COM Interop problem with 3rd Party DLL using C# under ASP.NET

[UKPostie]

I am writing a solution using C# under ASP.NET that must make use of a 3rd
party DLL (written in C I believe). This DLL handles messaging between my
location and a third party location.

I have built and tested a C# Win.Net DLL to access the 3rd party DLL.
Unfortunately, the method of the 3rd party DLL requires both input and output
parameters and those output parameters are strings. Hence my testing resulted
in a successful call to the 3rd party DLL, but I was under able to access the
output strings.

Research on the net lead me to built an intermediary VB6 DLL to had the
pointers required to enable the output parameters.

My test (Win Form Test Harness - Win.Net DLL - VB6 DLL - 3rd party DLL) was
successful.

I then built a ASP.NET DLL and replaced the Win.NET DLL and retested the
following:

ASP.NET Test Harness - ASP.NET DLL - VB6 DLL - 3rd Party DLL

Now I get an error from the call to the 3rd Party DLL method in the VB6 DLL
- "Error 28: Out of stack space".

On a hunch that it had something to do with Context of Execution or
permissions, I built another Test Harness, this time in ASP:

ASP Test Harness - VB6 DLL - 3rd Party DLL

This test was successful, so I conclude it is not to do with Context of
Execution or permissions and I am now stuck.

Is there something about ASP.NET and 3rd Party DLLs that doesn't apply to
Win.NET?