This guide is meant to help developers get familiar with the structure of a basic Revit add-in. The code specific to this particular add-in is not covered in detail.

This is what I learned building my first Plugin for Autodesk Revit.

The Problem

Per default door tags in Revit contain an auto-incrementing number starting at 1. Often it would be more useful if they showed the number of the associated room instead. This is also the naming scheme followed by the Revit Architecture sample Project.

To automatically label the door tags this way we need to

  1. Access a door element when it is added or iterate over every door (to rename all existing doors)
  2. Find out the room the door leads to
  3. Rename the door tag

The Tool – Why F#?

While I love F# for many reasons, I found it particularly nice for building a Revit add-in for these three reasons:

  1. Type inference is awesome. The F# compiler is smart. Usually it deduces most variable types and function signatures without requiring you to explicitly declare them.
  2. Clean syntax is cool. In F# you don’t need curly braces or semicolons for delimiting code blocks. Like in python, this happens through indentation only. This eliminates much of the noise that takes up many lines of code in a C# program.
  3. List functions and lambdas rule. As you will see later, F# makes it easy to perform complex operations on lists and sequences.

For more information on why F# is awesome I recommend reading Scott Wlaschin’s “Why Use F#?” Series.

Need To Know: Revit Add-Ins

Before we start building our add-in it is important to get familiar with the general structure of such an add-in. We need to know that:

A Revit add-in can either be a Command or an Application. Commands (Revit.DB.IExternalCommand) execute once when triggered. Applications (Revit.DB.IExternalApplication) start on Revit startup and run until Revit is closed.

All the important meta-information is saved in xml files with the .addin extension. These files are called add-in Manifests and contain the path to the add-in dll and information like the author’s name or a unique GUID. One add-in Manifest file can contain information about multiple add-ins.

On startup Revit (2017) checks the addin folders ProgramData\Autodesk\Revit\Addins\2017 and Users\[user]\AppData\Roaming\Autodesk\Revit\Addins\2017 for add-in manifests. It then tries to load the described add-in executables.

The Add-In: Rename Door Tags

To solve our problem we need to build two seperate add-ins:

  • A command that iterates over every door in the project and changes their door tags
  • An application that waits until a new doors is added and then changes its tag.

We start by building the command

Add-In Boilerplate

When using Visual Studio, we need to add references to RevitAPI.dll and RevitAPIUI.dll, both located in C:\Program Files\Autodesk\Revit 2017\. We then start by building a simple class that implements the IExternalCommand Interface.


open Autodesk.Revit.UI
open Autodesk.Revit.DB
open Autodesk.Revit.Attributes

[<Transaction( TransactionMode.Manual )>]
type RenameAll() =
    interface IExternalCommand with
        member x.Execute(commandData, message, elements) =

            // Our code goes here


If we now add the following add-in manifest to C:\ProgramData\Autodesk\Revit\Addins\2017

<?xml version="1.0" encoding="utf-8"?>
  <AddIn Type="Command">
    <Name>Label all doors</Name>
    Build Informed,

… we already have a valid Revit command!

Access Revit Elements

We can now start adding functionality to our empty command. First of all, we need to get a list of all door instances in the active Revit Model. To achieve this, we use a FilteredElementCollector:

// retrieve document from the commandData passed to x.Execute
let app = commandData.Application
let doc = app.ActiveUIDocument.Document

// get a list of all doors in the document
let getDoors doc =
    use collector =
        (new FilteredElementCollector(doc))

    |> Seq.cast<FamilyInstance>
    |> Seq.toList

Because the FilteredElementCollector implements IEnumerable (and we already filtered for FamilyInstances) we can cast the collector into an F# Sequence of FamilyInstances. Thanks to F#’s smart type inference we don’t have to declare any type explicitly. The compiler knows that the FilteredElementCollector constructor needs a Document and deduces that the argument of getDoors has to be a Document.

Now all that’s left to do is change each door’s mark property to the number of the door’s containing room.

A post about handling Events in Revit that follows up on this example will be available soon. It will released on our Blog and on our Social Media Channels.

How to Build a Revit Add-In Using F#

time to read: 6 min