Handling COM calls rejected by AutoCAD from an external .NET application

We’ve had a few reports from people implementing external .NET applications to drive AutoCAD – as shown in this previous post – experiencing intermittent failures once AutoCAD 2010 Update 1 has been applied. Here’s a typical error message:

It contains the text “Problem executing component: Call was rejected by callee. (Exception from HRESULT: 0x80010001 (RPC_E_CALL_REJECTED))” (to help people Googling this error message :-).

This “problem” was introduced as we addressed an issue with the way our WPF components in AutoCAD handle inbound messages, largely due to Microsoft’s decision not to support nested message loops in WPF. If WPF is in the middle of performing some kind of layout processing operation (which leads to Dispatcher.DisableProcessing() being called) and there’s an incoming COM call, then we were previously respecting it which could lead to a crash. Now we do the right thing and reject the COM call: effectively asking the caller to try again later.

The problem is that – while the VB6 runtime was very good at automatically retrying calls such as CreateObject() – WinForm applications are not. We need to implement an additional interface from our WinForm application to make sure it can handle failure (I could probably do with adding that one myself ;-).

As it’s hopefully clear: while the problem is likely to be more visible in AutoCAD 2010 once Update 1 has been applied, this is ultimately about correctly inappropriate expectations on the side of the calling application and teaching it to do the right thing.

Here’s the updated C# code from the previous post, with the changed lines in red, and here’s the source project.

    1 using Autodesk.AutoCAD.Interop;
    2 using System.Windows.Forms;
    3 using System.Runtime.InteropServices;
    4 using System.Reflection;
    5 using System;
    6 using LoadableComponent;
    7 
    8 // For more information on IMessageFilter:
    9 // http://msdn.microsoft.com/en-us/library/ms693740(VS.85).aspx
   10 
   11 namespace DrivingAutoCAD
   12 {
   13   [ComImport,
   14    InterfaceType(ComInterfaceType.InterfaceIsIUnknown),
   15    Guid("00000016-0000-0000-C000-000000000046")]
   16   public interface IMessageFilter
   17   {
   18     [PreserveSig]
   19     int HandleInComingCall(
   20       int dwCallType, IntPtr hTaskCaller,
   21       int dwTickCount, IntPtr lpInterfaceInfo
   22     );
   23     [PreserveSig]
   24     int RetryRejectedCall(
   25       IntPtr hTaskCallee, int dwTickCount, int dwRejectType
   26     );
   27     [PreserveSig]
   28     int MessagePending(
   29       IntPtr hTaskCallee, int dwTickCount, int dwPendingType
   30     );
   31   }
   32 
   33   public partial class Form1 : Form, IMessageFilter
   34   {
   35     [DllImport("ole32.dll")]
   36     static extern int CoRegisterMessageFilter(
   37       IMessageFilter lpMessageFilter,
   38       out IMessageFilter lplpMessageFilter
   39     );
   40 
   41     public Form1()
   42     {
   43       InitializeComponent();
   44       IMessageFilter oldFilter;
   45       CoRegisterMessageFilter(this, out oldFilter);
   46     }
   47 
   48     private void button1_Click(object sender, EventArgs e)
   49     {
   50       const string progID = "AutoCAD.Application.18";
   51 
   52       AcadApplication acApp = null;
   53       try
   54       {
   55         acApp =
   56           (AcadApplication)Marshal.GetActiveObject(progID);
   57       }
   58       catch
   59       {
   60         try
   61         {
   62           Type acType =
   63             Type.GetTypeFromProgID(progID);
   64           acApp =
   65             (AcadApplication)Activator.CreateInstance(
   66               acType,
   67               true
   68             );
   69         }
   70         catch
   71         {
   72           MessageBox.Show(
   73             "Cannot create object of type \"" +
   74             progID + "\""
   75           );
   76         }
   77       }
   78       if (acApp != null)
   79       {
   80         try
   81         {
   82           // By the time this is reached AutoCAD is fully
   83           // functional and can be interacted with through code
   84 
   85           acApp.Visible = true;
   86 
   87           INumberAddition app =
   88             (INumberAddition)acApp.GetInterfaceObject(
   89               "LoadableComponent.Commands"
   90             );
   91 
   92           // Now let's call our method
   93 
   94           string res = app.AddNumbers(5, 6.3);
   95 
   96           acApp.ZoomAll();
   97 
   98           MessageBox.Show(
   99             this,
  100             "AddNumbers returned: " + res
  101           );
  102         }
  103         catch (Exception ex)
  104         {
  105           MessageBox.Show(
  106             this,
  107             "Problem executing component: " +
  108             ex.Message
  109           );
  110         }
  111       }
  112     }
  113     #region IMessageFilter Members
  114 
  115     int IMessageFilter.HandleInComingCall(
  116       int dwCallType, IntPtr hTaskCaller,
  117       int dwTickCount, IntPtr lpInterfaceInfo
  118     )
  119     {
  120       return 0; // SERVERCALL_ISHANDLED
  121     }
  122 
  123     int IMessageFilter.RetryRejectedCall(
  124       IntPtr hTaskCallee, int dwTickCount, int dwRejectType
  125     )
  126     {
  127       return 1000; // Retry in a second
  128     }
  129 
  130     int IMessageFilter.MessagePending(
  131       IntPtr hTaskCallee, int dwTickCount, int dwPendingType
  132     )
  133     {
  134       return 1; // PENDINGMSG_WAITNOPROCESS
  135     }
  136 
  137     #endregion
  138   }
  139 }