The app uses DLLImport to call a legacy unmanaged dll. Let's call this dll Unmanaged.dll for the sake of this question. Unmanaged.dll has dependencies on 5 other legacy dll's. All of the legacy dll's are placed in the WebApp/bin/ directory of my ASP.NET application.

When IIS is running in 5.0 isolation mode, the app works fine - calls to the legacy dll are processed without error.

When IIS is running in the default 6.0 mode, the app is able to initiate the Unmanaged.dll (InitMe()), but dies during a later call to it (ProcessString()).

I'm pulling my hair out here. I've moved the unmanaged dll's to various locations, tried all kinds of security settings and searched long and hard for a solution. Help!

Sample code:

[DllImport("Unmanaged.dll", EntryPoint="initME", CharSet=System.Runtime.InteropServices.CharSet.Ansi, CallingConvention=CallingConvention.Cdecl)]
	internal static extern int InitME();
//Calls to InitMe work fine - Unmanaged.dll initiates and writes some entries in a dedicated log file
[DllImport("Unmanaged.dll", EntryPoint="processString", CharSet=System.Runtime.InteropServices.CharSet.Ansi, CallingConvention=CallingConvention.Cdecl)]
	internal static extern int ProcessString(string inStream, int inLen, StringBuilder outStream, ref int outLen, int maxLen);
//Calls to ProcessString cause the app to crash, without leaving much of a trace that I can find so far