A couple changes:

1. Don't prompt for the profile. Outlook will use the default if that's how it is configured.
2. Free up Outlook resources to not keep the Outlook process running on startup or when we sync.

Author: David-Engel

src/OutlookCalendarSync/MainForm.cs

@@ -45,13 +45,24 @@ public MainForm()
             numericUpDownDaysInThePast.Value = Settings.Instance.DaysInThePast;
             numericUpDownDaysInTheFuture.Value = Settings.Instance.DaysInTheFuture;
             textBoxMinuteOffsets.Text = Settings.Instance.MinuteOffsets;
-            OutlookHelper oh = new OutlookHelper();
+            OutlookHelper oh = null;
+
+            try
+            {
+                oh = new OutlookHelper();
+            }
+            catch (COMException)
+            {
+                //Just retry once
+                oh = new OutlookHelper();
+            }
+
             checkedListBoxCalendars.Items.Clear();
             List<OutlookCalendar> savedCalendarsToSync = new List<OutlookCalendar>();
             savedCalendarsToSync.AddRange(Settings.Instance.CalendarsToSync.ToArray());
             foreach (OutlookCalendar calendar in oh.CalendarFolders)
             {
-                int position = checkedListBoxCalendars.Items.Add(calendar);
+                int position = checkedListBoxCalendars.Items.Add(calendar.Name);
                 foreach (OutlookCalendar entry in savedCalendarsToSync)
                 {
                     if (entry.Name.Equals(calendar.Name))
@@ -61,6 +72,8 @@ public MainForm()
                 }
             }
 
+            freeCOMResources(oh);
+
             checkBoxSyncEveryHour.Checked = Settings.Instance.SyncEveryHour;
             checkBoxShowBubbleTooltips.Checked = Settings.Instance.ShowBubbleTooltipWhenSyncing;
             checkBoxStartInTray.Checked = Settings.Instance.StartInTray;
@@ -163,6 +176,7 @@ private void syncWorker_DoWork(object sender, DoWorkEventArgs e)
                 if (calendarsToSync.Count < 2)
                 {
                     MessageBox.Show("You need at least two calendars to sync on the 'Settings' tab.");
+                    freeCOMResources(oh);
                     return;
                 }
 
@@ -530,12 +544,12 @@ private void checkedListBoxCalendars_ItemCheck(object sender, ItemCheckEventArgs
             List<OutlookCalendar> checkedItems = new List<OutlookCalendar>();
             foreach (var item in checkedListBoxCalendars.CheckedItems)
             {
-                checkedItems.Add((OutlookCalendar)item);
+                checkedItems.Add(new OutlookCalendar((string)item, null));
             }
 
             if (e.NewValue == CheckState.Checked)
             {
-                checkedItems.Add((OutlookCalendar)checkedListBoxCalendars.Items[e.Index]);
+                checkedItems.Add(new OutlookCalendar((string)checkedListBoxCalendars.Items[e.Index], null));
             }
 
             Settings.Instance.CalendarsToSync = checkedItems;

src/OutlookCalendarSync/OutlookHelper.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Runtime.InteropServices;
+using System.Threading;
 //using Outlook = Microsoft.Office.Interop.Outlook;
 using Microsoft.Office.Interop.Outlook;
 
@@ -11,7 +12,6 @@ namespace OutlookCalendarSync
     /// </summary>
     public class OutlookHelper
     {
-        //private static OutlookHelper instance;
         private List<OutlookCalendar> _calendarFolders = new List<OutlookCalendar>();
 
         private string _accountName = string.Empty;
@@ -32,10 +32,10 @@ public OutlookHelper()
             Application oApp = new Application();
 
             // Get the NameSpace and Logon information.
-            NameSpace oNS = oApp.GetNamespace("mapi");
-
-            //Log on by using a dialog box to choose the profile.
-            oNS.Logon("","", true, true);
+            NameSpace oNS = (NameSpace)RetryInteropAction(new Func<NameSpace>(() =>
+            {
+                return oApp.GetNamespace("mapi");
+            }));
 
             Store store = oNS.DefaultStore;
             _accountName = store.DisplayName;
@@ -50,17 +50,42 @@ public OutlookHelper()
                     }
                     else
                     {
-                        Marshal.ReleaseComObject(folder);
+                        Marshal.FinalReleaseComObject(folder);
                     }
                 }
 
-                Marshal.ReleaseComObject(mailbox);
+                Marshal.FinalReleaseComObject(mailbox);
+            }
+
+            Marshal.FinalReleaseComObject(oApp);
+            Marshal.FinalReleaseComObject(oNS);
+            oApp = null;
+            oNS = null;
+        }
+
+        private object RetryInteropAction(Func<object> action)
+        {
+            int count = 0;
+            while (count < 5)
+            {
+                try
+                {
+                    count++;
+                    return action();
+                }
+                catch (COMException)
+                {
+                    if (count < 5)
+                    {
+                        Thread.Sleep(1000);
+                        continue;
+                    }
+
+                    throw;
+                }
             }
 
-            // Done. Log off.
-            oNS.Logoff();
-            Marshal.ReleaseComObject(oApp);
-            Marshal.ReleaseComObject(oNS);
+            return null;
         }
     }
 }