I don't know if there is another way in C#...
get_extents is implemented to call mdlView_getParameters and I think what you need here is to call mdlView_getParametersWithExpandedDepth passing true for "expandDepth".
/*---------------------------------------------------------------------------------**//**
* @Description Identical to ~mmdlView_getParameters, excep that it has an additional argument that will optionally expand the depth
* for a 3D view with front or back clipping turned off to encompass all of the geometry in the view.
* <p>All settings other than viewNumber are returned in the current coordinate system. If any settings
* are NULL, MicroStation does not fill them in.
* @Param origin OUT a Dpoint3d to receive the view's origin. The view origin is the point on the back
* clipping plane's lower left corner.
* @Param center OUT a Dpoint3d to receive the view's center, which is the center of the volume contained
* by the front and back clipping planes.
* @Param delta OUT a Dpoint3d to receive the size of the view in the view's X, Y and Z directions.
* @Param rMatrix OUT a RotMatrix to receive the view's rotation matrix.
* @Param activeZ OUT a double to receive the active depth. Active depth is defined as the positive distance
* from the back clipping plane to the active depth plane.
* @Param view IN view number of interest.
* @Param expandDepth IN if TRUE and the the front and back clipping is disabled, then the origin and delta
* are expanded to include the geometry.
* @Return SUCCESS if the information requested is valid. MDLERR_BADVIEWNUMBER is returned if viewNumber is invalid (not in the
* range 0-7). MDLERR_VIEWNOTDISPLAYED if viewNumber is valid, but the view is not currently displayed.
* @Group "View Functions"
* @bsimethod
+---------------+---------------+---------------+---------------+---------------+------*/
int mdlView_getParametersWithExpandedDepth
(
Dpoint3d* origin,
Dpoint3d* center,
Dpoint3d* delta,
RotMatrix* rMatrix,
double* activeZ,
int view,
BoolInt expandDepth
);
HTH
-B