Catching exceptions thrown from dialogs inside AutoCAD using .NET

Another juicy tidbit from an internal Q&A session.

The question...

How can I throw an exception from within a modal form on mine inside AutoCAD, and catch it in the calling command? I have tried try/catch, but nothing seems to work.

Here's my command definition:

using Autodesk.AutoCAD.ApplicationServices;

using Autodesk.AutoCAD.Runtime;

using System.Windows.Forms;

using MyApplication;

using acApp =

  Autodesk.AutoCAD.ApplicationServices.Application;

namespace CatchMeIfYouCan

{

  public class Commands

  {

    [CommandMethod("CATCH")]

    static public void CatchDialogException()

    {

      try

      {

        MyForm form = new MyForm();

        DialogResult res =

          acApp.ShowModalDialog(form);

      }

      catch (System.Exception ex)

      {

        MessageBox.Show(

          "Caught using catch: " +

          ex.Message,

          "Exception"

        );

      }

    }

  }

}

And here's the code that throws an exception from behind button inside my form:

using System;

using System.Windows.Forms;

namespace MyApplication

{

  public partial class MyForm : Form

  {

    public MyForm()

    {

      InitializeComponent();

    }

    private void button1_Click(

      object sender,

      EventArgs e

    )

    {

      throw new Exception(

        "Something bad happened."

      );

    }

  }

}

The answer, once again, came from our AutoCAD Engineering team, in this case from one of our API Test Engineers based in Singapore...

I was able to catch the exception using the ThreadExceptionEventHandler event. Here is your modified command:

using Autodesk.AutoCAD.ApplicationServices;

using Autodesk.AutoCAD.EditorInput;

using Autodesk.AutoCAD.Runtime;

using Autodesk.AutoCAD.Windows;

using System.Windows.Forms;

using System.Threading;

using MyApplication;

using acApp =

  Autodesk.AutoCAD.ApplicationServices.Application;

using sysApp =

  System.Windows.Forms.Application;

namespace CatchMeIfYouCan

{

  public class Commands

  {

    [CommandMethod("CATCH")]

    static public void CatchDialogException()

    {

      try

      {

        sysApp.ThreadException +=

          new ThreadExceptionEventHandler(

            delegate(

              object o,

              ThreadExceptionEventArgs args

            )

            {

              MessageBox.Show(

                "Caught using event: " +

                args.Exception.Message,

                "Exception"

              );

            }

          );

        MyForm form = new MyForm();

        DialogResult res =

          acApp.ShowModalDialog(form);

      }

      catch (System.Exception ex)

      {

        MessageBox.Show(

          "Caught using catch: " +

          ex.Message,

          "Exception"

        );

      }

    }

  }

}

I gave this a try, myself, and saw that when we click the button on the dialog shown by the CATCH command, our event handler picks up the exception and displays it via a MessageBox:

One thing to note: when running this from the debugger, Visual Studio will break, telling us that there's an exception that has gone unhandled:

This can be ignored - and probably disabled inside Visual Studio - but it could be annoying, if you use this technique throughout your code. Perhaps someone who's implemented this approach - or another that works for them - can add a comment on how they handle this scenario?