Question

I have below variables

public IEnumerable<SelectListItem> SelectedGroups { get; set; }
Dictionary<string,string> grpsList;

Below is my iteration

if(condition)
 viemodel.SelectedGroups = selectlist1;
else
  viemodel.SelectedGroups = selectlist2;

foreach (var item in viemodel.SelectedGroups) <<--Error message
       grpsList.Remove(item.Value);

I am not modifying selectedgroups but using it as a reference to remove from grpsList but its failing in foreach declaration.

---

Added code,
selectlist1 contains reference to grpsList, something like this (removed additional joins and all other code)

from g in grpsList
select g).ToSelectList(g => g.Value, g => g.Key);

selectlist2 is different list.

Answer 1

Most likely SelectedGroups point to a linq expression that involves grpsList.
To avoid this problem do a .ToList() before the assigment to SelectedGroups. (You can still keep the property signature as IEnumerable<SelectListItem>)

Using your added code:

from g in grpsList
select g).ToSelectList(g => g.Value, g => g.Key).ToList(); <-- here

Note that if you do not do the ToList() on a linq expression than every time you iterate the Enumerator the complete linq statement is evaluated. This can cause a lot of unnecessary performance issues.