Casting void far pointer to System::Text::StringBuilder

[Bob]

I am using an API that needs a function pointer with the following
prototype:

void CALLBACK DXLExportStreamFunc ( const BYTE *bytes, DWORD length,
void far *Context )

I get to pass it the Context and want to pass it a StringBuilder to
catch the "bytes" in. I tried:

System::Text::StringBuilder *sb = new System::Text::StringBuilder;
APIFunction ( DXLExportStreamFunc, &sb)

Then in the DXLExportStreamFunc, I tried among other things:
System::Text::StringBuilder *localBuilder =
(System::Text::StringBuilder *)Context;

but it does not work.
I need the solution to leave the memory in place as this function gets
called many times and the StringBuilder will become VERY large.
Thoughts?
Thank You,
Bob K.