The Parameter Engine and Params Class

Learn how Paracore transforms simple C# properties into interactive UI widgets without writing a single line of UI code.

1. The Params Class

To create an interactive tool, define a public class Params at the bottom of your script. Paracore scans this class and generates corresponding UI elements for every public property.

Supported Types

• Primitives: double, int, and string become standard input boxes.
• System Families: Use the Revit type directly (e.g., WallType or Wall) to generate list-based pickers.
• Loadable Components: Use the [RevitElements] attribute to list specific categories.

public class Params 
{
    // A standard numeric input
    public double OffsetDistance { get; set; } = 12.0;

    // A dropdown listing every Wall Type in the project
    public WallType TargetType { get; set; }

    // Multi-select list of Rooms
    public List<Room> MyRooms { get; set; }

    // Curation: Listing specific loadable families
    [RevitElements(Category = "Doors")]
    public FamilySymbol? DoorType { get; set; }
}

2. Element Hydration (The Magic)

When you use a Revit element as a parameter, you are interacting with Real Revit Objects.

• The Compute Button: Click the Compute icon in the UI to fetch real instances from the model.
• Direct Access: Because the elements are "Hydrated," you can access their BIM data immediately.

public class Params { ... } // Defined below

// Logic Space
Params p = new();
// p.MyRooms[0] is a real Autodesk.Revit.DB.Architecture.Room!
Println($"Area: {p.MyRooms[0].Area}");

3. Advanced Filtering

Suffix providers (like _Options or _Filter) allow you to curate your UI lists. These must be paired with a primary property.

Custom Lists (_Options)

Use the _Options suffix to provide a curated list of elements for a specific property.

public class Params
{
    // 1. The Primary Property (The UI Widget)
    public List<Room> SmallRooms { get; set; }

    // 2. The Suffix Provider (The Data Source)
    public List<Room> SmallRooms_Options => 
        new FilteredElementCollector(Doc)
            .OfCategory(BuiltInCategory.OST_Rooms)
            .Cast<Room>()
            .Where(r => r.Area < 10.0)
            .ToList();
}

Predicate Filters (_Filter)

Use a boolean _Filter method to validate elements before they reach the UI dropdown.

public class Params
{
    // The Primary Property
    public List<Room> LargeRooms { get; set; }

    // The Filter Logic: Automatically called by the engine
    public bool LargeRooms_Filter(Room r) => r.Area > 20.0;
}

---

Next Step: Practice building custom UIs in the 04 - Step-by-Step Exercise.