Dllimport a method returning point to a struct

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C#6.0

I have a c dll that i am trying to call from c# using Dllimport.
I continue to get "Attempted to read or write to protected memory" when the function is called. I have tried just about anything i can think of, so any help would be appreciated.

Here is the C code

struct DocProcHandle {
	unsigned long Size;//sizeof(DocProcHandle)
	void* UserHandle;
	const char* DeviceName;
	unsigned long DeviceTypeId;
	unsigned long DeviceState;
	unsigned long OpenFlags;
	void* Reserved1;
};
typedef struct DocProcHandle *DPHandle;

BPS_DP_API DPHandle CDECLCALL_CONV DPOpenHandleEx(unsigned long deviceTypeId, const char* trkBaseWrite, const char* trkBaseConfig, const unsigned long openFlags);

Here is my c# code

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
struct DocProcHandle
{
  ulong Size;//sizeof(DocProcHandle)
  IntPtr UserHandle;
  [MarshalAs(UnmanagedType.LPStr)] string DeviceName;
  ulong DeviceTypeId;
  ulong DeviceState;
  ulong OpenFlags;
  IntPtr Reserved1;
};

[DllImport("DocProc.dll", EntryPoint = "DPOpenHandleEx", ExactSpelling = false, CallingConvention =CallingConvention.Cdecl)]
private static extern IntPtr DPOpenHandleEx(ulong deviceTypeId, [MarshalAs(UnmanagedType.LPStr)] string trkBaseWrite, [MarshalAs(UnmanagedType.LPStr)] string trkBaseConfig, [In] ulong openFlags);

static public void OpenHandle()
{
  string s = "C:\\temp";
  IntPtr a = DPOpenHandleEx((ulong)7, s, s, 0);
  //DocProcHandle h = (DocProcHandle)Marshal.PtrToStructure(a, typeof(DocProcHandle));
}

It blows up calling the DPOpenHandleEx method.
Any ideas are appreciated.

What I have tried:

I added the additional items to the dllimport line to further refine it.
Changing the return type marshal it.