Commands.cs

using Autodesk.AutoCAD.ApplicationServices.Core;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.Runtime;
using Newtonsoft.Json;
using System.IO;

[assembly: CommandClass(typeof(UpdateDWGParam.Commands))]
[assembly: ExtensionApplication(null)]

namespace UpdateDWGParam
{
    public class Commands
    {
        [CommandMethod("UpdateParam", CommandFlags.Modal)]
        public static void UpdateParam()
        {
            //Get active document of drawing with Dynamic block
            var doc = Application.DocumentManager.MdiActiveDocument;
            var db = doc.Database;

            // read input parameters from JSON file
            InputParams inputParams = JsonConvert.DeserializeObject<InputParams>(File.ReadAllText("params.json"));

            using (Transaction t = db.TransactionManager.StartTransaction())
            {
                var bt = t.GetObject(db.BlockTableId, OpenMode.ForRead) as BlockTable;

                foreach (ObjectId btrId in bt)
                {
                    //get the blockDef and check if is anonymous
                    BlockTableRecord btr = (BlockTableRecord)t.GetObject(btrId, OpenMode.ForRead);
                    if (btr.IsDynamicBlock)
                    {
                        //get all anonymous blocks from this dynamic block
                        ObjectIdCollection anonymousIds = btr.GetAnonymousBlockIds();
                        ObjectIdCollection dynBlockRefs = new ObjectIdCollection();
                        foreach (ObjectId anonymousBtrId in anonymousIds)
                        {
                            //get the anonymous block
                            BlockTableRecord anonymousBtr = (BlockTableRecord)t.GetObject(anonymousBtrId, OpenMode.ForRead);
                            //and all references to this block
                            ObjectIdCollection blockRefIds = anonymousBtr.GetBlockReferenceIds(true, true);
                            foreach (ObjectId id in blockRefIds)
                            {
                                dynBlockRefs.Add(id);
                            }
                        }
                        if (dynBlockRefs.Count > 0)
                        {
                            //Get the first dynamic block reference, we have only one Dyanmic Block reference in Drawing
                            var dBref = t.GetObject(dynBlockRefs[0], OpenMode.ForWrite) as BlockReference;
                            UpdateDynamicProperties(dBref, inputParams);
                        }
                    }
                }
                t.Commit();
            }
            LogTrace("Saving file...");
            db.SaveAs("outputFile.dwg", DwgVersion.Current);
        }

        /// <summary>
        /// This updates the Dyanmic Blockreference with given Width and Height
        /// The initial parameters of Dynamic Blockrefence, Width =20.00 and Height =40.00
        /// </summary>
        /// <param Editor="ed"></param>
        /// <param BlockReference="br"></param>
        /// <param String="name"></param>
        private static void UpdateDynamicProperties(BlockReference br, InputParams inputParams)
        {
            // Only continue is we have a valid dynamic block
            if (br != null && br.IsDynamicBlock)
            {
                // Get the dynamic block's property collection
                DynamicBlockReferencePropertyCollection pc = br.DynamicBlockReferencePropertyCollection;
                foreach (DynamicBlockReferenceProperty prop in pc)
                {
                    switch (prop.PropertyName)
                    {
                        case "Width":
                            prop.Value = inputParams.Width;
                            break;
                        case "Height":
                            prop.Value = inputParams.Height;
                            break;
                        default:
                            break;
                    }
                }
            }
        }

        /// <summary>
        /// This will appear on the Design Automation output
        /// </summary>
        private static void LogTrace(string format, params object[] args) { Application.DocumentManager.MdiActiveDocument.Editor.WriteMessage(format, args); }
    }

    public class InputParams
    {
        public double Width { get; set; }
        public double Height { get; set; }
    }
}