Revit API primer

Application                 the Revit session
  Document                  one .rvt (or .rfa)
    Element                 everything is an Element
      ├─ ElementType        the "type" (Category → Family → Type → Instance)
      ├─ Parameter          named values on an element
      ├─ Category           classification
      ├─ View               a view is an Element too
      └─ Geometry           GeometryElement → Solid → Face → Edge → Curve
UIApplication / UIDocument  the UI wrapper — selection, prompts, active view

using (Transaction t = new Transaction(doc, "Renumber doors"))
{
    t.Start();
    foreach (var door in doors)
        door.get_Parameter(BuiltInParameter.ALL_MODEL_MARK).Set(mark);
    t.Commit();
}

Autodesk.Revit.Exceptions.InvalidOperationException:
Attempting to modify the model outside of transaction.

with revit.Transaction("Renumber doors"):
    ...   # rolls back if an exception escapes the block

public class MyHandler : IExternalEventHandler
{
    public string GetName() => "MyHandler";
    public void Execute(UIApplication app)
    {
        // valid API context — safe to open a transaction here
    }
}
 
// once, at startup:
var handler = new MyHandler();
ExternalEvent ev = ExternalEvent.Create(handler);
 
// from anywhere, any thread:
ev.Raise();   // Revit calls Execute() when it's safe

var doors = new FilteredElementCollector(doc)
    .OfCategory(BuiltInCategory.OST_Doors)
    .WhereElementIsNotElementType()
    .ToElements();

.WhereElementIsNotElementType()   // placed instances
.WhereElementIsElementType()      // the type definitions

// good
new FilteredElementCollector(doc)
    .OfCategory(BuiltInCategory.OST_Walls)      // quick
    .WhereElementIsNotElementType()             // quick
    .WherePasses(fireRatingFilter)              // slow, but on a small set
 
// bad — expands every element in the document
new FilteredElementCollector(doc)
    .WherePasses(fireRatingFilter)

new FilteredElementCollector(doc, view.Id)   // only what's visible in that view

// all instances of a specific family type
new FilteredElementCollector(doc)
    .WherePasses(new FamilyInstanceFilter(doc, symbolId));
 
// elements intersecting a bounding box
new FilteredElementCollector(doc)
    .WherePasses(new BoundingBoxIntersectsFilter(outline));
 
// elements in a given design option
new FilteredElementCollector(doc)
    .WherePasses(new ElementDesignOptionFilter(optionId));
 
// everything on a level
new FilteredElementCollector(doc)
    .WherePasses(new ElementLevelFilter(levelId));

// 1. Built-in enum — stable across versions and language packs
Parameter p = element.get_Parameter(BuiltInParameter.ALL_MODEL_MARK);
 
// 2. Shared parameter GUID — stable for your own data
Parameter p = element.get_Parameter(new Guid("9c2f4b7e-...."));
 
// 3. By name — convenient, breaks under localization and typos
Parameter p = element.LookupParameter("Mark");

switch (p.StorageType)
{
    case StorageType.String:    p.AsString();     break;
    case StorageType.Double:    p.AsDouble();     break;   // internal units!
    case StorageType.Integer:   p.AsInteger();    break;   // also Yes/No: 0 or 1
    case StorageType.ElementId: p.AsElementId();  break;
}
p.AsValueString();   // formatted per project units — for display

// convert out
double mm = UnitUtils.ConvertFromInternalUnits(p.AsDouble(), UnitTypeId.Millimeters);
 
// convert in
p.Set(UnitUtils.ConvertToInternalUnits(3000, UnitTypeId.Millimeters));

element.Id          // ElementId — an integer, unique within THIS document only
element.UniqueId    // string GUID-ish — stable across sessions and export

Wall wall = Wall.Create(doc, curve, levelId, structural: false);
Floor floor = Floor.Create(doc, profileLoops, floorTypeId, levelId);
FamilyInstance fi = doc.Create.NewFamilyInstance(point, symbol, level, StructuralType.NonStructural);
ViewSheet sheet = ViewSheet.Create(doc, titleBlockTypeId);
Viewport vp = Viewport.Create(doc, sheet.Id, viewId, location);

if (!symbol.IsActive) { symbol.Activate(); doc.Regenerate(); }

Options opts = new Options { ComputeReferences = true, DetailLevel = ViewDetailLevel.Fine };
GeometryElement geo = element.get_Geometry(opts);
 
foreach (GeometryObject obj in geo)
{
    if (obj is Solid solid && solid.Volume > 0)
        foreach (Face face in solid.Faces) { /* ... */ }
 
    if (obj is GeometryInstance inst)   // family instances nest their geometry
        foreach (GeometryObject nested in inst.GetInstanceGeometry()) { /* ... */ }
}

using Autodesk.Revit.Attributes;
using Autodesk.Revit.DB;
using Autodesk.Revit.UI;
 
[Transaction(TransactionMode.Manual)]
[Regeneration(RegenerationOption.Manual)]
public class CountWalls : IExternalCommand
{
    public Result Execute(ExternalCommandData data, ref string message, ElementSet elements)
    {
        UIDocument uidoc = data.Application.ActiveUIDocument;
        Document doc = uidoc.Document;
 
        var walls = new FilteredElementCollector(doc)
            .OfClass(typeof(Wall))
            .WhereElementIsNotElementType()
            .Cast<Wall>()
            .ToList();
 
        double feet = walls.Sum(w =>
            w.get_Parameter(BuiltInParameter.CURVE_ELEM_LENGTH).AsDouble());
 
        TaskDialog.Show("Walls", $"{walls.Count} walls, {feet:F1} ft total.");
        return Result.Succeeded;
    }
}

<?xml version="1.0" encoding="utf-8"?>
<RevitAddIns>
  <AddIn Type="Command">
    <Name>Count Walls</Name>
    <Assembly>C:\RevitAddins\2027\CountWalls.dll</Assembly>
    <AddInId>0f8a4c1e-6b3d-4a2f-9c8e-1d5b7a9e3f21</AddInId>
    <FullClassName>CountWalls</FullClassName>
    <VendorId>RBC</VendorId>
    <VendorDescription>Revit Bootcamp</VendorDescription>
  </AddIn>
</RevitAddIns>