c# - Getting around "Collection was modified; enumeration operation may not execute" with async - i.e. without locking -

i have ui displaying graphs held in observable collection. user can change time span graphs referencing - there's listener triggers loop through them , re-fetches data, can take while.

foreach (timeseriesdataviewmodel viewmodels in trends) {     viewmodels.timeseriesdatamodel = await modelprovider.gettimeseriesdatafor(viewmodels.kpiviewmodel.kpimodel,                                                                                 timespanviewmodel.starttime,                                                                                 timespanviewmodel.endtime); } 

my problem is, while happening user can modify collection adding more trends, or deleting them. results in "collection modified; enumeration operation may not execute" exception.

normally, i'd doing using background process, , i'd put lock around accessing collection. however, locks don't work asynchronous code (which i'm new to!). there standard way of doing this?

i'm using thread safe concurrent observable collections here.

edit:

after experimentation, seems work well. however, i'm lucky don't need await removing items, can keep await outside await lock statements.

        concurrentdictionary<itimeseriesdataviewmodel,task<itimeseriesdatamodel>> updatedatatasks = new concurrentdictionary<itimeseriesdataviewmodel,task<itimeseriesdatamodel>>();         lock (trends)         {             foreach (itimeseriesdataviewmodel viewmodels in trends)             {                 updatedatatasks.getoradd(viewmodels, modelprovider.gettimeseriesdatafor(viewmodels.kpiviewmodel.kpimodel,                                                                                     timespanviewmodel.starttime,                                                                                     timespanviewmodel.endtime));             }         }         await task.whenall(updatedatatasks.values);          foreach (keyvaluepair<itimeseriesdataviewmodel, task<itimeseriesdatamodel>> item in updatedatatasks)         {             item.key.timeseriesdatamodel = item.value.result;         } 

the problem lock can't contain await, data before awaiting?