<!--
<page url="..." abortOnError="TRUE|false">
  <param name="..." 
         type="Int|Text|Bool" 
         optional="FALSE|true"
         length="number (for Text type only)"
         casesensitive="false|true" />
</page>
-->

<querystring>

  <page url="/source/test.aspx" abortOnError="true">
    <param name="id" type="Int" optional="true" casesensitive="true" />
    <param name="code" type="Text" length="5" optional="false" />
    <param name="detailed" type="Bool" optional="false" />
  </page>

  <page url="/source/Test1.aspx" abortOnError="false">
    <param name="guid" type="Int" />
  </page>

</querystring>

Imports System
Imports System.Web
Imports System.IO

Public Class QueryStringModule : Implements IHttpModule

    Private _app As HttpApplication
    Private _queryStringData As Hashtable

    Public Sub Init(ByVal context As System.Web.HttpApplication) _
            Implements System.Web.IHttpModule.Init
        _app = context
        AddHandler _app.BeginRequest, AddressOf OnEnter

        ' Load and cache the XML querystring file
        Dim fileName As String = _
            HttpContext.Current.Server.MapPath("web.querystring")
        _queryStringData = QueryStringHelper.LoadFromFile(fileName)
    End Sub

    Public Sub Dispose() Implements System.Web.IHttpModule.Dispose
    End Sub

    Private Sub OnEnter(ByVal source As Object, ByVal e As EventArgs)
        ' Retrieve the query string data structure for the current page
        Dim currentPage As String = _
            HttpContext.Current.Request.Path.ToLower()
        Dim qsDesc As QueryStringDescriptor = _
            _queryStringData.Item(currentPage)

        ' Validate the query string
        Dim isValid As Boolean
        isValid = QueryStringHelper.Validate( _
           HttpContext.Current.Request.QueryString, qsDesc)

        ' Abort the request if validation fails
        If Not isValid Then
            If qsDesc.AbortOnError Then
                HttpContext.Current.Response.StatusCode = 500
                HttpContext.Current.Response.[End]()
            Else
                ' Add information about the error to Context.Items
                HttpContext.Current.Items( _
                    QueryStringHelper.QueryStringValidationStatus) = _
                        QueryStringHelper.GetErrorCode()
            End If
        Else
            ' Add information for the page to the Context.Items
            HttpContext.Current.Items( _
                QueryStringHelper.QueryStringValidationStatus) = _
                    QueryStringHelper.GetErrorCode()

            ' Add typed values  
            HttpContext.Current.Items( _
                QueryStringHelper.QueryStringValues) = _
                    QueryStringHelper.GetTypedValues( _
                        HttpContext.Current.Request.QueryString, qsDesc)
        End If
    End Sub
End Class

Friend Class QueryStringDescriptor
    Public Url As String
    Public AbortOnError As Boolean
    Public Parameters As QueryStringParamCollection
End Class

Friend Class QueryStringParamInfo
    Public Name As String
    Public [Type] As QueryStringParamTypes
    Public Length As Integer
    Public [Optional] As Boolean
    Public CaseSensitive As Boolean
End Class

Friend Class QueryStringParamCollection : Inherits Collection( _
        Of QueryStringParamInfo)

    Public Overloads Function Contains(ByVal name As String) As Boolean
        For i As Integer = 0 To Count - 1
            Dim comparison As StringComparison = _
                StringComparison.OrdinalIgnoreCase
            Dim currentItem As QueryStringParamInfo = Item(i)
            If currentItem.CaseSensitive Then
                comparison = StringComparison.Ordinal
            End If

            If String.Equals(currentItem.Name, name, comparison) Then
                Return True
            End If
        Next

        Return False
    End Function

    Public Function Find(ByVal name As String) As QueryStringParamInfo
        For i As Integer = 0 To Count - 1
            Dim currentItem As QueryStringParamInfo = Item(i)
            If String.Equals(currentItem.Name, name, _
                    StringComparison.OrdinalIgnoreCase) Then
                Return currentItem
            End If
        Next

        Return Nothing
    End Function
End Class

<Flags()> _
Public Enum QueryStringErrorCodes
    NoError = 0
    TooManyParameters = 1
    InvalidQueryParameter = 2
    MissingRequiredParameter = 4
    InvalidContent = 8
End Enum