sdaiGetMemberCount

public const string ifcenginedll = @"ifcengine.dll";

[DllImport(IFCEngineDLL, EntryPoint = "sdaiGetMemberCount")]
public static extern Int32 sdaiGetMemberCount(int_t aggregate);    

using RDF;      //  include at least engine.cs within your solution

void GetColumns(Int64 model)
{
    Int64 ifcColumnInstances = ifcengine.sdaiGetEntityExtentBN(model, "IFCCOLUMN"),
          noIfcColumnInstances = ifcengine.sdaiGetMemberCount(ifcColumnInstances);
    if (noIfcColumnInstances != 0)
    {
        for (Int64 i = 0; i < noIfcColumnInstances; i++)
        {
            Int64 ifcColumnInstance = 0;
            ifcengine.sdaiGetAggrByIndex(ifcColumnInstances, i, ifcengine.sdaiINSTANCE, out ifcColumnInstance);

            string globalId;
            ifcengine.sdaiGetAttrBN(ifcColumnInstance, "GlobalId", ifcengine.sdaiUNICODE, out globalId);

            string name;
            ifcengine.sdaiGetAttrBN(ifcColumnInstance, "Name", ifcengine.sdaiUNICODE, out name);

            string description;
            ifcengine.sdaiGetAttrBN(ifcColumnInstance, "Description", ifcengine.sdaiUNICODE, out description);
        }
    }
}