how to pass void * (pointing to double or int) from C to C#

Question

I have general callback type in C:

typedef int(*OverrideFieldValueSetCB_t)(const char *Dialog, const char *FieldName, void *Value);

and callback:

OverrideFieldValueSetCB_t       gOverrideFieldValueSetCB;

and function I call in C-code to pass value to C# :

int DllGuiSetFieldValue(const char *Dialog, const char *FieldName, void *pValue)
{   
    return gOverrideFieldValueSetCB(Dialog, FieldName, pValue);
}

In C# code I set this kind of delegate:

private static int OverrideFieldValueSetCb(string dialogName, string fieldName, IntPtr value)
{
    ///...
}

In above I would like to marshal/cast value to int or double depending on fieldName.

Questions:

1. Is IntPtr correct?
2. If IntPtr is correct, how to cast/marshal it to double or int ?

Answer 1

"Pointing to double or int" is just asking for trouble.

But if you're certain that's the way you want to go, have a look at the Marshal class - Marshal.ReadInt32 for int, and Marshal.PtrToStructure<double> for double. Make sure you don't mess the two up :)

Of course, if you can use unsafe code, you don't need to use Marshal. Just do the cast as you would in C.

Example:

double val = 123.45d;
double second;
double third;

unsafe
{
  void* ptr = &val;

  second = *(double*)ptr;
  third = Marshal.PtrToStructure<double>(new IntPtr(&val));
}

second.Dump();