C# app calling 64-bit C/C++ DLL crashes, but 32-bit version runs

Question

My C/C++ DLL is built for 32- and 64-bit Windows environments. Two programmers working in .NET (C# and VB.NET) say when they build their clients for 32-bit, everything runs correctly. But when they build for 64-bit, the run ends in an access violation.

I am not a .NET programmer, but I can load their process and step through the DLL code in C++ debugger. The .NET run definitely encounters corrupt memory that does not occur when the client is one I wrote in C++/VCL.

My question: Is there something special in the .NET project settings for interfacing to a 64-bit C/C++ DLL that differs from what works under 32-bit ?

Possibly relevant but I can't prove it: The DLL is built with Character Setting=Not Set for UTF-8/ASCII. I don't know whether the VB.NET client is built for Unicode, but I assume that would create problems.

EDITED TO ADD declarations from the VB.NET, C# clients and the C++ DLL

Declare Function Edit_RunEdits Lib "EDITS50.DLL" (ByVal smfID As Integer, _
                                                  ByVal edit_set_tag As String, _
                                                  ByVal layout_tag As String, _
                                                  ByVal data As String, _
                                                  ByVal edit_options As Integer, _
                                                  ByRef errors_count As Integer, _
                                                  ByVal owner As IntPtr, _
                                                  ByVal callback_func As IntPtr) As Integer

This the C# declaration:

[DllImportAttribute("EDITS50.dll", EntryPoint = "Edit_RunEdits")]
    public static extern int Edit_RunEdits(int smfID, 
         [InAttribute()] [MarshalAsAttribute(UnmanagedType.LPStr)] string edit_set_tag, 
         [InAttribute()] [MarshalAsAttribute(UnmanagedType.LPStr)] string layout_tag, 
         [InAttribute()] [MarshalAsAttribute(UnmanagedType.LPStr)] string data, 
         int edit_options, 
         ref int errors_count, 
         System.IntPtr owner, 
         System.IntPtr callback_func);

This is the C++ side:

#define EDIT_API  __declspec(dllexport) __stdcall
extern "C" int EDIT_API Edit_RunEdits( const int smfID, const char* edit_set_tag, 
const char* layout_tag, 
const char* data, 
const int edit_options, 
int* errors_count, 
void* owner, 
void* callback_func);

Answer 1

I believe I may have found the problem. The project was built for AnyCPU, but under the x64 settings the Prefer32Bit setting was "true". I am asking the .NET programmers to try building explicitly for x64, and to look for and override all instances of Prefer32Bit (set them to "false").

Thank you, everyone, for your feedback.