Example of creating a parameter for ventilation

CreateUnitValueParameter.cs

// in a family document
if (!CurrentDocument.IsFamilyDocument)
    throw new Exception();

var mgr = CurrentDocument.FamilyManager;


// get the UNIT_TYPE parameter
var param = mgr.get_Parameter("UNIT_TYPE");

if(param == null)
{
    // If it doesn't exist, create it
    using(var tr = new Transaction(CurrentDocument, "PARAM CREATION"))
    {
        if (!tr.HasStarted())
            tr.Start();

        // name it measurements or whatever
        param = mgr.AddParameter("UNIT_TYPE", BuiltInParameterGroup.PG_ADSK_MODEL_PROPERTIES, ParameterType.Text, false);

        tr.Commit();
    }
}


// input value as something "imperial/metric"
mgr.Set(param, "IMPERIAL");