Can i directly use PInvoke DllImport attribute to call a function from a COM DLL?

steven

it is a COM dll, so i can call the function in javascript like this:

         var typelib = new ActiveXObject("scriptlet.typelib");
        typelib.Reset();
        document.getElementById("myID").innerHTML = typelib.guid;

It works well.

But i tried to call a function from the COM, it did not work.

[DllImport("scrobj.dll")]
internal static extern void Reset();

...
...

Reset();

It said Unable to find an entry point named 'Reset' in DLL 'scrobj.dll'.

1) Can i directly use PInvoke DllImport attribute to call a function from a COM DLL?
2) WHY?

Thanks.

6 Replies

Know the answer? Post it — somebody with the same question will find it here.

Vulpes15y ago
Accepted answer
P/Invoke doesn't work here because the Reset() function is actually inside an interface called IGenScriptletTLib (see http://www.webtropy.com/articles/art14-2.asp?Interop=Scriptlet). It's not a global function which is exported by scrobj.dll.

You'd need to use COM interop and create an object which implemented IGenScriptletTLib in order to call the Reset() function and then obtain its GUID property.

However, if all you're trying to do is to generate a GUID, then you can do this very easily using the .NET Guid structure:

string guid = Guid.NewGuid().ToString();
steven15y ago
Sam, much thanks for your quick reply. I have got Dependency Walker, looks easy to go with it.
Sam Hobbs15y ago
You cannot call it as a regular DLL because it is not a regular DLL.

As for knowing if it is a COM object, one great tool is Visual Studio. If it appears in the COM tab when you try to add a reference, then you know it is a COM object. Simple. Another way is using the OLE-Com Object Viewer. Look for it in the Windows SDK Tools in the Visual Studio menu in the Start Menu.

The thing that makes a COM object a COM object is the type library. Both of the techniques above will recognize it as a COM object (if it is) mainly because of the type library. Sometimes a type library is in the same DLL as the COM object and sometimes the type library is in a tlb file.

There is a tool called Depends that can show you what is in a DLL. Depends is no longer provided by Microsoft but it is available elsewhere. Most COM objects have a DllRegisterServer entry point.

I have a few uncompleted articles in COM And ActiveX in my web site that might help. I use the term COM Automation but that is essentially the same as a COM object.
steven15y ago
Thanks Vulpes & Sam. I knew scrobj.dll is a COM object, and also i knew how to call it in .Net.

I just want to know why i cannot call it by PInvoke way. So the reason is as you said, it's not a global function which is exported by scrobj.dll, right...

I have aother question, how to know a DLL is COM object or general dynamic libarary? i can look into a COM object using OLE/COM object Viewer, How can i look into a general DLL, is there some tool like .Net reflector?
Vulpes15y ago
As it may not be immediately obvious, it's the Microsoft Scriptlet Library (scrobj.dll) that you need to add a reference to which should be there under the COM tab.

The code to get the GUID would then be:

using Scriptlet;

// ....

Type t = Type.GetTypeFromProgID("Scriptlet.TypeLib");
IGenScriptletTLib tlib = (IGenScriptletTLib)Activator.CreateInstance(t);
tlib.Reset();
string guid = tlib.GUID.Substring(0, tlib.GUID.Length - 2);

For some unknown reason you seem to get a couple of extra characters at the end of the GUID which I've trimmed off.

If you're using C# 4.0, the code is slightly easier as you can use the new 'dynamic' keyword to leave the runtime to figure out the type of tlib:

Type t = Type.GetTypeFromProgID("Scriptlet.TypeLib");
dynamic tlib = Activator.CreateInstance(t);
tlib.Reset();
string guid = tlib.GUID.Substring(0, tlib.GUID.Length - 2);
Sam Hobbs15y ago
You need to use the library as a COM object. The DLL is developed to be used that way and if you call it directly it will be very confused.

So in your C# or VB .Net project, go to the Solution Explorer and right-click on the References tab. Select Add Reference. Then look at the COM tab and look for your COM object. If you find it, select it and click on Okay. If you don't find it, then go to the Browse tab and browse to the DLL or whatever, then click okay.