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DataSourceView.ExecuteSelect Method

Gets a list of data from the underlying data storage.

Namespace: System.Web.UI
Assembly: System.Web (in system.web.dll)

'Declaration
Protected Friend MustOverride Function ExecuteSelect ( _
	arguments As DataSourceSelectArguments _
) As IEnumerable
'Usage
Dim arguments As DataSourceSelectArguments
Dim returnValue As IEnumerable

returnValue = Me.ExecuteSelect(arguments)
protected abstract IEnumerable ExecuteSelect (
	DataSourceSelectArguments arguments
)
protected internal abstract function ExecuteSelect (
	arguments : DataSourceSelectArguments
) : IEnumerable
Not applicable.

Parameters

arguments

A DataSourceSelectArguments that is used to request operations on the data beyond basic data retrieval.

Return Value

An 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

Windows 98, Windows Server 2000 SP4, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0

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