DllImport works 32-bit, fails 64-bit (AccessViolationException)

Question

I have built an extension for MapInfo Pro using .Net in C#. Older versions of MapInfo are 32-bit applications, the newest are 64-bit. I want my extension to work with both. While a lot functionality of MapInfo can be called using .Net, some of them only work with unmanaged dlls. One of those dlls is called migrid.dll, which I have in a 32-bit and a 64-bit version. However I do not have the source code, so I'm not able to get the exact definitions of the functions in that file.

One of the functions I want to use is GE_WriteContinuousValue. The only documentation I have about that function is how to declare it using MapBasic (the default programming language for MapInfo):

Declare Function GE_WriteContinuousValue Lib "Migrid.dll" (

ByVal hGrid As Integer, 'grid handle

ByVal lCol As Integer, 'column number

ByVal lRow As Integer, 'row number

ByVal dValue As Float 'z value

) As Logical 'FALSE if there is an error

'TRUE on success

In my c# code i'm using this function with a DllImport:

[DllImport("migrid.dll", CallingConvention = CallingConvention.StdCall)]
public static extern bool GE_WriteContinuousValue(int hGrid, int lCol, int lRow, double dValue);

and then call it for example with:

GE_WriteContinuousValue(iGridID, c, r, value);

Which works fine using the 32-bit dll. When I use the same for the 64-bit dll I get a "'System.AccessViolationException' Attempted to read or write protected memory" at this line:

GE_WriteContinuousValue(iGridID, c, r, value);

Other functions in the same dll work without problems for both 32-bit and 64-bit. So it must be something specific for this function.

Hoping there is a way to get the exact definition of this function I have searched the internet and found a tool called snowman. Which gave me some information, but not anything useful for me. Maybe it helps you?

Result from the 32-bit dll:

struct s43 {
signed char[4] pad4;
int32_t f4;
};

struct s44 {
signed char[4] pad4;
int32_t f4;
};

/* migridu.dll:222 */
int32_t migridu_dll_222 = 0x800000de;

void GE_WriteContinuousValue(int32_t a1, int32_t a2, int32_t a3) {
int32_t* edi4;
int32_t* esi5;
struct s43* edi6;
struct s44* esi7;

*edi4 = *esi5;
edi6->f4 = esi7->f4;
migridu_dll_222();
goto a2;
}

Result from the 64-bit dll:

void fun_180001b50() {
goto GE_WriteContinuousValue;
}

int64_t GE_WriteContinuousValue = 0x577c;

void GE_WriteContinuousValue() {
goto GE_WriteContinuousValue;
}

Can someone help me get rid of the System.AccessViolationException or point me a little bit in the right direction?