[quote user="clever_anthony"]I guess that the option is relative to DIMSTYLE_PROP_ExtensionLine_ShowAny_BOOLINT[/quote]
With software development, guessing is bad.
[quote user="clever_anthony"]However, the "mdlDimStyle_setBoolIntProp" is not workingmdlDimStyle_setBoolIntProp(0, 1, 211)[/quote]
It's not working because you're not setting a dimension style.
The first parameter is a pointer (in MDL) to a DgnDimStyle. You've passed value zero, which is a pointer to nothing. You need the address of an MDL DgnDimStyle. You can get a pointer to (i.e. the address of) the active dimension style using mdlDimStyle_getActive. Here's the VBA declaration, which you must modify for C#...
Declare Function mdlDimStyle_getActive Lib "stdmdlbltin.dll" ( ) As Long ' Returns a pointer to a structure
Modify your call to this...
mdlDimStyle_setBoolIntProp (mdlDimStyle_getActive (), 1, 211);