Why might my GetRawData declared function return a correct value when called from my VB.NET application, but return zero when called from my ASP.NET page?

The code is exactly the same except for class type difference (Form / Page) and calling event handler (Form1_Load, Page_Load).

Note: In the actual code, #DLL# and #RAWDATAFILE# are both absolute filenames to my DLL and raw data file respectively.

Note: The DLL file was not created by Visual Studio.

Form1.vb

Public Class Form1

    Declare Auto Function GetRawData Lib "#DLL#" (ByVal filename() As Byte, _
                                                  ByVal byteArray() As Byte, _
                                                  ByVal length As Int32) As Int32


    Private Sub Form1_Load(ByVal sender As System.Object, _
                           ByVal e As System.EventArgs) Handles MyBase.Load

        Dim buffer(10485760) As Byte
        Dim msg As String, length As Integer = 10485760
        Dim filename As String = "#RAWDATAFILE#"
        length = GetRawData(Encoding.Default.GetBytes(filename), buffer, length)

Default.aspx.vb

Partial Public Class _Default
    Inherits System.Web.UI.Page

    Declare Auto Function GetRawData Lib "#DLL#" (ByVal filename() As Byte, _
                                                  ByVal byteArray() As Byte, _
                                                  ByVal length As Int32) As Int32


    Protected Sub Page_Load(ByVal sender As Object, _
                            ByVal e As System.EventArgs) Handles Me.Load

        Dim buffer(10485760) As Byte
        Dim msg As String, length As Integer = 10485760
        Dim filename As String = "#RAWDATAFILE#"
        length = GetRawData(Encoding.Default.GetBytes(filename), buffer, length)