Issues import C DLL into C# program

I've been reading on this problem a lot before i decided to post it here. I have C dll that i need to access inside of a C# program. The DLL's code is pretty simple. As it's nothing more then a hook into biometrics driver (it's C because it's has to include lib and header files from driver for the code to work). Here is the code for the dll:

#include <stdio.h> #include <windows.h> #include <DPCN_TM.h> __declspec(dllexport) unsigned char * ConvertPlatinumTemplateToDpTemplate(const unsigned char* inputData); HRESULT Convert( const unsigned char* inputData, const size_t size, DPCN_DATA_TYPE inputDataType, DPCN_DATA_TYPE outputDataType, DPCN_PURPOSE purpose, unsigned char** ppbOutputData, const void * outputParameters, size_t * pcbData) { HRESULT hr = 0; size_t cbData = 0; if (FAILED(hr = DPCNConvertFingerprintData(inputData, size, inputDataType, purpose, NULL, outputDataType, outputParameters, NULL, &cbData))) { return hr; } if (!(*ppbOutputData = (unsigned char *)malloc(cbData))) { return DPCN_ERR_NO_MEMORY; } hr = DPCNConvertFingerprintData(inputData, size, inputDataType, purpose, NULL, outputDataType, outputParameters, *ppbOutputData, &cbData); *pcbData = cbData; return hr; } unsigned char * ConvertPlatinumTemplateToDpTemplate(const unsigned char* inputData) { HRESULT hr = 0; const size_t inputSize = sizeof(inputData); DPCN_DATA_TYPE inputDataType = DPCN_DT_DP_TEMPLATE; DPCN_DATA_TYPE outputDataType = DPCN_DT_DP_PLATINUM_TEMPLATE; unsigned char *pbOutputData = NULL; size_t cbData = 0; hr = Convert(inputData, inputSize, inputDataType, outputDataType, DPCN_PURPOSE_IDENTIFICATION, &pbOutputData, NULL, &cbData); return pbOutputData; } 

As you can see the contents of the DLL is pretty straight forward. From the code you can see I'm needing to access this function inside of the C# program.

unsigned char * ConvertPlatinumTemplateToDpTemplate(const unsigned char* inputData); 

Now in my C# code I have done this:

[DllImport(@"path_to_dll\DPFPTemplateConvert.dll")] public extern byte[] ConvertPlatinumTemplateToDpTemplate(byte[] inputData); 

When I call the function I end up getting this error:

A first chance exception of type 'System.Runtime.InteropServices.MarshalDirectiveException' occurred in DLLImportTest.exe Additional information: Cannot marshal 'return value': Invalid managed/unmanaged type combination. 

What am I doing wrong?