How to use C# -openstaad get beam force?I need an sample?

if I use openstaad as follow, there is mistake, objOpenSTAAD =null, I have import  LibBentley.manifest file. Ineed you to seed me an an sample, thalnk you ,

OpenSTAAD objOpenSTAAD = Marshal.GetActiveObject("StaadPro.OpenStaad") as OpenSTAAD;
objOpenSTAAD.Analyze();
double[] StartForces = new Double[6] {-1, -1, -1, -1, -1, -1 };

objOpenSTAAD.Output.GetMemberEndForces(1, 0, 21, StartForces);