Populate a DataSet from a DataAdapter
Applies to: 
.NET Framework
.NET
.NET Standard
The ADO.NET DataSet is a memory-resident representation of data that provides a consistent relational programming model independent of the data source. The DataSet represents a complete set of data that includes tables, constraints, and relationships among the tables. Because the DataSet is independent of the data source, a DataSet can include data local to the application, and data from multiple data sources. Interaction with existing data sources is controlled through the DataAdapter.
The SelectCommand property of the DataAdapter is a Command object that retrieves data from the data source. The InsertCommand, UpdateCommand, and DeleteCommand properties of the DataAdapter are Command objects that manage updates to the data in the data source according to modifications made to the data in the DataSet. These properties are covered in more detail in Update Data Sources with DataAdapters.
The Fill method of the DataAdapter is used to populate a DataSet with the results of the SelectCommand of the DataAdapter. Fill takes as its arguments a DataSet to be populated, and a DataTable object, or the name of the DataTable to be filled with the rows returned from the SelectCommand.
Note
Using the DataAdapter to retrieve all of a table takes time, especially if there are many rows in the table. This is because accessing the database, locating and processing the data, and then transferring the data to the client is time-consuming. Pulling all of the table to the client also locks all of the rows on the server. To improve performance, you can use the WHERE clause to greatly reduce the number of rows returned to the client. You can also reduce the amount of data returned to the client by only explicitly listing required columns in the SELECT statement. Another good workaround is to retrieve the rows in batches (such as several hundred rows at a time) and only retrieve the next batch when the client is finished with the current batch.
The Fill method uses the DataReader object implicitly to return the column names and types that are used to create the tables in the DataSet, and the data to populate the rows of the tables in the DataSet. Tables and columns are only created if they do not already exist; otherwise Fill uses the existing DataSet schema. Column types are created as .NET Framework types according to the tables in Data Type Mappings in ADO.NET. Primary keys are not created unless they exist in the data source and DataAdapter.MissingSchemaAction is set to MissingSchemaAction.AddWithKey. If Fill finds that a primary key exists for a table, it will overwrite data in the DataSet with data from the data source for rows where the primary key column values match those of the row returned from the data source. If no primary key is found, the data is appended to the tables in the DataSet. Fill uses any mappings that may exist when you populate the DataSet (see DataAdapter, DataTable, and DataColumn Mappings).
Note
If the SelectCommand returns the results of an OUTER JOIN, the DataAdapter does not set a PrimaryKey value for the resulting DataTable. You must define the PrimaryKey yourself to make sure that duplicate rows are resolved correctly.
The following code example creates an instance of a SqlDataAdapter that uses a SqlConnection to the Microsoft SQL Server Northwind database and populates a DataTable in a DataSet with the list of customers. The SQL statement and SqlConnection arguments passed to the SqlDataAdapter constructor are used to create the SelectCommand property of the SqlDataAdapter.
Example
// Assumes that connection is a valid SqlConnection object.
string queryString =
"SELECT CustomerID, CompanyName FROM dbo.Customers";
SqlDataAdapter adapter = new SqlDataAdapter(queryString, connection);
DataSet customers = new DataSet();
adapter.Fill(customers, "Customers");
Note
The code shown in this example does not explicitly open and close the Connection. The Fill method implicitly opens the Connection that the DataAdapter is using if it finds that the connection is not already open. If Fill opened the connection, it also closes the connection when Fill is finished. This can simplify your code when you deal with a single operation such as a Fill or an Update. However, if you are performing multiple operations that require an open connection, you can improve the performance of your application by explicitly calling the Open method of the Connection, performing the operations against the data source, and then calling the Close method of the Connection. You should try to keep connections to the data source open as briefly as possible to free resources for use by other client applications.
Multiple result sets
If the DataAdapter encounters multiple result sets, it creates multiple tables in the DataSet. The tables are given an incremental default name of TableN, starting with "Table" for Table0. If a table name is passed as an argument to the Fill method, the tables are given an incremental default name of TableNameN, starting with "TableName" for TableName0.
Populate a DataSet from multiple DataAdapters
Any number of DataAdapter objects can be used with a DataSet. Each DataAdapter can be used to fill one or more DataTable objects and resolve updates back to the relevant data source. DataRelation and Constraint objects can be added to the DataSet locally, which enables you to relate data from dissimilar data sources. For example, a DataSet can contain data from a Microsoft SQL Server database, an IBM Db2 database exposed through OLE DB, and a data source that streams XML. One or more DataAdapter objects can handle communication to each data source.
Example
The following code example populates a list of customers from the Northwind database on Microsoft SQL Server, and a list of orders from the Northwind database stored in Microsoft Access. The filled tables are related with a DataRelation, and the list of customers is then displayed with the orders for that customer.
// Assumes that customerConnection and orderConnection are valid SqlConnection objects.
SqlDataAdapter custAdapter = new SqlDataAdapter(
"SELECT * FROM dbo.Customers", customerConnection);
SqlDataAdapter ordAdapter = new SqlDataAdapter(
"SELECT * FROM Orders", orderConnection);
DataSet customerOrders = new DataSet();
custAdapter.Fill(customerOrders, "Customers");
ordAdapter.Fill(customerOrders, "Orders");
DataRelation relation = customerOrders.Relations.Add("CustOrders",
customerOrders.Tables["Customers"].Columns["CustomerID"],
customerOrders.Tables["Orders"].Columns["CustomerID"]);
foreach (DataRow pRow in customerOrders.Tables["Customers"].Rows)
{
Console.WriteLine(pRow["CustomerID"]);
foreach (DataRow cRow in pRow.GetChildRows(relation))
Console.WriteLine("\t" + cRow["OrderID"]);
}
SQL Server Decimal type
By default, the DataSet stores data by using .NET data types. For most applications, these provide a convenient representation of data source information. However, this representation may cause a problem when the data type in the data source is a SQL Server decimal or numeric data type. The .NET decimal data type allows a maximum of 28 significant digits, whereas the SQL Server decimal data type allows 38 significant digits. If the SqlDataAdapter determines during a Fill operation that the precision of a SQL Server decimal field is larger than 28 characters, the current row is not added to the DataTable. Instead the FillError event occurs, which enables you to determine whether a loss of precision will occur, and respond appropriately. For more information about the FillError event, see Handle DataAdapter Events. To get the SQL Server decimal value, you can also use a SqlDataReader object and call the GetSqlDecimal method.
ADO.NET also includes enhanced support for System.Data.SqlTypes in the DataSet. For more information, see SqlTypes and the DataSet.