Imports System.Xml

xmldoc.LoadXml(xmlString)
theNode = xmldoc.SelectSingleNode(”/xpath/query”)
myDataSet.ReadXml(New System.Xml.XmlNodeReader(theNode))

This little example just catches all unhandled exceptions and forwards them to a page, ErrorHandler.aspx, which then displays things a little prettier.

Global.asax.vb

Imports System.Web.SessionState

Public Class Global_asax
    Inherits System.Web.HttpApplication

    Sub Application_Start(ByVal sender As Object, ByVal e As EventArgs)
        ' Fires when the application is started
    End Sub

    Sub Session_Start(ByVal sender As Object, ByVal e As EventArgs)
        ' Fires when the session is started
    End Sub

    Sub Application_BeginRequest(ByVal sender As Object, ByVal e As EventArgs)
        ' Fires at the beginning of each request
    End Sub

    Sub Application_AuthenticateRequest(ByVal sender As Object, ByVal e As EventArgs)
        ' Fires upon attempting to authenticate the use
    End Sub

    Sub Application_Error(ByVal sender As Object, ByVal e As EventArgs)
        ' Fires when an error occurs
        Dim ctx As HttpContext = HttpContext.Current
        Dim oException As Exception = ctx.Server.GetLastError()
        ' do not redirect for 404
        If oException.GetType().FullName = "System.Web.HttpException" Then
            Dim exHttp As HttpException = DirectCast(oException, HttpException)
            If exHttp.GetHttpCode() = 404 Then
                Throw exHttp
            End If
        End If

        ctx.Items("LastError") = oException
        ctx.Server.ClearError()
        ctx.Server.Transfer("ErrorHandler.aspx")

    End Sub

    Sub Session_End(ByVal sender As Object, ByVal e As EventArgs)
        ' Fires when the session ends
    End Sub

    Sub Application_End(ByVal sender As Object, ByVal e As EventArgs)
        ' Fires when the application ends
    End Sub

End Class

ErrorHandler.aspx

    <form id="form1" runat="server">
    <div>
    <asp:Panel ID="panel1" runat="server">
    <h2>An error occurred.</h2>
    <p><asp:Label ID="lblUserMsg" runat="server" /></p>
    <hr />
    <p><asp:Label ID="lblErrType" runat="server" Text="<b>Type:</b> " /></p>
    <p><asp:Label ID="lblErrMsg" runat="server" text="<b>Message:</b> " /></p>
    <p><asp:Label ID="lblErrDetail" runat="server" text="<b>Detail:</b> " /></p>
    <p><asp:Label ID="lblTrace" runat="server" Text="<b>Trace:</b> " /></p>
    <p><asp:Label ID="lblErrSrc" runat="server" Text="<b>Source:</b> " /></p>
    </asp:Panel>
    </div>
    </form>

ErrorHandler.aspx.vb

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        ' application_onerror redirects here; if there is any compilation error,
        ' you get an infinite loop.
        ' SO BE CAREFUL CHANGING THIS PAGE
        '
        Dim lastException, baseException As Exception
        Try
            lastException = DirectCast(Context.Items("LastError"), Exception)

            Dim detail As String = DirectCast(Context.Items("ErrorDetail"), String)
            If Not lastException Is Nothing Then
                baseException = lastException.GetBaseException()
                lblErrType.Text += baseException.GetType().ToString()
                lblErrMsg.Text += baseException.Message
                If Not String.IsNullOrEmpty(detail) Then
                    lblErrDetail.Text += detail
                End If
                lblTrace.Text += baseException.StackTrace
                lblErrSrc.Text += baseException.Source
            Else
                lblErrType.Text += "Not available"
                lblErrMsg.Text += "Not available"
                lblTrace.Text += "Not available"
                lblErrSrc.Text += "Not available"
            End If
        Catch oException As Exception
            'mostly this is here in case we did something stupid in that try block up there.
            lblErrMsg.Text = "Error Message: " + oException.Message
            lblTrace.Text = "Stack Trace: " + oException.StackTrace
            lblErrSrc.Text = "Error Source: " + oException.Source
        End Try

    End Sub

Note that this would catch all exceptions, including compile errors, so be sure to precompile before deploying! If you don’t, and there’s a compile error on the error page itself, you have fun with an endless loop. 

For more information, see How to: Handle Application-Level Errors

Sometimes SqlDataSource doesn’t work for what you want to do. It’s a great control, but as I’ve mentioned before, it can interfere with your plans because of the page lifecycle. You should also know how to run queries and get the results using the ADO.NET classes from System.Data namespace.

The following example shows you how to set up a simple ASPX page with a GridView. The GridView (bound with an easy SqlDataSource) grabs the employees from AdventureWorks. When you select a row, the manager’s details will show up in the Label controls. We make use of the DataKeyNames collection to make grabbing the ManagerID easier in the SelectedIndexChanged event. We use the ManagerID as a parameter to the query, then use SqlDataReader to read the results and populate the Label Text.

On to the code.

