Building a SolidWorks Add-in from Scratch in C#

Why Build an Add-in?

Macros and standalone apps are fine for simple automation. But add-ins run inside the SolidWorks process, giving you:

• Speed: No COM marshaling overhead. Direct access to the API objects.
• UI integration: Add menus, toolbar buttons, and TaskPane panels that look native.
• Event handling: React to document open, save, rebuild, selection changes, and more.
• Persistent state: The add-in stays loaded for the entire SolidWorks session.

Step 1: Project Setup

Create a Class Library (.NET Framework) project. Add-ins must target .NET Framework (4.7.2+), not .NET 5+.

Add references to:

• SolidWorks.Interop.sldworks
• SolidWorks.Interop.swconst
• SolidWorks.Interop.swpublished (contains ISwAddin interface)

Mark the assembly as COM-visible in project properties, and add a strong name key.

Step 2: Implement ISwAddin

[ComVisible(true)]
[Guid("YOUR-GUID-HERE")]
public class MyAddin : ISwAddin
{
    private SldWorks _swApp;
    private int _addinCookie;

    public bool ConnectToSW(object thisSW, int cookie)
    {
        _swApp = (SldWorks)thisSW;
        _addinCookie = cookie;
        _swApp.SetAddinCallbackInfo2(0, this, cookie);
        // Create command groups, TaskPanes, register events
        return true;
    }

    public bool DisconnectFromSW()
    {
        // Cleanup: remove commands, release COM objects
        return true;
    }
}

Step 3: Register the Add-in

SolidWorks discovers add-ins through the Windows registry. Add these registration attributes:

[ComRegisterFunction]
public static void RegisterFunction(Type t)
{
    var key = Registry.LocalMachine.CreateSubKey(
        @"SOFTWARE\SolidWorks\Addins\{" + t.GUID + "}");
    key.SetValue(null, 0);  // 0 = not loaded at startup
    key.SetValue("Title", "My Add-in");
    key.SetValue("Description", "My custom add-in");
}

Then run regasm:

regasm /codebase MyAddin.dll

Step 4: Add Commands and Menus

In ConnectToSW, create a command group with buttons:

var cmdGroup = _swApp.GetCommandManager(_addinCookie)
    .CreateCommandGroup2(0, "My Tools", "My custom tools",
        "Tooltip", -1, true, ref errors);
cmdGroup.AddCommandItem2("Run Tool",
    -1, "Run my tool", "Run my tool",
    0, "OnRunTool", "CanRunTool",
    0, (int)swCommandItemType_e.swToolbarItem);
cmdGroup.Activate();

The callback strings ("OnRunTool", "CanRunTool") are method names on your add-in class that SolidWorks will call via reflection.

Step 5: Add a TaskPane

For a rich UI panel, create a WPF UserControl and host it in a TaskPane:

var taskPane = _swApp.CreateTaskpaneView3(
    iconPath,  // 16x16 and 20x20 bitmaps
    "My Panel");
taskPane.AddControl(
    "MyAddin.MyUserControl",
    "");

The UserControl must be COM-visible with its own GUID.

Step 6: Handle Events

Register for document events in ConnectToSW:

_swApp.ActiveDocChangeNotify += OnActiveDocChanged;
_swApp.FileOpenPostNotify += OnFileOpened;

For document-specific events (rebuild, save, selection), register on each ModelDoc2:

((ModelDoc2)doc).DestroyNotify2 += OnDocClosing;

Debugging

• Build the project, run regasm, then start SolidWorks
• In Visual Studio: Debug > Attach to Process > select SLDWORKS.exe
• Set breakpoints in your code — they'll hit when your add-in methods are called
• Use Debug.Print or a log file for output (Console.WriteLine doesn't show in SolidWorks)

Deployment

To install on another machine:

1. Copy the DLL and all dependencies to a folder
2. Run regasm /codebase on the target machine (requires admin rights)
3. SolidWorks will discover the add-in at next startup

For production deployment, create an MSI installer that handles regasm and file copying automatically.