How to Use AutoCAD API: Syntax, Example, and Tips

To use the AutoCAD API, you typically write code in C# or VB.NET that interacts with AutoCAD objects via the Autodesk.AutoCAD.ApplicationServices and Autodesk.AutoCAD.DatabaseServices namespaces. You start by creating a command method, accessing the current document and database, then manipulating drawing elements programmatically.

Syntax

The basic syntax to create a command in AutoCAD API involves defining a public method with the CommandMethod attribute. Inside this method, you access the current document and database, then use transactions to modify drawing objects safely.

• CommandMethod: Marks the method as an AutoCAD command.
• Document: Represents the active drawing.
• Database: Holds the drawing data.
• Transaction: Ensures safe changes to the drawing.

csharp

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

public class Commands
{
    [CommandMethod("MyCommand")]
    public void MyCommand()
    {
        Document doc = Application.DocumentManager.MdiActiveDocument;
        Database db = doc.Database;
        Editor ed = doc.Editor;

        using (Transaction tr = db.TransactionManager.StartTransaction())
        {
            // Access and modify drawing objects here

            tr.Commit();
        }
    }
}

Example

This example shows how to create a simple line in the current drawing using the AutoCAD API. It demonstrates opening the block table, starting a transaction, creating a line object, adding it to the model space, and committing the transaction.

csharp

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

public class Commands
{
    [CommandMethod("DrawLine")]
    public void DrawLine()
    {
        Document doc = Application.DocumentManager.MdiActiveDocument;
        Database db = doc.Database;
        Editor ed = doc.Editor;

        using (Transaction tr = db.TransactionManager.StartTransaction())
        {
            BlockTable bt = (BlockTable)tr.GetObject(db.BlockTableId, OpenMode.ForRead);
            BlockTableRecord btr = (BlockTableRecord)tr.GetObject(bt[BlockTableRecord.ModelSpace], OpenMode.ForWrite);

            Line line = new Line(new Point3d(0, 0, 0), new Point3d(100, 100, 0));
            btr.AppendEntity(line);
            tr.AddNewlyCreatedDBObject(line, true);

            tr.Commit();
        }

        ed.WriteMessage("\nLine created from (0,0,0) to (100,100,0).");
    }
}

Output

Line created from (0,0,0) to (100,100,0).

Common Pitfalls

Common mistakes when using the AutoCAD API include:

• Not starting or committing a transaction, which prevents changes from saving.
• Accessing objects without proper open mode (read vs write), causing exceptions.
• Forgetting to add new objects to the block table record and transaction.
• Running commands outside AutoCAD environment or without proper references.

Always ensure you use using blocks for transactions and open objects with correct permissions.

csharp

/* Wrong way: Missing transaction commit and adding entity */
using (Transaction tr = db.TransactionManager.StartTransaction())
{
    BlockTable bt = (BlockTable)tr.GetObject(db.BlockTableId, OpenMode.ForRead);
    BlockTableRecord btr = (BlockTableRecord)tr.GetObject(bt[BlockTableRecord.ModelSpace], OpenMode.ForWrite);

    Line line = new Line(new Point3d(0, 0, 0), new Point3d(50, 50, 0));
    // Missing btr.AppendEntity(line);
    // Missing tr.AddNewlyCreatedDBObject(line, true);

    // Missing tr.Commit();
}

/* Right way: */
using (Transaction tr = db.TransactionManager.StartTransaction())
{
    BlockTable bt = (BlockTable)tr.GetObject(db.BlockTableId, OpenMode.ForRead);
    BlockTableRecord btr = (BlockTableRecord)tr.GetObject(bt[BlockTableRecord.ModelSpace], OpenMode.ForWrite);

    Line line = new Line(new Point3d(0, 0, 0), new Point3d(50, 50, 0));
    btr.AppendEntity(line);
    tr.AddNewlyCreatedDBObject(line, true);

    tr.Commit();
}

Quick Reference

Here is a quick reference for common AutoCAD API classes and methods:

Class/Method	Purpose
Application.DocumentManager.MdiActiveDocument	Access current drawing document
Database	Represents the drawing database
TransactionManager.StartTransaction()	Start a transaction to edit drawing safely
BlockTable	Access block definitions like ModelSpace
BlockTableRecord	Container for drawing entities
BlockTableRecord.AppendEntity()	Add new entity to drawing
Transaction.Commit()	Save changes made in transaction

Key Takeaways

Always use transactions to safely modify AutoCAD drawings.

Mark your methods with CommandMethod attribute to create AutoCAD commands.

Open objects with correct read/write permissions to avoid errors.

Add new entities to the block table record and transaction before committing.

Test your commands inside AutoCAD to see immediate results.