SAPIEN Solutions

PowerShell and COM objects.

While developing a COM object for scripting use I wanted to provide a VBScript sample as well as a PowerShell sample.

While creating the PowerShell sample I received an unexpected error message when setting a property value. Some quick review shows that PowerShell 1.0 (I did not check with the 2.0 CTP yet) cannot handle write-only COM properties.

Consider the following PowerShell script:

$obj = New-Object -comObject “PropertyTest.PropTest”
$obj.ReadWrite = “Some text”
$readonly = $obj.ReadOnly
$obj.WriteOnly = “some more text”

The object’s properties behave as they are named. If you run these instructions one by one in PowerShell you get the  following error message:

Exception setting “WriteOnly”: “Type mismatch. (Exception from HRESULT: 0×80020
005 (DISP_E_TYPEMISMATCH))”
At line:1 char:6
+ $obj.W <<<< riteOnly = “some more text”

Read-write and read-only properties work fine, only write-only properties are affected. I have uploaded the test component here: PropertyTest.dll

Obviously you need to run regsvr32 PropertyTest.dll if you want to test this.

Technorati Tags: com, powershell, scripting

Tags: com, powershell, scripting

This entry was posted on Thursday, August 7th, 2008 at 12:04 am and is filed under Windows PowerShell. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

9 Responses to “PowerShell and COM objects.”

1. Shay Levy Says:
   August 7th, 2008 at 12:35 am

   I registered the dll and run the code without the writeOnly call, it seems that none of the properties gets updated:

   PS > $obj = New-Object -comObject “PropertyTest.PropTest”
   PS > $obj.ReadWrite = “Some text”
   PS > $obj.ReadWrite
   # no output

   PS > $readonly = $obj.ReadOnly
   PS > $readonly
   # no output

   PS > $obj

   ReadWrite ReadOnly WriteOnly
   ——— ——– ———

   -Shay

2. Alex Riedel Says:
   August 7th, 2008 at 1:30 am

   The object doesn’t really do anything. It’s just there to illustrate the error PowerShell spits out. I can add the source for the component if there is interest.

3. Shay Levy Says:
   August 7th, 2008 at 3:00 am

   Ya, PowerShell COM interoperability is not that good, and that’s an under statment :), BTW, I can ’set’ the WriteOnly member this way:

   $obj.gettype().InvokeMember(”WriteOnly”,”SetProperty”,$null,$obj,”some text”)

4. Marco Shaw Says:
   August 7th, 2008 at 3:45 am

   Same error on XP SP2 with v2 CTP2.

   I’d like to see the source.

5. Alex Riedel Says:
   August 7th, 2008 at 9:16 am

   You can download it at
   ftp://ftp.sapien.com/propertytest.zip

6. Hal Rottenberg Says:
   August 7th, 2008 at 6:56 pm

   Bruce Payette admits their COM support is lacking in his book.

7. Alex Riedel Says:
   August 7th, 2008 at 8:35 pm

   I didn’t want to point any fingers, I just think it’s important to know what exactly happens, specially since the error message does not provide any relevant information.
   A non-programmer could get quite frustrated with this

8. Marco Shaw Says:
   August 8th, 2008 at 1:05 pm

   Yes, PowerShell does lack a bit on the COM side. Ugly workaround, call VBScript to call the COM object. The built-in disk quota COM object suffers here also when called from PowerShell directly.

   I’ve also have similar problems with a COM object from HP for their OpenView Operations agent.

   Is COM dead?

9. Alexander Riedel Says:
   August 8th, 2008 at 3:30 pm

   It will be as soon as can call CmdLets directly from VBScript or there is a “WScript.LoadAssembly”