I would like to call a method in an unmanaged C dll from C# using
VS2003. However, I haven't been able to
successfully do this so far. I am hoping that someone can help me get
this working.

The method signature for the C function is:
HRESULT __stdcall FuncFromCDll(ISomeInterface** ppISomeInterface)

Here is what I've tried:
1)
[DllImport("somedll.DLL", EntryPoint="FuncFromCDll", ExactSpelling=
true, CallingConvention=CallingConvention.StdCall)]
public static extern void FuncFromCDll(ref IntPtr ppISomeInterface);

I then called the above as shown below:

IntPtr myIntPtr = IntPtr.Zero;
FuncFromCDll(ref myIntPtr);
object myObject = Marshal.GetObjectForIUnknown(myIntPtr);

This successfully calls FuncFromCDll. (I confirmed by debugging into
the code above that myIntPtr has a non-zero value after the
FuncFromCDll call.) However, it fails in Marshal.GetObjectForIUnknown
call giving a
System.ExecutionEngineException. Note that ISomeInterface does derive
from IUnknown.

Don't know why but if I initialize myIntPtr as IntPtr myIntPtr = new
IntPtr(300) or some other
non-zero value, then the call to FuncFromCDll fails causing the
application to immediate shut down -- I guess it might
be because IntPtr is pointer to a memory location and we don't know
what is at memory location that we are pointing to when we assign it a
non-zero value.

2) I tried using IntPtr instead of ref IntPtr.
[DllImport("somedll.DLL", EntryPoint="FuncFromCDll", ExactSpelling=
true, CallingConvention=CallingConvention.StdCall)]
public static extern void FuncFromCDll(IntPtr ppISomeInterface);

I then called the above as shown below:

IntPtr myIntPtr = IntPtr.Zero;
FuncFromCDll(myIntPtr);
object myObject = Marshal.GetObjectForIUnknown(myIntPtr);

This fails in FuncFromCDll call with "Object reference not set to an
instance of an object" error.

3) I tried a variation of (1) above using ref Object instead of ref
IntPtr in my P/Invoke call.
I got an error in FuncFromCDll call saying that "Specified OLE variant
is invalid."

4) I also tried the code below which was suggested in the following
thread:
http://groups-beta.google.com/group/microsoft.public.dotnet.framework.interop/br
owse_thread/thread/a1becfa8dc6d46aa/434911737fb7ad58?q=%22pointer+to+a+pointer%2
2&_done=%2Fgroup%2Fmicrosoft.public.dotnet.framework.interop%2Fsearch%3Fgroup%3D
microsoft.public.dotnet.framework.interop%26q%3D%22pointer+to+a+pointer%22%26qt_
g%3D1%26&_doneTitle=Back+to+Search&&d#434911737fb7ad58

[DllImport("somedll.DLL")]
public static extern void FuncFromCDll(
[MarshalAs(UnmanagedType.Interface)]
out ISomeInterface ppISomeInterface);

//Defined ISomeInterface interface in C#
[
ComImport,
InterfaceType(ComInterfaceType.InterfaceIsIUnknown),
Guid("12345678-1234-4323-7878-984783787873")
]
public interface ISomeInterface
{
//Defined placeholders for all the methods on this
//interface
void Method1();
void Method2();
...
}

ISomeInterface someInterface;
FuncFromCDll(out someInterface);

The call to FuncFromCDll fails giving
System.ExecutionEngineException.

I've spent quite a lot of time looking at the newsgroups and websites
(including MSDN) trying to find a solution to this problem, with no
luck.
Any ideas on what else I can try or where I can find examples of how to
do this?

Thanks!
Nauman