/// <summary>
/// Creates a global parameter and uses it to label the set of given dimension elements 
/// </summary>
/// <param name="document">Revit project document that contains the dimension.</param>
/// <param name="gpname">A name of the global parameter to create.</param>
/// <param name="value">Initila value of the global parameter</param>
/// <param name="dimset">A collection of dimension elements to drive by a global parameter</param>
/// <returns>Number of actually labeled dimension</returns>
public int DriveSelectedDimensions(Document document, string name, double value, ISet<ElementId> dimset)
{
    if (!GlobalParametersManager.AreGlobalParametersAllowed(document))
        throw new System.InvalidOperationException("Global parameters are not permitted in the given document");

    if (!GlobalParametersManager.IsUniqueName(document, name))
        throw new System.ArgumentException("Global parameter with such name already exists in the document", "name");

    if (value <= 0.0)
        throw new System.ArgumentException("Value of a global parameter that drives dimension must be a positive number", "value");

    int nLabeledDims = 0;   // number of labeled dimensions (for testing)

    // creation of any element must be in a transaction
    using (Transaction trans = new Transaction(document, "Create Global Parameter"))
    {
        trans.Start();

        // create a GP with the given name and type Length
        // Note: Length (or Angle) is required type of global parameters that are to label a dimension
        GlobalParameter newgp = GlobalParameter.Create(document, name, ParameterType.Length);
        if (newgp != null)
        {
            newgp.SetValue(new DoubleParameterValue(value));

            // use the parameter to label the given dimensions
            foreach (ElementId elemid in dimset)
            {
                // not just any dimension is allowed to be labeled
                // check first to avoid exceptions
                if (newgp.CanLabelDimension(elemid))
                {
                    newgp.LabelDimension(elemid);
                    nLabeledDims += 1;
                }
            }

            trans.Commit();
        }
    }

    // for illustration purposes only, we'll test the results of our modifications 

    // 1.) Check the new parameter can be found

    ElementId gpid = GlobalParametersManager.FindByName(document,name);
    if (gpid == ElementId.InvalidElementId)
    {
        TaskDialog.Show("Error", "Failed to find a newly created global parameter");
    }

    GlobalParameter gp = document.GetElement(gpid) as GlobalParameter;

    // 2. Check the number of labeled dimension is as expected

    ISet<ElementId> labeledSet = gp.GetLabeledDimensions();
    if (labeledSet.Count != nLabeledDims)
    {
        TaskDialog.Show("Error", "Have not found all the dimension that were labeled.");
    }

    return labeledSet.Count;
}