ASP.NET Web API calling unmanaged Dll hanging

Hi all, 

I'm trying to dynamically call unmanaged C++ Dll from within my Web API. For this reason, I created 2 Controller Action:

(1) One is starting the dll: It uses the LoadLibrary to load the dll, and call an init-method in the dll, initializing some parameters for calculation.

(2) Here's is were I need the calculation result from my Dll (using the handle from LoadLibrary in Action 1, I use GetProcAdress to locate the function and create the delegate).

It seems to work fine, but when calling it multiple times, it hangs up - and this very sporadically and unpredictable. When I pause the debugging with Visual Studio, it is paused at my delegate-call for the function...

Can someone please help me?

Here's my code:

[DllImport("kernel32.dll", SetLastError = true)]
private static extern IntPtr LoadLibrary(string dllToLoad);

[DllImport("kernel32.dll", SetLastError = true)]
private static extern IntPtr GetProcAddress(IntPtr hModule, string procedureName);


[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
private delegate bool DllTransfer(ref WebCalcInfo Info);

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
private delegate bool InitDll([MarshalAs(UnmanagedType.LPStr)]string pExternPath);


//This method is called in 1. Action
public static bool LoadInitLibrary(string path)
        {
            if (!IsLoaded)
            {
                PATH = path;
                if (!Directory.Exists(PATH) && !File.Exists(PATH_DLL))
                    throw new Exception(string.Format("Dll not found: {0}.", PATH_DLL));

                //LoadLibrary
                hModule = DllInterop.LoadLibrary(PATH_DLL);
                IsLoaded = (hModule != IntPtr.Zero);
                if (!IsLoaded)
                {
                    int errno = Marshal.GetLastWin32Error();
                    throw new Exception(string.Format("Failed to load library. Error:{0}.", errno));
                }

                //GetPointer: PROC_InitDll
                IntPtr intPtrInitDll = DllInterop.GetProcAddress(hModule, PROC_InitDll);
                if (intPtrInitDll == IntPtr.Zero)
                {
                    int errno = Marshal.GetLastWin32Error();
                    FreeLibrary();
                    throw new Exception(string.Format("Process Address not found: PROC_InitDll is IntPtr.Zero. Error:{0}.", errno));
                }
                DllInterop.InitDll initDll = (DllInterop.InitDll)Marshal.GetDelegateForFunctionPointer(intPtrInitDll, typeof(DllInterop.InitDll));

                //InitDLL
                IsInitialized = initDll(string.Format("P={0}", PATH));
                if (!IsInitialized)
                {
                    FreeLibrary();
                    throw new Exception("PROC_InitDll returned FALSE.");
                }

                return true;
            }
            return false;
        }

//This method is for the 2.Action
public static CalcInfo TransferData(WebCalcInfo calcInfo)
        {
            if (!IsLoaded && !string.IsNullOrEmpty(PATH))
            {
                System.Diagnostics.Trace.WriteLine("Library was not loaded, start loading and init Dll.", "TransferData");
                LoadInitLibrary(PATH);
            }

            IntPtr intPtrTransfer = DllInterop.GetProcAddress(DllInterop.hModule, PROC_DllTransfer);
            if (intPtrTransfer == IntPtr.Zero)
            {
                int errno = Marshal.GetLastWin32Error();
                FreeLibrary();
                throw new Exception(string.Format("Process Address not found: PROC_DllTransfer is IntPtr.Zero. Error:{0}.", errno));
            }
            DllInterop.DllTransfer dllTransfer = (DllInterop.DllTransfer)Marshal.GetDelegateForFunctionPointer(intPtrTransfer, typeof(DllInterop.DllTransfer));

            bool transferResult = dllTransfer(ref Info); // HERE IT HANGS
           
            if (!transferResult)
            {
                FreeLibrary();
                throw new Exception("PROC_DllTransfer returned FALSE.");
            }
}