Civil 3D API in C#: first steps to programming Civil 3D, from your first command to a plugin

Civil 3D is built on AutoCAD, and so is its API. A Civil 3D program uses two layers:

• The AutoCAD .NET API: documents, transactions, the command line, lines, polylines, blocks and layers.
• The Civil 3D .NET API: the engineering objects: alignments, profiles, surfaces, corridors, sections, COGO points, parcels and pipe networks.

The alternatives, depending on what you need:

• Dynamo, to automate without writing code. Since Civil 3D 2025.1 it ships more than a thousand out-of-the-box nodes; we cover it in 10 Dynamo scripts for Civil 3D.
• Python, inside Dynamo's Python nodes, when the visual nodes fall short.
• The COM API, older, still used by some LISP and VBA routines.
• C# on the .NET API, for production tools the whole office uses. That's the path in this guide.

1. Windows with Civil 3D installed. The API only runs inside Civil 3D.
2. Visual Studio Community, free, with the .NET desktop development workload.
3. The right framework for your release:

4. A class library project, referencing the libraries in the Civil 3D install folder: AcCoreMgd, AcDbMgd and AcMgd from AutoCAD, and AecBaseMgd and AeccDbMgd from Civil 3D.

This command walks through the drawing's alignments and writes each name and length to the command line:

using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Runtime;
using Autodesk.Civil.ApplicationServices;
using Autodesk.Civil.DatabaseServices;

[assembly: CommandClass(typeof(ZeistDemo.Commands))]

namespace ZeistDemo
{
    public class Commands
    {
        [CommandMethod("ZLISTALIGNMENTS")]
        public void ListAlignments()
        {
            Document doc = Application.DocumentManager.MdiActiveDocument;
            Editor ed = doc.Editor;
            CivilDocument civil = CivilApplication.ActiveDocument;

            using (Transaction tr = doc.Database.TransactionManager.StartTransaction())
            {
                foreach (ObjectId id in civil.GetAlignmentIds())
                {
                    var alignment = (Alignment)tr.GetObject(id, OpenMode.ForRead);
                    ed.WriteMessage($"\n{alignment.Name}: {alignment.Length:F2}");
                }
                tr.Commit();
            }
        }
    }
}

What each part does:

• CommandMethod turns the method into a command you type in Civil 3D.
• CivilApplication.ActiveDocument gives access to the Civil 3D objects in the open drawing.
• GetAlignmentIds() returns the IDs of every alignment.
• The transaction opens each alignment for reading, and Commit closes it.

To test it: build, open Civil 3D with a drawing that has alignments, type NETLOAD, pick your DLL and run ZLISTALIGNMENTS.

Almost everything starts at the CivilDocument, the Civil 3D document of the open drawing:

From each alignment you reach its profiles and sample line groups; from each corridor, its baselines and regions. The pattern never changes: get the ID, open the object inside a transaction, then read or modify it. An example that creates sample lines from a station list is in automated Civil 3D sample lines.

• "Cannot load assembly" on NETLOAD. The project targets a different framework than your Civil 3D release uses. Check the table in step 2.
• Odd errors when Civil 3D loads or closes. The Autodesk references were left with Copy Local set to Yes.
• Changes aren't saved. tr.Commit() is missing: an uncommitted transaction is discarded entirely.
• eLockViolation. It happens when code runs from your own window instead of a command: lock the document before modifying it.
• Works on your release, not on your colleague's. Every Civil 3D release requires building against its own libraries.

A command that works on your machine isn't a tool yet. For the office to use it you need:

• A button on the Civil 3D top toolbar, instead of a typed command.
• A bundle: the folder in Autodesk's official format that Civil 3D loads at startup.
• Error handling and logging, so a failure never leaves the drawing half-done.
• A layered structure, so an Autodesk release change touches only one part of the code.

We explain it in Revit and Civil 3D add-in development, and if you want to get there faster with AI, in building a Civil 3D plugin with Claude Code.