Programmatically accessing data from DataSource controls

For the main part, the DataSource controls are incredibly easy to work with. You just drag them onto your page in Design View, wire them up to a connection and provide some kind of command (ad hoc sql query or the name of a stored procedure - saved query in Access), then link them to a databound control on the page using its DataSource property. Then you run the page and all your data appears. But what if you want to get at one or more values from the returned data to do something else with it, like display it in a control that doesn't expose a DataSource property?

To programmatically access the contents of a Sql or Access DataSource control you need to explicitly call its Select() method. This method accepts a single input parameter of type DataSourceSelectArguments. This parameter can contain information regarding the filters to apply or the column to Order By. For example, when working with a sortable GridView, sorting a column calls the Select() method, and passes in a DataSourceSelectArguments instance with its SortExpression property set to the column name the user chose to sort by. If you don't want the DataSource to sort or filter, you pass in DataSourceSelectArguments.Empty.

Depending on the DataSourceMode of the DataSource control, one of two objects are returned when the Select() method is called. If the DataSourceMode is set to DataReader, a DataReader object is returned. The type of DataReader (SqlDataReader, OleDbDataReader, OdbcDataReader etc) that is returned depends entirely on the provider type used - in other words, whether you are using the OleDb provider, SqlClient provider etc. It has nothing to do with the type of DataSource control. The examples below both query versions of the NorthWind database. The AccessDataSource control connects to the mdb (Access) version, while the SqlDataSource shows SqlClient classes in operation - it connects to an mdf version of NorthWind.

If the DataSourceMode is set to Dataset, or not set at all (which means that the default setting of Dataset is used) the object that is returned is a DataView. A DataView is like a DataTable on steroids. It exposes methods that allow you to filter and sort data, for example, and bind it. A DataView contains a collection of DataRowView objects, which represent each row in the returned results.

So, with a DataReader, you would access the values during the DataReader.Read() operation, in very much the same way as if you are using plain ADO.NET code, whereas with the DataSet, you would need to create an object of the appropriate type - DataView, then iterate the DataRowView collection to access the values. In this, the code is remarkably similar to accessing values directly from a DataSet's table collection using plain ADO.NET.

The code below shows the contents of an aspx file, which contains four label controls, the two AccessDataSource controls and two SqlDataSource controls. Each type of DataSource control has its DataSource mode set to alternative values - DataReader and DataSet, and they all have an OnSelecting event defined in which the value of the EmployeeID parameter is assigned:


<asp:Label ID="Label1" runat="server" /> <asp:Label ID="Label2" runat="server" />
<br />
<asp:Label ID="Label3" runat="server" /> <asp:Label ID="Label4" runat="server" />


<asp:AccessDataSource 
ID="AccessDataSource1" 
runat="server" 
DataFile="~/App_Data/Northwind.mdb" 
SelectCommand="SELECT [LastName], [FirstName] FROM [Employees] WHERE ([EmployeeID] = ?)" 
DatasourceMode="DataSet"
OnSelecting="AccessDataSource1_Selecting">
    <SelectParameters>
        <asp:Parameter Name="EmployeeID" Type="Int32" />
    </SelectParameters>
</asp:AccessDataSource>

<asp:AccessDataSource 
ID="AccessDataSource2" 
runat="server" 
DataFile="~/App_Data/Northwind.mdb" 
SelectCommand="SELECT [LastName], [FirstName] FROM [Employees] WHERE ([EmployeeID] = ?)" 
DataSourceMode="DataReader"
OnSelecting="AccessDataSource2_Selecting">
    <SelectParameters>
        <asp:Parameter Name="EmployeeID" Type="Int32" />
    </SelectParameters>
</asp:AccessDataSource>

<asp:SqlDataSource 
ID="SqlDataSource1" 
runat="server" 
ConnectionString="<%$ ConnectionStrings:NorthWindConnectionString2 %>"
ProviderName="<%$ ConnectionStrings:NorthWindConnectionString2.ProviderName %>" 
DatasourceMode="DataSet"
SelectCommand="SELECT [LastName], [FirstName] FROM [Employees] WHERE ([EmployeeID] = @EmployeeID)" 
OnSelecting="SqlDataSource1_Selecting">
    <SelectParameters>
        <asp:Parameter Name="EmployeeID" Type="Int32" />
    </SelectParameters>
</asp:SqlDataSource>

<asp:SqlDataSource
ID="SqlDataSource2" 
runat="server"
ConnectionString="<%$ ConnectionStrings:NorthWindConnectionString2 %>"
ProviderName="<%$ ConnectionStrings:NorthWindConnectionString2.ProviderName %>" 
DatasourceMode="DataReader"
SelectCommand="SELECT [LastName], [FirstName] FROM [Employees] WHERE ([EmployeeID] = @EmployeeID)" 
OnSelecting="SqlDataSource2_Selecting">
    <SelectParameters>
        <asp:Parameter Name="EmployeeID" Type="Int32" />
    </SelectParameters>
</asp:SqlDataSource>

The following code snippet shows the aspx.cs file contents, where the parameter values are set in the Selecting event handler. In the Page_Load method, the data returned by each of the Sql DataSource controls is accessed and a value consigned to a label. The method of access depends on the DataSource mode, but is identical for both SqlDataSource and AccessDataSource:


protected void Page_Load(object sender, EventArgs e)
{
    //AccessDataSource DateSet Mode
    DataView dvAccess = (DataView)AccessDataSource1.Select(DataSourceSelectArguments.Empty);
    Label1.Text = dvAccess[0][1].ToString();

    //AccessDataSource DataReader Mode
    using (OleDbDataReader rdrAccess = (OleDbDataReader)AccessDataSource2.Select(DataSourceSelectArguments.Empty))
    {
        while (rdrAccess.Read())
            Label2.Text = rdrAccess["LastName"].ToString();
    }

    //SqlDataSource DateSet Mode
    DataView dvSql = (DataView)SqlDataSource1.Select(DataSourceSelectArguments.Empty);
    Label3.Text = dvSql[0][1].ToString();

    //SqlDataSource DataReader Mode
    using (SqlDataReader rdrSql = (SqlDataReader)SqlDataSource2.Select(DataSourceSelectArguments.Empty))
    {
        while (rdrSql.Read())
            Label4.Text = rdrSql["LastName"].ToString();
    }
}

protected void AccessDataSource1_Selecting(object sender, SqlDataSourceSelectingEventArgs e)
{
    e.Command.Parameters["EmployeeID"].Value = 1;
}

protected void AccessDataSource2_Selecting(object sender, SqlDataSourceSelectingEventArgs e)
{
    e.Command.Parameters["EmployeeID"].Value = 1;
}

protected void SqlDataSource1_Selecting(object sender, SqlDataSourceSelectingEventArgs e)
{
    e.Command.Parameters["@EmployeeID"].Value = 2;
}

protected void SqlDataSource2_Selecting(object sender, SqlDataSourceSelectingEventArgs e)
{
    e.Command.Parameters["@EmployeeID"].Value = 2;
}

If you try to use these methods with your control in DataSet mode (the default), don't forget to reference System.Data in your page, as that is required to provide access to the DataView class.

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