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Architecture

This document explains how TotalLength is laid out and how the pieces fit together. Reading this before making changes will save you time.

High-level overview

TotalLength is a single AutoCAD .NET add-on that ships as codehaks.TotalLength.dll. The same source code is compiled three times — once per supported AutoCAD release — using a shared project (.shproj) imported by three thin per-version csprojs.

                  ┌────────────────────────────┐
                  │   src/Shared/Shared.shproj │
                  │   (all source + XAML)      │
                  └─────────────┬──────────────┘
                                │ projitems import
        ┌───────────────────────┼───────────────────────┐
        ▼                       ▼                       ▼
┌────────────────┐     ┌────────────────┐     ┌────────────────┐
│ TotalLength    │     │ TotalLength    │     │ TotalLength    │
│ 2024.csproj    │     │ 2025.csproj    │     │ 2026.csproj    │
│ net48          │     │ net8.0-windows │     │ net8.0-windows │
│ AutoCAD 2024   │     │ AutoCAD 2025   │     │ AutoCAD 2026   │
└────────────────┘     └────────────────┘     └────────────────┘
        │                       │                       │
        └────────► codehaks.TotalLength.dll ◄───────────┘
                  (one binary per version)

Target frameworks

AutoCAD Project Target framework Project format
2024 src/TotalLength 2024/ .NET Framework 4.8 Old-style .csproj with Microsoft.CSharp.targets
2025 src/TotalLength 2025/ net8.0-windows SDK-style (Microsoft.NET.Sdk, <UseWPF>true</UseWPF>)
2026 src/TotalLength 2026/ net8.0-windows SDK-style (Microsoft.NET.Sdk, <UseWPF>true</UseWPF>)

The 2024 / 2025 split mirrors the AutoCAD .NET runtime change: AutoCAD 2024 still hosts .NET Framework 4.8; AutoCAD 2025+ hosts .NET 8 (Core).

Looking ahead: AutoCAD 2027 will require .NET 10. When that target is added, the pattern repeats — clone the 2025/2026 csproj, change <TargetFramework> to net10.0-windows, point hint paths at AutoCAD 2027.

Layering

All shared source lives under src/Shared/. The codebase follows a light layered structure:

src/Shared/
├── AcadApplication.cs          ← IExtensionApplication entry (registers ribbon)
├── AcadCommand.cs              ← [CommandMethod("ZLen")] — opens the WPF window
│
├── Application/
│   └── SelectService.cs        ← AutoCAD selection + length math
│
├── Core/
│   └── SelectOptions.cs        ← user-toggled object-type filter (DTO)
│
├── Common/
│   └── RelayCommand.cs         ← MVVM ICommand<T> implementation
│
├── Commands/
│   └── DrawCircleCommand.cs    ← DEBUG-only smoke test command
│
└── Presentation/
    ├── Ribbons/
    │   ├── RibbonManager.cs        ← builds the "Codehaks" tab + button
    │   └── RibbonMainCommand.cs    ← ribbon button → SendStringToExecute("ZLen")
    ├── ViewModels/
    │   └── MainViewModel.cs        ← orchestrates SelectService, exposes commands
    └── Views/
        ├── MainWindow.xaml
        └── MainWindow.xaml.cs

Responsibilities

  • AcadApplication — implements IExtensionApplication. Runs on every AutoCAD session start (after NETLOAD); calls RibbonManager.AddRibbons().
  • AcadCommand.ShowMainWindow — the ZLen command. Constructs MainWindow, parents it to the AutoCAD main window via WindowInteropHelper, calls ShowDialog().
  • SelectService.SelectObjects — wraps editor.GetSelection(filter), walks the resulting ObjectIdCollection inside a transaction, and sums each Curve's length using GetDistanceAtParameter(EndParam) - GetDistanceAtParameter(StartParam). Returns (SelectedCount, TotalLength, Message).
  • SelectOptions — POCO carrying three booleans (lines / polylines / arcs). Its ToString() produces a comma-separated DXF type filter (e.g. "LINE,LWPOLYLINE,ARC").
  • MainViewModel — wires SelectCommand, exposes TotalLength, SelectedObjectCount, StatusMessage, Title.
  • RelayCommand<T> — generic MVVM command; CanExecuteChanged is hooked to CommandManager.RequerySuggested.
  • RibbonManager / RibbonMainCommand — programmatically build a RibbonTab + RibbonPanel + RibbonButton. The button's command issues SendStringToExecute("Zlen ", true, false, false) which routes through AutoCAD's command queue back into AcadCommand.ShowMainWindow.

Namespace note

The C# root namespace is MyApp.* (e.g. MyApp.Services, MyApp.Presentation.ViewModels). The assembly name is codehaks.TotalLength. Don't rely on the namespace matching the assembly name — the WPF pack:// URIs intentionally reference the assembly name (pack://application:,,,/codehaks.TotalLength;component/Resources/large.png).

AutoCAD interop notes

  • Window ownership. MainWindow is always parented to Application.MainWindow.Handle via WindowInteropHelper. This keeps the dialog modal to AutoCAD, gives it correct z-order, and prevents focus-stealing oddities.
  • Modal vs modeless. The current dialog is modal (ShowDialog). If you switch to Show() for a modeless palette, you must also use Application.ShowModelessWindow or the AutoCAD PaletteSet API, not raw WPF.
  • Transactions. Every read of an ObjectId must happen inside a Transaction. SelectService uses a single short-lived transaction over the whole selection.
  • Ribbon assemblies. Ribbon types come from AcWindows.dll and AdWindows.dll (note: Autodesk.Windows.RibbonControl, not System.Windows.Controls.Ribbon). Both versions of the assemblies are referenced from C:\Program Files\Autodesk\AutoCAD <year>\.

Build-time mechanics

  • All three csprojs <Import Project="..\Shared\Shared.projitems" Label="Shared" />. The shared project compiles its files into each consumer assembly — there is no separate Shared.dll.
  • Shared.projitems lists every file once with $(MSBuildThisFileDirectory)… so it's neutral to the consumer's location.
  • The 2024 csproj uses old-style <Reference Include="…"> items with <HintPath> for both AutoCAD assemblies and WPF (PresentationCore, PresentationFramework, WindowsBase, System.Xaml). The 2025/2026 csprojs replace those WPF references with a single <UseWPF>true</UseWPF> property.
  • <Private>False</Private> on every AutoCAD reference prevents AutoCAD's own DLLs from being copied into the output. They're loaded by AutoCAD itself at runtime.

Conditional compilation

The codebase deliberately avoids ACAD20XX symbols. The only #if directives are:

If you find yourself adding #if ACAD2024, stop and ask whether the AutoCAD API you're calling is actually different across versions — most of the managed surface used here is stable.