MyFirstCadPlugin.cs

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


[assembly: CommandClass(typeof(MyFirstCadPlugin.Class1))]
namespace MyFirstCadPlugin
{
    public class Class1
    {
        [CommandMethod("hello")]
        public void HelloCommand()
        {
            // Here we connect to the active AutoCAD Document and Database
            Document acDoc = Application.DocumentManager.MdiActiveDocument;
            Database acCurDb = acDoc.Database;

            // Here we create a Transaction in the current Database
            using (Transaction acTrans = acCurDb.TransactionManager.StartTransaction())
            {
                // Access to the Model Blocktable for write
                BlockTable acBlkTbl;
                acBlkTbl = acTrans.GetObject(acCurDb.BlockTableId,
                                             OpenMode.ForWrite) as BlockTable;
                BlockTableRecord acBlkTblRec;
                acBlkTblRec = acTrans.GetObject(acBlkTbl[BlockTableRecord.ModelSpace],
                                                OpenMode.ForWrite) as BlockTableRecord;

                // Create a Circle with its center in the coordinates origin and radius = 50.
                Circle myCircle = new Circle();
                myCircle.Center = new Point3d(0, 0, 0);
                myCircle.Radius = 50;

                // Create text with "Hello!" in the middle of the circle
                DBText myText = new DBText();
                myText.SetDatabaseDefaults();
                myText.Height = 20;
                myText.TextString = "Hello!";
                myText.Justify = AttachmentPoint.MiddleCenter;

                
                // Append Circle and Text to the Blocktable record and Database
                acBlkTblRec.AppendEntity(myCircle);
                acTrans.AddNewlyCreatedDBObject(myCircle, true);
                acBlkTblRec.AppendEntity(myText);
                acTrans.AddNewlyCreatedDBObject(myText, true);

                // Finish Transaction
                acTrans.Commit();
            }
        }
    }
}