I have a Page where I have placed a User Control. The User Control is responsible for dynamically showing a menu based on values in a database. The code for adding the menu looks like this: 

1    private void AddMenuContent(Control container, Menu menu, List<Tab> tabs, BasePageAdmin myPage)
2    {
3        Version currentSystemVersion = SessionUtils.SessionContainer.CurrentDepartment.Version;
4        List currentUserRoles = SessionUtils.SessionContainer.User.Roles;
5        
6        foreach (Tab t in tabs)
7        {
8            // Only add tab to this menu if it is supposed
9            // to be shown here, is in the current version and this user has access to it.
10           if (t.Menu != null && menu.Id == t.Menu.Id && !t.IsExcluded(currentSystemVersion) && t.IsAuthorized(currentUserRoles))
11           {
12   
13               // Mark as isActiveTab if this Tab is the currently displayed tab.
14               bool isActiveTab = false;
15               if (t.Id == myPage.ActiveTab.Id)
16               {
17                   isActiveTab = true;
18               }
19   
20               // Decide what kind of menu to display
21               switch (_menuType)
22               {
23                   case MenuTypes.Image:
24                       this.AddImageElement(t, container, isActiveTab);
25                       break;
26                   case MenuTypes.Text:
27                       this.AddTextElement(t, container, isActiveTab);
28                       break;
29                   default:
30                       this.AddTextElement(t, container, isActiveTab);
31                       break;
32               }
33   
34               // Decide if we should display the menu as vertical or horizontal
35               if (_orientation == Orientations.Vertical)
36               {
37                   MenuContent.Controls.Add(new LiteralControl("&lt;br />"));
38               }
39   
40           }
41       } // End foreach
42   }
43   

This method is called from Page_Load of the User Control. Some times, this method throws this exception:

System.Web.HttpUnhandledException: Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections.Generic.List`1.Enumerator.MoveNext()

Can anyone explain why this exception is being thrown? And why is it not thrown on every request?

 

Hi There,

If you wish to add/edit/delete item from <list> best to use following to prevent modified enumeration collection.

For ( int i = 0; i < tabs.Count; i++ )

{

tabs[i] ......

}

Hope it helps!

But I'm not adding/editing/deleting any of the items in the tabs collection inside the foreach loop? I'm only adding to the Control called container that I'm passing as a parameter. Or is it maybe that one that is causing the problem?

Hi There,

What are you doing in this function ?

this.AddTextElement(t, container, isActiveTab); // i doubt you are adding something to your tab ? if yes, is this editing?

In addition of that, if you put the Foreach inside try and catch

Try

{

foreach ....

}

catch

{

// do nothing

}

The operation will still complete and catches the error.

You can't add or remove items for a collection in a foreach loop. Try converting the collection to an array (use Collection.ToArray()), and iterate over the array. Then you can add items to the collection, since you are iterating over the array, not the collection.

Here is the code of the functions being called inside the foreach loop

 

1            private void AddImageElement(Tab t, Control c, bool isActiveTab)
2            {
3                Theme currTheme = SessionUtils.SessionContainer.Theme;
4                string themePath = "~/Themes/" + currTheme.Path + "/images/menu/";
5    
6                string languageExtension = "en";
7                if (_currentSystemLanguage != null)
8                {
9                    languageExtension = _currentSystemLanguage.Culture;
10               }
11   
12               string linkimage = "";
13               linkimage += @"<img src=";
14               if (isActiveTab)
15               {
16                   linkimage += Page.ResolveUrl(themePath + t.TabAlias + ".3." + languageExtension + ".gif");
17               }
18               else
19               {
20                   linkimage += Page.ResolveUrl(themePath + t.TabAlias + ".1." + languageExtension + ".gif");
21               }
22               linkimage += " alt=\"" + TabBLL.GetLocalizedTabTooltip(t) + "\"";
23               linkimage += " border=\"0\">";
24   
25               c.Controls.Add(new LiteralControl("<a href=\"DesktopDefault.aspx?tabalias=" + t.TabAlias + "\">" + linkimage + "</a>"));
26           }
27   
28           private void AddTextElement(Tab t, Control c, bool isActiveTab)
29           {
30               c.Controls.Add(new LiteralControl("<a href=\"DesktopDefault.aspx?tabalias=" + t.TabAlias + "\">" + TabBLL.GetLocalizedTabName(t) + "</a>"));
31           }
32   
33   

As far as I can see, it is only the Control collection c that is being modified when new controls are added to it. But that's perfectly ok isn't it?

Yes I can catch the exception, but I don't think that is very nice. I would rather know why it's telling me that the collection is being modified.

 

that does certainly seem to be the case (that you are only modifying Controls). So either there is some wierd reference foo (did you add the Tabs objects to the Control's collection?), or you are somehow modifying a tab indirectly. I *think* that most collections have a "dirty" property for their items, and if that flag is set during an iteration, it freaks out.... anyhow, try the array thing I mentioned... it should work?