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TableAdapters provide communication between your application and a database. More specifically, a TableAdapter connects to a database, executes queries or stored procedures, and either returns a new data table populated with the returned data or fills an existingwith the returned data. TableAdapters are also used to send updated data from your application back to the database.
Users of previous versions of Visual Studio can think of a TableAdapter as awith a built-in connection object and the ability to contain multiple queries. Each query added to a TableAdapter is exposed as a public method that is simply called like any other method or function on an object.
In addition to the standard functionality of a DataAdapter, TableAdapters provide additional typed methods that encapsulate queries that share a common schema with the associated typed DataTable. In other words, you can have as many queries as you want on a TableAdapter as long as they return data that conforms to the same schema.
In the previous version of Visual Studio,were used for communicating between an application and a database. While data adapters are still a main component of , TableAdapters are designer-generated components that improve upon the functionality of DataAdapters. TableAdapters typically contain Fill and Update methods to fetch and update data in a database.
TableAdapters are created with the Dataset Designer inside of strongly typed datasets. You can create TableAdapters during creation of a new dataset with the. You can also create TableAdapters in existing datasets with the or by dragging database objects from Server Explorer onto the Dataset Designer. For more information, see .
While TableAdapters are designed with the Dataset Designer, the TableAdapter classes generated are not generated as nested classes of the. They are located in a separate namespace specific to each dataset. For example, if you have a dataset named NorthwindDataSet, the TableAdapters associated with the DataTables in the NorthwindDataSet would be in the NorthwindDataSetTableAdapters namespace. To access a particular TableAdapter programmatically, you must declare a new instance of the TableAdapter. For example:
Dim northwindDataSet As New NorthwindDataSet() Dim customersTableAdapter As New NorthwindDataSetTableAdapters.CustomersTableAdapter() customersTableAdapter.Fill(northwindDataSet.Customers)
NorthwindDataSet northwindDataSet = new NorthwindDataSet(); NorthwindDataSetCustomersTableAdapter customersTableAdapter = new NorthwindDataSetCustomersTableAdapter(); customersTableAdapter.Fill(northwindDataSet.get_Customers());
Associated DataTable Schema
When creating a TableAdapter, the initial query or stored procedure is used to define the schema of the TableAdapter's associated DataTable. You execute this initial query or stored procedure by calling the TableAdapter's main Fill method (which fills the TableAdapter's associated DataTable). Any changes made to the TableAdapter's main query are reflected in the schema of the associated data table. For example, removing a column from the main query removes the column from the associated data table. If any additional queries on the TableAdapter use SQL statements returning columns that are not in the main query, then the designer will attempt to synchronize the column changes between the main query and any additional queries. For more information, see.
TableAdapter Update Commands
The update functionality of a TableAdapter is dependent on how much information is available based on the main query provided in the TableAdapter Wizard. For example, TableAdapters that are configured to fetch values from multiple tables (JOINs), scalar values, views, or the results of aggregate functions are not initially created with the ability to send updates back to the underlying database. However, you can configure the INSERT, UPDATE and DELETE commands manually in the Properties window.
Unlike standard data adapters, TableAdapters can contain multiple queries to fill their associated data tables. You can define as many queries for a TableAdapter as your application requires, as long as each query returns data that conforms to the same schema as its associated data table. This enables loading of data that satisfies differing criteria. For example, if your application contains a table of customers, you can create a query that fills the table with every customer whose name begins with a certain letter, and another query that fills the table with all customers located in the same state. To fill a Customers table with customers in a given state you can create a FillByState query that takes a parameter for the state value: SELECT * FROM Customers WHERE State = @State. You execute the query by calling the FillByState method and passing in the parameter value like this: CustomerTableAdapter.FillByState("WA"). For more information, see.
In addition to queries that return data of the same schema as the TableAdapter's data table, you can add queries that return scalar (single) values. For example, creating a query that returns a count of customers (SELECT Count(*) From Customers) is valid for a CustomersTableAdapter even though the data returned does not conform to the table's schema.
The TableAdapter adds a property not available on the base DataAdapter class. By default, every time you execute a query to fill a TableAdapter's data table, the data is cleared and only the results of the query are loaded into the table. Set the TableAdapter's ClearBeforeFill property to false if you want to add or merge the data returned from a query to the existing data in a data table. Regardless of whether you clear the data, you need to explicitly send updates back to the database, if desired. So remember to save any changes made to the data in the table before executing another query that fills the table. For more information, see.
TableAdapters extend the functionality of standard data adapters by encapsulating a configured DataAdapter. By default, the TableAdapter inherits fromand cannot be cast to the DataAdapter class. Casting a TableAdapter to a DataAdapter results in an . To change the base class of a TableAdapter, you can type a class that derives from Component in the Base Class property of the TableAdapter in the Dataset Designer.
TableAdapter Methods and Properties
The TableAdapter class is not part of the .NET Framework, and as such you cannot look it up in the documentation or the Object Browser. It is created at design time when you use one of the wizards mentioned above. The name assigned to a TableAdapter when you create it is based on the name of the table you are working with. For example, when creating a TableAdapter based on a table in a database named Orders, the TableAdapter would be named OrdersTableAdapter. The class name of the TableAdapter can be changed using the Name property in the Dataset Designer.
The following are the commonly used methods and properties of TableAdapters:
Populates the TableAdapter's associated data table with the results of the TableAdapter's SELECT command. For more information, see.
Sends changes back to the database. For more information, see.
Returns a new DataTable filled with data.
Creates a new row in the data table. For more information, see.
Determines whether a data table is emptied before you call one of the Fill methods.
TableAdapter Update Method
TableAdapters use data commands to read to and write from the database. The TableAdapter's initial Fill (main) query is used as the basis for creating the schema of the associated data table, as well as the InsertCommand, UpdateCommand, and DeleteCommand commands associated with the TableAdapter.Update method. This means that calling a TableAdapter's Update method executes the statements created when the TableAdapter was originally configured, and not one of the additional queries added with the TableAdapter Query Configuration Wizard.
When you use a TableAdapter, it effectively performs the same operations with the commands that you typically would perform. For example, when you call the adapter's Fill method, the adapter executes the data command in its SelectCommand property and uses a data reader (for example,) to load the result set into the data table. Similarly, when you call the adapter's Update method, it executes the appropriate command (in the UpdateCommand, InsertCommand, and DeleteCommand properties) for each changed record in the data table.
If there is enough information in the main query, the InsertCommand, UpdateCommand, and DeleteCommand commands are created by default when the TableAdapter is generated. If the TableAdapter's main query is more than a single table SELECT statement, it is possible the designer will not be able to generate the InsertCommand, UpdateCommand, and DeleteCommand. If these commands are not generated, you may receive an error when executing the TableAdapter.Update method.
In addition to the InsertCommand, UpdateCommand, and DeleteCommand, TableAdapters are created with methods that can be executed directly against the database. These methods (TableAdapter.Insert, TableAdapter.Update, and TableAdapter.Delete) can be called directly to manipulate data in the database.
If you do not want to create these direct methods, set the TableAdapter's GenerateDbDirectMethods property to false (in the Properties window). Additional queries added to the TableAdapter are standalone queries — they do not generate these methods.
TableAdapter Support for Nullable Types
The TableAdapters support nullable types Nullable(Of T) and T?. For more information on nullable types in Visual Basic, see. For more information on nullable types in C#, see .