Problems Determining Patch.State - WindowsInstaller

Hi all,
I hope someone can help me here. There aren't many good code examples out there dealing with Windows Installer patches (.msp). We have an update widget that will install or apply a patch simply if the .msp is located in the application's update folder location. I want to try to change that to first, determine if the patch actually needs to be applied.

First, I need to grab the Patch/Product codes from the .msp, then check the Patch.State against the Product Code. The first part is working fine, but I'm having problems checking the State...

1.	Const MinMsiVersion = "3.0" 'Minimum version to support functionality
2.	Const MSIPATCHSTATE_APPLIED = 1 'Patch is applied to this product instance.
3.	Const msiInstallContextMachine = 4 'Enumerate products that are under the machine account.
4.	 
5.	Dim iInstaller As WindowsInstaller.Installer
6.	Dim pPatch 'As WindowsInstaller.Patch
7.	        '= CreateObject("WindowsInstaller.Installer")
8.	 
9.	Dim strPatchPath, _
10.	    strPatchCode, _
11.	    strProdCode As String
12.	Dim strPatchXML As String
13.	Dim xmlMsiPatch As XmlDocument = New XmlDocument()
14.	 
15.	Try       
16.	    If MinMsiVersion > iInstaller.Version Then
17.	        MsgBox("Minimum Windows Installer version " & MinMsiVersion & " required.  Current version is " & iInstaller.Version, MsgBoxStyle.OkOnly, "Windows Installer version problem...")
18.	    End If
19.	 
20.	    iInstaller = CType(CreateObject("WindowsInstaller.Installer"),  _
21.	                   WindowsInstaller.Installer)
22.	 
23.	    strPatchPath = "C:\Program Files (x86)\MyComp\Adept80\MyApp\Install\MyAppPatch.msp"
24.	    strPatchXML = iInstaller.ExtractPatchXMLData(strPatchPath)
25.	 
26.	    xmlMsiPatch.LoadXml(strPatchXML)
27.	 
28.	    strPatchCode = xmlMsiPatch.DocumentElement.Attributes("PatchGUID"
).Value
29.	    strProdCode = xmlMsiPatch.GetElementsByTagName("TargetProductCod
e").Item(0).InnerText
30.	 
31.	    pPatch = iInstaller.Patch(strPatchCode, strProdCode, "", msiInstallContextMachine)
32.	 
33.	    If pPatch.State = MSIPATCHSTATE_APPLIED Then 'already applied
34.	        MsgBox("Write to log... Patch has already been applied (" & pPatch.State & ")!")
35.	    Else
36.	        MsgBox("I'm installing the patch")
37.	    End If
38.	 
39.	Catch ex As Exception
40.	    MsgBox(Err.Number)
41.	    MsgBox(Err.Description)
42.	    'MsgBox(ex.Message)
43.	    MsgBox(ex.ToString)
44.	 
45.	    'ERROR_PATCH_NOT_APPLIED
46.	End Try

This code is failing on line 31 with an error 13 - Return Argument Has An Invalid Type. Can anyone help get me around this? I've attached an image with some more info from this error.

Since all of this may have to be ported to C++, I wonder if its better to try to get the following code up and running...

1.	Const MinMsiVersion = "3.0" 'Minimum version to support functionality
2.	Const MSIPATCHSTATE_APPLIED = 1 'Patch is applied to this product instance.
3.	Const msiInstallContextMachine = 4 'Enumerate products that are under the machine account.
4.	 
5.	Dim iInstaller = CreateObject("WindowsInstaller.Installer")
6.	 
7.	Dim strPatchPath, _
8.	    strPatchCode, _
9.	    strProdCode As String
10.	Dim strPatchXML As String
11.	Dim xmlMsiPatch As XmlDocument = New XmlDocument()
12.	 
13.	Try
14.	 
15.	If MinMsiVersion > iInstaller.Version Then
16.	     MsgBox("Minimum Windows Installer version " & MinMsiVersion & " required.  Current version is " & iInstaller.Version, MsgBoxStyle.OkOnly, "Windows Installer version problem...")
17.	End If
18.	 
19.	strPatchPath = "C:\Program Files (x86)\MyComp\MyApp\Client\Install\MyAppPatch.msp"
20.	strPatchXML = iInstaller.ExtractPatchXMLData(strPatchPath)
21.	 
22.	xmlMsiPatch.LoadXml(strPatchXML)
23.	 
24.	strPatchCode = xmlMsiPatch.DocumentElement.Attributes("PatchGUID"
).Value
25.	strProdCode = xmlMsiPatch.GetElementsByTagName("TargetProductCod
e").Item(0).InnerText
26.	 
27.	Dim pPatch = iInstaller.Patch(strPatchCode, strProdCode, "", msiInstallContextMachine)
28.	 
29.	If pPatch.State = MSIPATCHSTATE_APPLIED Then 'already applied
30.	    MsgBox("Write to log... Patch has already been applied (" & pPatch.State & ")!")
31.	Else
32.	    MsgBox("I'm installing the patch")
33.	End If
34.	 
35.	Catch ex As Exception
36.	    MsgBox(Err.Number)
37.	    MsgBox(Err.Description)
38.	    'MsgBox(ex.Message)
39.	    MsgBox(ex.ToString)
40.	 
41.	    'ERROR_PATCH_NOT_APPLIED
42.	End Try

However this code snippet was failing on...

If pPatch.State = MSIPATCHSTATE_APPLIED Then

The error number I eventually was able to get from it was -21470223249 which after digging a bit I found was ERROR_PATCH_NOT_APPLIED.

I'm not currently able to handle if the patch has not been applied with this code. If the patch is installed, the return, 1, is fine and the log write would occur (in this case msgbox). If not installed or applied, BOOM!

In the first code snipped, these were the imports...

Imports WindowsInstaller
Imports System
Imports System.Xml

In the second code segment, Windowsinstaller was commented out.

I hope someone can help me as I think/hope I am very close to having some workable code to detect if a patch has been applied!

Thanks in advance for any help!!!

THANKS FOR YOUR HELP!!