The DataSet Class

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The DataSet Class You use an object of the DataSet class to represent a local copy of the information stored in the database.

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The DataSet Class You use an object of the DataSet class to represent a local copy of the information stored in the database. You can make changes to that local copy in your DataSet and then later synchronize those changes with the database through a DataAdapter. A DataSet can represent database structures such as tables, rows, and columns. You can even add constraints to your locally stored tables to enforce unique and foreign key constraints. Figure 10.1 shows the DataSet and its relationship to some of the objects you can store within it. As you can see from this figure, you can store multiple DataTable objects in a DataSet, and so on. Figure 10.1: Some of the DataSet objects Table 10.4 shows some of the DataSet properties. Table 10.4: DataSet PROPERTIES PROPERTY TYPE DESCRIPTION CaseSensitive bool Gets or sets a bool value that indicates whether string comparisons within DataTable objects are case-sensitive. DataSetName string Gets or sets the name of the current DataSet object. DefaultViewManager DataViewManager Gets a custom view of the data stored in
Table 10.4: DataSet PROPERTIES PROPERTY TYPE DESCRIPTION the DataSet object. You use a view to filter, search, and navigate the DataSet. EnforceConstraints bool Gets or sets a bool value that indicates whether constraint rules are followed when updating information in the DataSet object. ExtendedProperties PropertyCollection Gets a collection (PropertyCollection) of user information. You can use the PropertyCollection to store strings with any additional information you want. You use the Add() method through ExtendedProperties to add a string. HasErrors bool Gets a bool value that indicates whether there are errors in any of the rows in the tables of the DataSet object. Locale CultureInfo Gets or sets a CultureInfo object for the DataSet. A CultureInfo object contains information about a specific culture including its name, writing system, and calendar. Namespace string Gets or sets the namespace for the DataSet object. The namespace is a string that is used when reading and writing an XML document using the ReadXml(), WriteXml(), ReadXmlSchema(), and WriteXmlSchema() methods. The namespace is used to scope the XML attributes and elements. Prefix string Gets or sets the XML prefix for the DataSet namespace. The prefix is used in an XML document to identify the elements that belong to the DataSet object's namespace. Relations DataRelationCollection Gets the collection of relations (DataRelationCollection) that allows navigation from a parent table to a child table. A DataRelationCollection consists of DataRelation objects.
Table 10.4: DataSet PROPERTIES PROPERTY TYPE DESCRIPTION Tables DataTableCollection Gets the collection of tables (DataTableCollection) that contains the DataTable objects stored in the DataSet. Table 10.5 shows some of the DataSet methods. Table 10.5: DataSet METHODS METHOD RETURN DESCRIPTION TYPE AcceptChanges() void Commits all the changes made to the DataSet object since it was loaded or since the last time the AcceptChanges() method was called. BeginInit() void Used by the Visual Studio .NET designer to initialize a DataSet used in a form or component. Clear() void Removes all rows from all tables in the DataSet object. Clone() DataSet Clones the structure of the DataSet object and returns that clone. The clone contains all the schemas, relations, and constraints. Copy() DataSet Copies the structure and data of the DataSet object and returns that copy. The copy contains all the schemas, relations, constraints, and data. EndInit() void Used by the Visual Studio .NET designer to end initialization of a DataSet used in a form or component. GetChanges() DataSet Overloaded. Gets a copy of all the changes made to the DataSet object since it was last loaded or since the last time the AcceptChanges() method was called. GetXml() string Returns the XML representation of the data stored in the DataSet object. GetXmlSchema() string Returns the XML representation of the schema for the DataSet object. HasChanges() bool Overloaded. Returns a bool value that indicates whether the DataSet object has changes that haven't been committed.
Table 10.5: DataSet METHODS METHOD RETURN DESCRIPTION TYPE Merge() void Overloaded. Merges this DataSet with another specified DataSet object. ReadXml() XmlReadMode Overloaded. Loads the data from an XML file into the DataSet object. ReadXmlSchema() void Overloaded. Loads a schema from an XML file into the DataSet object. RejectChanges() void Undoes all the changes made to the DataSet object since it was created or since the last time the AcceptChanges() method was called. Reset() void Resets the DataSet object to its original state. WriteXml() void Overloaded. Writes out the data from the DataSet object to an XML file. WriteXmlSchema() void Overloaded. Writes out the schema of the DataSet object to an XML file. Table 10.6 shows one of the DataSet events. Table 10.6: DataSet EVENT EVENT EVENT HANDLER DESCRIPTION MergeFailed MergeFailedEventHandler Fires when an attempt is made add a DataRow to a DataSet when a DataRow with the same primary key value already exists in that DataSet. In the next section, you'll learn how to create a DataSet object. Creating a DataSet Object You create a DataSet object using one of the following DataSet constructors: DataSet() DataSet(string dataSetNameString) where dataSetNameString is the string assigned to the DataSetName property of your DataSet object. The setting of the DataSetName property is optional. The following example uses the DataSet() constructor to create a DataSet object:
DataSet myDataSet = new DataSet(); The next example uses the DataSet(string dataSetNameString) constructor to create a DataSet object: DataSet myDataSet = new DataSet("myDataSet"); Populating a DataSet Object In this section, you'll learn how to populate a DataSet using the Fill() method of a DataAdapter. Specifically, you'll see how to populate a DataSet using • A SELECT statement • A range of rows • A stored procedure Using a SELECT Statement Before you populate a DataSet you first need a Connection, a Command, and a DataAdapter: SqlConnection mySqlConnection = new SqlConnection( "server=localhost;database=Northwind;uid=sa;pwd=sa" ); SqlCommand mySqlCommand = mySqlConnection.CreateCommand(); mySqlCommand.CommandText = "SELECT TOP 5 ProductID, ProductName, UnitPrice " + "FROM Products " + "ORDER BY ProductID"; SqlDataAdapter mySqlDataAdapter = new SqlDataAdapter(); mySqlDataAdapter.SelectCommand = mySqlCommand; DataSet myDataSet = new DataSet(); mySqlConnection.Open(); Notice the mySqlCommand object contains a SELECT statement that retrieves the ProductID, ProductName, and UnitPrice columns of the top five rows from the Products table. RETRIEVING FROM MULTIPLE TABLES Of course, you're not limited to a SELECT statement that retrieves from a single table. You can use a SELECT statement that retrieves from multiple tables using a join,
however, you should typically avoid doing that because a DataTable is meant to be used to store rows from a single database table. Next, to populate myDataSet with the rows from the Products table, you call the Fill() method of mySqlDataAdapter. For example: int numberOfRows = mySqlDataAdapter.Fill(myDataSet, "Products"); The int returned by the Fill() method is the number of rows synchronized between the DataSet and the database via the DataAdapter. In the previous example, the int is the number of rows copied from the Products table to myDataSet and is set to 5-the number of rows retrieved by the SELECT statement shown earlier. The first parameter to the Fill() method is your DataSet, and the second parameter is a string containing the name you want to assign to the DataTable created in your DataSet. Note The name you assign to your DataTable doesn't have to be the same as the name of the database table. You can use any string of text, though typically you should still use the same name, since it will help you keep track of what database table was used to populate the DataTable. When you call the Fill() method for the first time, the following steps are performed by ADO.NET: 1. The SELECT statement in your SqlCommand is executed. 2. A new DataTable object is created in your DataSet. 3. Your DataTable is populated with the result set returned by the SELECT statement. If you're finished with the database after calling the Fill() method, you should close your Connection object using the Close() method: mySqlConnection.Close();