DataSourceView.ExecuteSelect Method (DataSourceSelectArguments)
Gets a list of data from the underlying data storage.
Assembly: System.Web (in System.Web.dll)
Protected Friend MustOverride Function ExecuteSelect ( arguments As DataSourceSelectArguments ) As IEnumerable
Parameters
- arguments
-
Type:
System.Web.UI.DataSourceSelectArguments
A DataSourceSelectArguments that is used to request operations on the data beyond basic data retrieval.
Return Value
Type: System.Collections.IEnumerableAn IEnumerable list of data from the underlying data storage.
The ExecuteSelect method is called to retrieve data from the underlying data store and return it as an IEnumerable object. All data source controls support data retrieval from their underlying data storage, even if other operations such as insertion and sorting are not supported. Because a data-bound control can request a list of data at any time as a result of a DataSourceChanged event or a DataBind method call, the data retrieval must be performed on demand.
The following code example demonstrates how to override the ExecuteSelect method in a class that extends the DataSourceView class. The CsvDataSourceView opens a comma-separated value (.csv) file, parses it line by line, and creates a DataTable object and a DataView object to hold the data in memory. Finally, a sort expression is applied if one is supplied by the DataSourceSelectArguments object, and the DataView object is returned as an IEnumerable instance. This code example is part of a larger example provided for the DataSourceView class.
' Get data from the underlying data source. ' Build and return a DataView, regardless of mode. Protected Overrides Function ExecuteSelect(selectArgs As DataSourceSelectArguments) _ As System.Collections.IEnumerable Dim dataList As IEnumerable = Nothing ' Open the .csv file. If File.Exists(Me.SourceFile) Then Dim data As New DataTable() ' Open the file to read from. Dim sr As StreamReader = File.OpenText(Me.SourceFile) Try ' Parse the line Dim dataValues() As String Dim col As DataColumn ' Do the following to add schema. dataValues = sr.ReadLine().Split(","c) ' For each token in the comma-delimited string, add a column ' to the DataTable schema. Dim token As String For Each token In dataValues col = New DataColumn(token, System.Type.GetType("System.String")) data.Columns.Add(col) Next token ' Do not add the first row as data if the CSV file includes column names. If Not IncludesColumnNames Then data.Rows.Add(CopyRowData(dataValues, data.NewRow())) End If ' Do the following to add data. Dim s As String Do s = sr.ReadLine() If Not s Is Nothing Then dataValues = s.Split(","c) data.Rows.Add(CopyRowData(dataValues, data.NewRow())) End If Loop Until s Is Nothing Finally sr.Close() End Try data.AcceptChanges() Dim dataView As New DataView(data) If Not selectArgs.SortExpression Is String.Empty Then dataView.Sort = selectArgs.SortExpression End If dataList = dataView Else Throw New System.Configuration.ConfigurationErrorsException("File not found, " + Me.SourceFile) End If If dataList is Nothing Then Throw New InvalidOperationException("No data loaded from data source.") End If Return dataList End Function 'ExecuteSelect Private Function CopyRowData([source]() As String, target As DataRow) As DataRow Try Dim i As Integer For i = 0 To [source].Length - 1 target(i) = [source](i) Next i Catch iore As IndexOutOfRangeException ' There are more columns in this row than ' the original schema allows. Stop copying ' and return the DataRow. Return target End Try Return target End Function 'CopyRowData
Available since 2.0