OS. Use Geometry Methods

Type OpenSTAADUI.OSGeometryUI geometry = os.Geometry; and then press Return.

This defines the geometry variable as the Geometry group of methods in OpenSTAAD.
Type int nodeCount = geometry.GetNodeCount(); and then press Return.

This uses the OpenSTAAD method GetNodeCount() to return the number of nodes in the active STAAD.Pro model into the specified variable.
Type int beamCount = geometry.GetMemberCount(); and then press Return.

Similarly, this uses the OpenSTAAD method GetMemberCount() to return the number of members in the active STAAD.Pro model into the specified variable.

Tip: Reference the OpenSTAAD API Documentation for details on available methods and what values are accepted as input or returned from each.

Your program at this point should look like:

using System;
using System.Runtime.InteropServices;
	

  	namespace OSEOMConsoleApp
{
class Program
        	{
        	static void Main (string[] args)
            	{
            	OpenSTAADUI.OpenSTAAD os = Marshal.GetActiveObject("StaadPro.OpenSTAAD") as OpenSTAADUI.OpenSTAAD;
            	OpenSTAADUI.OSGeometryUI geometry = os.Geometry;
            	int nodeCount = geometry.GetNodeCount();
           	 int beamCount = geometry.GetMemberCount();
            	}
        }
    }