/// <summary>
/// Modifies values of certain types of global parameters so that
/// - integer values will be incremented by one
/// - double values will be multiplied by two
/// - boolean values will be negated
/// </summary>
/// <param name="document">Revit project document in which we create global parameters.</param>
/// <param name="gpset">A set of ElementIds of global parameter elements</param>
/// <returns>Number of actually modified values</returns>
public int ModifyGlobalParametersValues(Document document, ISet<ElementId> gpset)
{
    int nChangedValues = 0;     // number of values changed

    // we need a transaction to modify the model
    using (Transaction trans = new Transaction(document, "Change global parameters values"))
    {
        trans.Start();

        foreach(ElementId gpid in gpset)
        {
            // filter out element Ids that are not of a global parameters
            // (in production code, this would be an assert-able situation)
            if (!GlobalParametersManager.IsValidGlobalParameter(document, gpid))
                continue;

            // get the current value of the global parameter
            GlobalParameter gp = document.GetElement(gpid) as GlobalParameter;
            ParameterValue gpvalue = gp.GetValue();

            // test whether it is a Double or Integer
            if (gp.GetType() == typeof(DoubleParameterValue))
            {
                DoubleParameterValue dvalue = gpvalue as DoubleParameterValue;
                dvalue.Value *= 2.0;
                gp.SetValue(dvalue);
                nChangedValues += 1;
            }
            else if (gp.GetType() == typeof(IntegerParameterValue))
            {
                // Integer values may represent Boolean parameters too
                IntegerParameterValue ivalue = gpvalue as IntegerParameterValue;
                if (gp.GetDefinition().ParameterType == ParameterType.YesNo)
                {
                    ivalue.Value = (ivalue.Value == 0) ? 1 : 0;
                }
                else   // common integers
                {
                    ivalue.Value += 1;
                }
                gp.SetValue(ivalue);
                nChangedValues += 1;
            }
        }

        trans.Commit();
    }

    return nChangedValues;
}