its contents are, on first access, identical to the table s contents Thereafter, you can set the Filter and Sort properties to anything you want Or you can create a new view of a table, specifying the table, row selection criteria, sort sequence, and row status criteria, as in the following statement:
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new DataView(dsetTimeTrackerTables["Projects"], "strIdent = '17'", "dateStart ASC, dateEnd DESC", DataViewRowStateCurrentRows);
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Having discussed the DataSet object and its contained objects, it s time to cover moving data between the data set and the presentation tier in other words, data binding
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622 Data Binding Data binding is the ability to associate an object that holds data with a control that displays and updates that data We covered the subject in 4, but now we need to apply it to the DataTable and DataView classes specifically The following list summarizes what we covered in 4
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The DataGrid control can display the entire contents of a bound data
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object You bind to a DataGrid control by setting its DataSource property Unlike the desktop DataGrid control, the NET Compact Framework version is read-only We presented a workaround for this limitation in 4 The ListBox and ComboBox controls can display one column of a DataTable/DataView and can use one other column for an identifying key
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version is read-only That is, the user can select from the list of choices but cannot enter text into the ComboBox control Single-item controls, such as the TextBox, Label, RadioButton, and CheckBox, can bind to a single data element of a data object You can bind to a single-item control by adding entries to its DataBindings collection The DataBindings collection specifies the column to be bound to but not the row A current row of a DataTable/DataView is always used for binding Data binding is a two-way street Changes to data made in the control are automatically pushed back to the bound data table
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