ASPX

<%@ Page Language="VB" AutoEventWireup="false" CodeFile="Default4.aspx.vb" Inherits="Default4" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "<a href="http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd</a>">

<html xmlns="<a href="http://www.w3.org/1999/xhtml">http://www.w3.org/1999/xhtml</a>" >
<head runat="server">
    <title>Untitled Page</title>
</head>
<body>
    <form id="form1" runat="server">
            <asp:SqlDataSource ID="sqldsEmployees" runat="server" ConnectionString="<%$ ConnectionStrings:AdventureWorksConnectionString %>"
            SelectCommand="SELECT Employee.EmployeeID, Employee.ManagerID, Employee.Title, Contact.FirstName, Contact.LastName FROM Employee INNER JOIN Contact ON Employee.ContactID = Contact.ContactID">
        </asp:SqlDataSource>
               <div style="z-index: 101; left: 17px; width: 370px; position: absolute; top: 83px;
            height: 355px">
            <asp:GridView ID="gvEmployees" runat="server" AllowPaging="True" AllowSorting="True"
                AutoGenerateColumns="False" AutoGenerateSelectButton="True" Caption="Employee List"
                CellPadding="4" DataKeyNames="EmployeeID,ManagerID" DataSourceID="sqldsEmployees" ForeColor="#333333"
                GridLines="None" Width="461px">
                <FooterStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White" />
                <RowStyle BackColor="#F7F6F3" ForeColor="#333333" />
                <Columns>
                    <asp:BoundField DataField="EmployeeID" HeaderText="EmployeeID" InsertVisible="False"
                        ReadOnly="True" SortExpression="EmployeeID" Visible="False" />
                    <asp:BoundField DataField="ManagerID" HeaderText="ManagerID" SortExpression="ManagerID" />
                    <asp:BoundField DataField="Title" HeaderText="Title" SortExpression="Title" />
                    <asp:BoundField DataField="FirstName" HeaderText="FirstName" SortExpression="FirstName" />
                    <asp:BoundField DataField="LastName" HeaderText="LastName" SortExpression="LastName" />
                </Columns>
                <PagerStyle BackColor="#284775" ForeColor="White" HorizontalAlign="Center" />
                <SelectedRowStyle BackColor="#E2DED6" Font-Bold="True" ForeColor="#333333" />
                <HeaderStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White" />
                <EditRowStyle BackColor="#999999" />
                <AlternatingRowStyle BackColor="White" ForeColor="#284775" />
            </asp:GridView>
        </div>
        <div style="z-index: 102; left: 502px; width: 310px; position: absolute; top: 85px;
            height: 364px">
            <h3>Manager Info</h3>
            Title:
            <asp:Label ID="lblManagerTitle" runat="server"></asp:Label><br />
            Name:
            <asp:Label ID="lblManagerName" runat="server"></asp:Label><br />
            E-mail:
            <asp:Label ID="lblManagerEmail" runat="server"></asp:Label><br />
            Phone:
            <asp:Label ID="lblManagerPhone" runat="server"></asp:Label></div>
    </form>
</body>
</html>

Code Behind

Imports System.Collections.Specialized
Imports System.Data.Sql
Imports System.Data.SqlClient
Partial Class Default4
    Inherits System.Web.UI.Page

    ' use the SelectedIndexChanged event, not RowCommand, because the properties for the selected row aren't populated yet in row command
    Protected Sub gvEmployees_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles gvEmployees.SelectedIndexChanged
        Dim managerID As New Integer
        ' we can get the manager ID because we set it in the GridView's DataKeys collection as the second one
        Dim selectedManger As Object = Me.gvEmployees.SelectedDataKey.Values("ManagerID")
        If Not selectedManger Is Nothing Then
            managerID = CType(selectedManger, Integer)
            Dim oSqlDataReader As SqlDataReader = Nothing
            Try
                Using oSqlConnection As New SqlConnection
                    oSqlConnection.ConnectionString = ConfigurationManager.ConnectionStrings("AdventureWorksConnectionString").ConnectionString
                    Using oSqlCommand As New SqlCommand
                        oSqlCommand.Connection = oSqlConnection
                        oSqlCommand.CommandText = "SELECT Employee.Title, Contact.FirstName, Contact.LastName, Contact.EmailAddress, Contact.Phone FROM Employee INNER JOIN Contact ON Employee.ContactID = Contact.ContactID WHERE <a href="mailto:EmployeeID=@EmployeeID">EmployeeID=@EmployeeID</a>"
                        oSqlCommand.CommandType = Data.CommandType.Text
                        oSqlCommand.Parameters.Clear()
                        oSqlCommand.Parameters.AddWithValue("@EmployeeID", managerID)
                        oSqlConnection.Open()
                        oSqlDataReader = oSqlCommand.ExecuteReader(Data.CommandBehavior.CloseConnection)
                        While oSqlDataReader.Read()
                            ' there should only be one match
                            Me.lblManagerTitle.Text = oSqlDataReader("Title")
                            Me.lblManagerName.Text = oSqlDataReader("FirstName") & " " & oSqlDataReader("LastName")
                            Me.lblManagerEmail.Text = oSqlDataReader("EmailAddress")
                            Me.lblManagerPhone.Text = oSqlDataReader("Phone")
                        End While
                    End Using
                End Using
            Finally
                If Not oSqlDataReader Is Nothing AndAlso Not oSqlDataReader.IsClosed Then
                    oSqlDataReader.Close()
                End If
            End Try
        End If
    End Sub
End Class

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