Running Network I/O in AutoCAD .NET Commands

Traditionally, AutoCAD .NET plugins work with geometry and the drawing database.
But modern workflows often require pulling data from the web —
for example, fetching design inputs, configuration, or geometry parameters from a service.

Since the AutoCAD .NET API is not thread-safe, you cannot call it directly
from a background thread. This becomes tricky when performing network I/O,
which is inherently asynchronous.

The solution:

• Perform network requests in a background task
  (Task.Run or HttpClient async calls).
• Use SynchronizationContext.Current captured on the AutoCAD UI thread
  to post results back.
• Modify the AutoCAD drawing safely on the main thread.

Example: Drawing Lines from Web Data

We host a simple JSON file on GitHub Pages:

{
  "lines": [
    { "start": { "x": 0, "y": 0, "z": 0 }, "end": { "x": 25, "y": 15, "z": 0 } },
    { "start": { "x": 10, "y": 5, "z": 0 }, "end": { "x": 40, "y": 30, "z": 0 } }
  ]
}

In AutoCAD, the command WEBLINES downloads this file and creates corresponding
Line entities in ModelSpace:

public async void CreateLinesFromWeb()
{
    Document doc = Application.DocumentManager.MdiActiveDocument;
    Editor ed = doc.Editor;

    // Capture AutoCAD main thread SynchronizationContext
    System.Threading.SynchronizationContext uiContext =
        Autodesk.AutoCAD.Runtime.SynchronizationContext.Current;

    try
    {
        // Fetch and parse JSON in background
        var lines = await Task.Run(async () =>
        {
            using HttpClient client = new HttpClient();
            string url = "https://madhukarmoogala.github.io/linepoints.json";
            string json = await client.GetStringAsync(url);

            JsonDocument docRoot = JsonDocument.Parse(json);
            var lineList = new System.Collections.Generic.List();

            foreach (var lineElem in docRoot.RootElement.GetProperty("lines").EnumerateArray())
            {
                Point3d start = new Point3d(
                    lineElem.GetProperty("start").GetProperty("x").GetDouble(),
                    lineElem.GetProperty("start").GetProperty("y").GetDouble(),
                    lineElem.GetProperty("start").GetProperty("z").GetDouble()
                );
                Point3d end = new Point3d(
                    lineElem.GetProperty("end").GetProperty("x").GetDouble(),
                    lineElem.GetProperty("end").GetProperty("y").GetDouble(),
                    lineElem.GetProperty("end").GetProperty("z").GetDouble()
                );
                lineList.Add((start, end));
            }
            return lineList;
        });

        // Post back to UI thread to modify AutoCAD Database
        uiContext.Post(_ =>
        {
            using (doc.LockDocument())
            using (Transaction tr = doc.Database.TransactionManager.StartTransaction())
            {
                BlockTable bt =
                    (BlockTable)tr.GetObject(doc.Database.BlockTableId, OpenMode.ForRead);
                BlockTableRecord ms =
                    (BlockTableRecord)tr.GetObject(bt[BlockTableRecord.ModelSpace],
                                                   OpenMode.ForWrite);

                foreach (var linePair in lines)
                {
                    Point3d start = linePair.start;
                    Point3d end = linePair.end;
                    Line line = new Line(start, end);
                    ms.AppendEntity(line);
                    tr.AddNewlyCreatedDBObject(line, true);
                    ed.WriteMessage($"nCreated line from {start} → {end}");
                }

                tr.Commit();
                ed.WriteMessage("nLines created successfully from web data.");
                ed.PostCommandPrompt();
            }
        }, null);
    }
    catch (Exception ex)
    {
        ed.WriteMessage($"nError fetching/drawing lines: {ex.Message}");
    }
}

This small pattern opens the door to connecting AutoCAD commands with
cloud services, APIs, and dynamic data — safely mixing modern async programming
with AutoCAD’s single-threaded API.