Adding Helper Data Access Methods in Visual Studio .NET

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Adding Helper Data Access Methods
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At this point, we have a data access layer class library with type definitions for a CustomersDataSet, which can contain Customers, Orders, and Order_Details table data and maintain the relations between those tables We also have a table adapter for each table that lets us individually Fill and Update the tables in the data set The designer wrote all of that code for us We extended the table adapter for Customers to allow transactional updates, and we added a helper class that allows us to externally get or set the connection used by an instance of a CustomersTableAdapter What more could you possibly need Well, in this case we have three tables in the data set that are related in a hierarchical manner Orders is a child table forCustomers, and Order_Details is a child table forOrders In many situations, you will likely want to have all three tables populated at once, and it's a pain for a client to have to create three table adapter instances and make sure they fill them in the right order to ensure that they don't violate foreign key constraints To avoid this, you can add some code to the data access layer to provide convenience methods to populate the whole data set in one simple method call You could put this code in a couple of places I am going to do it as a factory method on the CustomersDataSet class itself by adding to the partial class definition for that class If you add a new class file to the project and name it CustomersDataSetcs, you can add the following partial class extension to the typed data set class: using System; using SystemData; using NorthwindDataAccessCustomersDataSetTableAdapters; namespace NorthwindDataAccess { public partial class CustomersDataSet : DataSet { public static CustomersDataSet GetCustomers() { CustomersTableAdapter custAdapter = new CustomersTableAdapter(); OrdersTableAdapter orderAdapter = new OrdersTableAdapter(); Order_DetailsTableAdapter detailsAdapter = new Order_DetailsTableAdapter(); CustomersDataSet customers = new CustomersDataSet(); custAdapterFill(customersCustomers); orderAdapterFill(customersOrders); detailsAdapterFill(customersOrder_Details); return customers; } } }
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The GetCustomers static factory method is pretty straightforward in implementation It simply creates an instance of each of the table adapters for the contained tables in the data set and uses them to fill the tables in the proper order to avoid foreign key constraint violations within the data set This allows client code to simply call the factory method on the class to get back a fully populated instance of the data set to work with: CustomersDataSet customers = CustomersDataSetGetCustomers();
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Updating a nested data set like this gets a little complicated, because you have to worry about performing the inserts, deletes, and updates to the database in the correct order so you won't violate constraints Encapsulating the details of that in a helper method would be a good idea and will depend on the scenarios in which you are going to use master-details data sets for performing updates
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Basing Table Adapters on Stored Procedures or Views
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Using stored procedures for all data access against a database has a number of advantages
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It promotes looser coupling between the data access layer and the explicit schema of the database, because certain kinds of changes can be made to the underlying table schema, and the client code won't have to change if the signature of the stored procedure didn't change Stored procedures improve your ability to secure the database to stored procedure-only access, which prevents clients from directly accessing tables and potentially violating data contracts through that access You may benefit from improved execution performance in many scenarios
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You can also use views to expose logical tables of data that map to more complex schemas at the database level You can easily base your strongly typed data sets and table adapters on either stored procedures or views as well If the stored procedure returns a result set (some combination of rows and columns), you can use that result set's schema to create a strongly typed data set that contains that schema and a table adapter capable of loading the rows returned from the stored procedure into the data set If the stored procedure takes in parameters to perform an update, insert, or delete, but doesn't return any rows, it can also be used to configure the corresponding command on a table adapter to update the database from a data set And finally, because a database view simply returns rows with a defined schema, it too can be used to fill a typed data set that matches the schema The method for using either stored procedures or views with typed data sets in the data set designer is very similar to what you have already seen with direct table access Dragging a database view onto the data set designer surface gives you a strongly typed data table definition based on the schema of the rows that the view returns It will also have a table adapter created to populate the data set from the database However, because the views of data in underlying tables in the database are read-only in the general case, the created table adapter's insert, update, and delete commands are set to null, so you can only use the table adapter to fill or retrieve a data set You will have to use some other means to perform updates When you drag a stored procedure onto the data set designer surface, what you get depends on the kind of stored procedure If the stored procedure returns rows, then the schema of the first result set of rows that the stored procedure returns will be used to create a typed data table and corresponding table adapter for that schema, which only has its select command configured for filling the data set If the stored procedure is one that doesn't return a result set, then a QueriesTableAdapter class will be created with methods exposed for each stored procedure that you drag onto the surface
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