.NET (Core and Framework)

8-Aug-11 8:52

Hi,

I am trying to enumerate system devices using .Net and keep receiving a null on the following code. How do I fix the problem? I saw several examples on the web, all showing similar code constructs.

try
 {
 // Defined in "C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Include\uuids.h" on line 960
 clsidSystemDeviceEnum = new Guid("62BE5D10-60EB-11d0-BD3B-00A0C911CE86");
 
// Get the COM type from the class ID.
 Type comType = Type.GetTypeFromCLSID(clsidSystemDeviceEnum);
 
// Create the COM instance for this object.
 object oEnum;
 oEnum = null;
 oEnum = Activator.CreateInstance(comType);
 
// Create the device enumerator (COM) object.
 pSysDevEnum = oEnum as ICreateDevEnum; // GetFromClsid(clsidSystemDeviceEnum) as ICreateDevEnum;
 
// Get an enumerator for the device indicated.
 if (null != pSysDevEnum)
 pSysDevEnum.CreateClassEnumerator(clsidDeviceClass, out ppEnumMoniker, dwFlags);
 else
 return;
 }
 
catch (Exception ex)
 {
 MessageBox.Show(ex.Message, Application.ProductName, MessageBoxButtons.OK);
 }

Thanks in advance,

Sarah

Shameel10-Aug-11 5:20

10-Aug-11 5:20

Which line throws the NullReferenceException?

SarahJMR10-Aug-11 5:36

10-Aug-11 5:36

Hi,

Thank you for the response. Here is a better description of the problem. To answer your specific question, oEnum, the Activator.CreateInstance call.

I am trying to enumerate system devices using .Net. The problem that I have is that Activator.CreateInstance returns null, even though I pass in a valid System Device Enumeration GUIID class. You can search the registry and find it.

The result of Type.GetTypeFromCLSID(clsidSystemDeviceEnum, true) copy and pasted from the debugger window holding the oEnum variable is:

{Name = "__ComObject" FullName = "System.__ComObject"}

A Google search yielded a couple of hits that showed that Activator.CreateInstance will sometimes return a null when the type is "__ComObject" but the hits never went deeper. Several articles talk about device enumeration and all gave the same code, namely to start by enumerating the System Device Enumeration object.

You can see the code below. Please ignore the clsidDeviceClass. That is not relevant in the code below. That is used below in the part of the code not visible (in the etc., etc., etc. part). That variable will hold the SCSI Storage object, which is one of the system devices. Just to be specific, this variable will have the value, clsidDeviceClass = new Guid("2ACCFE60-C130-11D2-B082-00A0C91EFB8B");.

First thing is first and that is to resolve why I receive a null from the call to Activator.CreateInstance and of course how I can resolve the problem so that I get a valid object back.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;
using System.Windows.Forms;
using System.Runtime.InteropServices.ComTypes;
using System.Collections;


public void CreateClassEnumerator([In] System.Guid clsidDeviceClass, [Out] out System.Runtime.InteropServices.ComTypes.IEnumMoniker ppEnumMoniker, [In] uint dwFlags)
{
                Guid clsidSystemDeviceEnum;
//            ICreateDevEnum pSysDevEnum;


                // Initialize the return value.
                ppEnumMoniker = null;

                try
                {
                                // Defined in "C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Include\uuids.h" on line 960
                                clsidSystemDeviceEnum = new Guid("62BE5D10-60EB-11d0-BD3B-00A0C911CE86");

                                // Get the COM type from the class ID.
                                Type comType = Type.GetTypeFromCLSID(clsidSystemDeviceEnum, true);

                                // Create the COM instance for this object.
                                object oEnum;
                                oEnum = Activator.CreateInstance(comType);

                                // Create the device enumerator (COM) object.
//                            pSysDevEnum = oEnum as ICreateDevEnum;

                                // etc., etc., etc.
                }

                catch (Exception ex)
                {
                                MessageBox.Show(ex.Message, Application.ProductName, MessageBoxButtons.OK);
                }
}

Thanks,

Sarah

Shameel10-Aug-11 5:54

10-Aug-11 5:54

I just created a console project with your code and it did not return a null. Did you ensure that the C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Include\uuids.h files exists as mentioned in the comments? Could be a missing COM interface.

SarahJMR10-Aug-11 6:02

10-Aug-11 6:02

Hi,

Thanks for the super quick response.

The header file is for informational purposes only and is not needed for compilation. I merely lifted the definition of the System Device Enumeration GUIID from that file. I figure that whomever wants to look at the code in the future should know where I got that GUIID from. Also, C# does not support header files, so...

I am glad that the code works on your end and that Activator.CreateInstance does not return a null on your end. My thought is that either the version of Visual Studio, OS version, or some other difference.

I use Microsoft Windows 7 Professional, 64-bit and Visual Studio 2010.

It might be a Visual Studio 2010 thing. I know that Microsoft ditched DirectShow from Visual Studio 2010, not that I am using DirectShow or need to, so might not be related, but mention. The reason the thought crossed my head is only because Direct<whatever> touches hardware in some form or fashion and that System Device Enumeration also touches hardware, although I am creating a COM object instance at this point in the code, so Activator.CreateInstance should return something other than null.

Thanks in advance,

Sarah

Shameel10-Aug-11 6:10

10-Aug-11 6:10

Yes, of course, C# doesn't need the header file to compile. But a missing header file can signal a missing COM component in the system. I use Win XP with Visual Studio 2010. And since you mentioned that you could in fact find the GUID in the registry, I was wondering what else could be wrong. Let me try with a Win 7 system.

SarahJMR10-Aug-11 6:17

10-Aug-11 6:17

I did a search to find the GUIID. I never even heard of that file before a search turned it up. As far as missing stuff, to my knowledge I have everything. I even installed a couple of days ago the latest Microsoft Windows SDK (v7.1) on my system.

Thanks for helping out Shameel!

Shameel10-Aug-11 9:10