C#: Referencing a windows shell interface

Question

I'm pretty new to C#, I'm trying to complete a little side project I've been working on that uses a small amount of C# code to assist the development of a Windows Desktop Gadget. Basically, I'm trying to implement the IDesktopGadget interface so that I can use the RunGadget method.

Here's what I got so far from reading information about similar interfaces:

[ComImport]
[Guid("C1646BC4-F298-4F91-A204-EB2DD1709D1A")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
interface IDesktopGadget
{
    uint RunGadget([MarshalAs(UnmanagedType.LPWStr)] string gadgetPath);
}

Unfortunately, I get an error when I try and create an object from it: "Cannot create an instance of the abstract class or interface 'GadgetTask.IDesktopGadget'"

Can someone point me in the right direction and maybe help me understand what I'm doing wrong at the same time?

tyranid · Accepted Answer · 2009-12-17 22:24:43Z

1

You actually need an implementation of the DesktopGadget object in order to use the interface. MS provide a standard COM object to do it on Windows 7. You can create an instance by doing something like:

Type t = Type.GetTypeFromCLSID(new Guid("924ccc1b-6562-4c85-8657-d177925222b6"));
IDesktopGadget dg = (IDesktopGadget)Activator.CreateInstance(t);

answered Dec 17, 2009 at 22:24

tyranid

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7 Comments

Andy E Over a year ago

Thanks for the answer. Am I supposed to replace the Guid there with the class id of the interface?

Andy E Over a year ago

I get a "class not registered error" and I'm not sure why. I plucked the class id of the interface right out of my registry.

tyranid Over a year ago

No, the Guid is the CLSID of the object not the interface. I got that Guid from the CLSID_DesktopGadget definition in the SDK headers.

Andy E Over a year ago

Thanks I'll check into it some more then!

Andy E Over a year ago

Thanks for your help so far... I get a new error now:

System.InvalidCastException: Unable to cast COM object of type 'System.__ComObject' to interface type 'IDesktopGadget'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{924CCC1B-6562-4C85-8657-D177925222B6}' failed due to the following error: No such interface supported (Exception from HRESULT: 0x80004002 (E_NOINTERFACE)).

|

Jason · Accepted Answer · 2011-07-28 01:51:48Z

Thanks for the guidance. For a little more straight forward help, this is what worked for me:

IDesktopGadget.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;

namespace GadgetActivator
{
    [ComImport]
    [Guid("C1646BC4-F298-4F91-A204-EB2DD1709D1A")]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]

    interface IDesktopGadget
    {
        uint RunGadget([MarshalAs(UnmanagedType.LPWStr)] string gadgetPath);
    }
}

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;


namespace GadgetActivator
{
    class Program
    {
        static void Main(string[] args)
        {
            Type t = Type.GetTypeFromCLSID(new Guid("924ccc1b-6562-4c85-8657-d177925222b6"));
            IDesktopGadget dg = (IDesktopGadget)Activator.CreateInstance(t);
            dg.RunGadget(@"C:\Program Files\Windows Sidebar\Gadgets\xxxxxxxxx.Gadget");
        }
   }
}

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