Casting problem with C# DLL

Not sure if this is best place for this problem, but here it is. I have a
project that is simply a C# class that interfaces with an IFilter. This is
so I can retreive the text from Word docs. I'm able to use this DLL without
any problems within my test windows app, but not within my windows service
(that's when I receive the casting exception).

Here's the code (sorry it's long):

VB Function within windows app and windows service:

Function OfficeFilter(ByVal fname As String) As String
Dim content As String
Dim ifilt As IFilters.DocFilters = CType(New IFilters.CFilter,
IFilters.DocFilters) 'Casting exception here in Windows Service, but not in
test app
Dim ipf As System.Runtime.InteropServices.UCOMIPersistFile =
CType(ifilt, System.Runtime.InteropServices.UCOMIPersistFile)
Try
ipf.Load(fname, 0)

Dim i As System.UInt32 = Convert.ToUInt32(0)
Dim ps As New IFilters.STAT_CHUNK
ifilt.Init(0, Convert.ToUInt32(0), Nothing, i)

Dim hr As Integer = 0
While hr = 0
ifilt.GetChunk(ps)

If ps.flags = IFilters.CHUNKSTATE.CHUNK_TEXT Then
Dim pcwcBuffer As System.UInt32 = Convert.ToUInt32(1000)
Dim hr2 As Integer = 0
While hr2 = IFilters.Constants.FILTER_S_LAST_TEXT OrElse
hr2 = 0
pcwcBuffer = Convert.ToUInt32(1000)
Dim sbBuffer As New System.Text.StringBuilder(1000)
hr2 = ifilt.GetText(pcwcBuffer, sbBuffer)
content &= sbBuffer.ToString
End While
End If
End While
Catch ex As System.Exception
Console.WriteLine(ex.Message)
'Return
End Try
ifilt = Nothing
ipf = Nothing
OfficeFilter = content
End Function 'OfficeFilter

DLL C# class:

using System;
using System.Text;
using System.Runtime.InteropServices;


namespace IFilters
{
[Flags]
public enum IFILTER_INIT
{
NONE = 0,
CANON_PARAGRAPHS = 1,
HARD_LINE_BREAKS = 2,
CANON_HYPHENS = 4,
CANON_SPACES = 8,
APPLY_INDEX_ATTRIBUTES = 16,
APPLY_CRAWL_ATTRIBUTES = 256,
APPLY_OTHER_ATTRIBUTES = 32,
INDEXING_ONLY = 64,
SEARCH_LINKS = 128,
FILTER_OWNED_VALUE_OK = 512
}


[Flags]
public enum IFILTER_FLAGS
{
OLE_PROPERTIES = 1
}

public enum CHUNK_BREAKTYPE
{
CHUNK_NO_BREAK = 0,
CHUNK_EOW = 1,
CHUNK_EOS = 2,
CHUNK_EOP = 3,
CHUNK_EOC = 4
}

[Flags]
public enum CHUNKSTATE
{
CHUNK_TEXT = 0x1,
CHUNK_VALUE = 0x2,
CHUNK_FILTER_OWNED_VALUE = 0x4
}


public enum PSKIND
{
LPWSTR = 0,
PROPID = 1
}


[StructLayout(LayoutKind.Sequential)]
public struct PROPSPEC
{
public uint ulKind;
public uint propid;
public IntPtr lpwstr;
}


[StructLayout(LayoutKind.Sequential)]
public struct FULLPROPSPEC
{
public Guid guidPropSet;
public PROPSPEC psProperty;
}


[StructLayout(LayoutKind.Sequential)]
public struct STAT_CHUNK
{
public uint idChunk;
[MarshalAs(UnmanagedType.U4)] public CHUNK_BREAKTYPE breakType;
[MarshalAs(UnmanagedType.U4)] public CHUNKSTATE flags;
public uint locale;
[MarshalAs(UnmanagedType.Struct)] public FULLPROPSPEC attribute;
public uint idChunkSource;
public uint cwcStartSource;
public uint cwcLenSource;
}


[StructLayout(LayoutKind.Sequential)]
public struct FILTERREGION
{
public uint idChunk;
public uint cwcStart;
public uint cwcExtent;
}


[ComImport]
[Guid("89BCB740-6119-101A-BCB7-00DD010655AF")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface DocFilters
{
void Init([MarshalAs(UnmanagedType.U4)] IFILTER_INIT grfFlags,
uint cAttributes,
[MarshalAs(UnmanagedType.LPArray, SizeParamIndex=1)] FULLPROPSPEC[]
aAttributes,
ref uint pdwFlags);

void GetChunk([MarshalAs(UnmanagedType.Struct)] out STAT_CHUNK pStat);

[PreserveSig] int GetText(ref uint pcwcBuffer,
[MarshalAs(UnmanagedType.LPWStr)] StringBuilder buffer);

void GetValue(ref UIntPtr ppPropValue);

void BindRegion([MarshalAs(UnmanagedType.Struct)]FILTERREGION origPos, ref
Guid riid, ref UIntPtr ppunk);

}


[ComImport]
[Guid("f07f3920-7b8c-11cf-9be8-00aa004b9986")]
public class CFilter
{
}

public class Constants

{
public const uint PID_STG_DIRECTORY =0x00000002;

public const uint PID_STG_CLASSID =0x00000003;
public const uint PID_STG_STORAGETYPE =0x00000004;

public const uint PID_STG_VOLUME_ID =0x00000005;
public const uint PID_STG_PARENT_WORKID =0x00000006;
public const uint PID_STG_SECONDARYSTORE =0x00000007;

public const uint PID_STG_FILEINDEX =0x00000008;
public const uint PID_STG_LASTCHANGEUSN =0x00000009;
public const uint PID_STG_NAME =0x0000000a;
public const uint PID_STG_PATH =0x0000000b;

public const uint PID_STG_SIZE =0x0000000c;
public const uint PID_STG_ATTRIBUTES =0x0000000d;
public const uint PID_STG_WRITETIME =0x0000000e;
public const uint PID_STG_CREATETIME =0x0000000f;
public const uint PID_STG_ACCESSTIME =0x00000010;
public const uint PID_STG_CHANGETIME =0x00000011;

public const uint PID_STG_CONTENTS =0x00000013;
public const uint PID_STG_SHORTNAME =0x00000014;

public const int FILTER_E_END_OF_CHUNKS
=(unchecked((int)0x80041700));
public const int FILTER_E_NO_MORE_TEXT
=(unchecked((int)0x80041701));
public const int FILTER_E_NO_MORE_VALUES
=(unchecked((int)0x80041702));

public const int FILTER_E_NO_TEXT
=(unchecked((int)0x80041705));
public const int FILTER_E_NO_VALUES
=(unchecked((int)0x80041706));

public const int FILTER_S_LAST_TEXT
=(unchecked((int)0x00041709));
}
}