DataSourceView.ExecuteSelect(DataSourceSelectArguments) Method
Definition
Important
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Gets a list of data from the underlying data storage.
protected public:
abstract System::Collections::IEnumerable ^ ExecuteSelect(System::Web::UI::DataSourceSelectArguments ^ arguments);protected internal abstract System.Collections.IEnumerable ExecuteSelect (System.Web.UI.DataSourceSelectArguments arguments);abstract member ExecuteSelect : System.Web.UI.DataSourceSelectArguments -> System.Collections.IEnumerableProtected Friend MustOverride Function ExecuteSelect (arguments As DataSourceSelectArguments) As IEnumerableParameters
- arguments
- DataSourceSelectArguments
A DataSourceSelectArguments that is used to request operations on the data beyond basic data retrieval.
Returns
An IEnumerable list of data from the underlying data storage.
Examples
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 override IEnumerable ExecuteSelect(DataSourceSelectArguments selectArgs) {
IEnumerable dataList = null;
// Open the .csv file.
if (File.Exists(this.SourceFile)) {
DataTable data = new DataTable();
// Open the file to read from.
using (StreamReader sr = File.OpenText(this.SourceFile)) {
// Parse the line
string s = "";
string[] dataValues;
DataColumn col;
// Do the following to add schema.
dataValues = sr.ReadLine().Split(',');
// For each token in the comma-delimited string, add a column
// to the DataTable schema.
foreach (string token in dataValues) {
col = new DataColumn(token,typeof(string));
data.Columns.Add(col);
}
// Do not add the first row as data if the CSV file includes column names.
if (! IncludesColumnNames)
data.Rows.Add(CopyRowData(dataValues, data.NewRow()));
// Do the following to add data.
while ((s = sr.ReadLine()) != null) {
dataValues = s.Split(',');
data.Rows.Add(CopyRowData(dataValues, data.NewRow()));
}
}
data.AcceptChanges();
DataView dataView = new DataView(data);
if (!string.IsNullOrEmpty(selectArgs.SortExpression)) {
dataView.Sort = selectArgs.SortExpression;
}
dataList = dataView;
}
else {
throw new System.Configuration.ConfigurationErrorsException("File not found, " + this.SourceFile);
}
if (null == dataList) {
throw new InvalidOperationException("No data loaded from data source.");
}
return dataList;
}
private DataRow CopyRowData(string[] source, DataRow target) {
try {
for (int i = 0;i < source.Length;i++) {
target[i] = source[i];
}
}
catch (System.IndexOutOfRangeException) {
// There are more columns in this row than
// the original schema allows. Stop copying
// and return the DataRow.
return target;
}
return target;
}
' 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
Remarks
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.
