c# - Task cancelling for an autocomplete field doesn't cancel all previous tasks -

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i have search method returns search suggestions ui. method fired every time user enters new character search box.

i have added cancellation code cancel previous search request. works not time.

private cancellationtokensource cancellationtokensource;  private async task usersearch(string searchcriteria) {     debug.writeline("searching {0}....", searchcriteria);      try     {         var cts = new cancellationtokensource();         this.suggestions = await this.searchasync(searchcriteria, cts.token);     }     catch (operationcanceledexception)     {         debug.writeline("search({0}) cancelled", searchcriteria);     } }  private async task<ilist<string>> searchasync(string searchcriteria, cancellationtoken canceltoken) {     cancellationtokensource previouscts = this.cancellationtokensource;     cancellationtokensource linkedcts = cancellationtokensource.createlinkedtokensource(canceltoken);      this.cancellationtokensource = linkedcts;      // if previous task running cancel     if (previouscts != null)     {         previouscts.cancel();     }      linkedcts.token.throwifcancellationrequested();      list<string> results =         (await this.searchprovider.searchasync(searchcriteria, linkedcts.token)).tolist();      debug.writeline("search({0}) returned {1} records", searchcriteria, results.count);      linkedcts.dispose();     this.cancellationtokensource = null;      return results; }

for example. following debug messages:

searchterm changing to: di  searching di.... searchterm changing to: dia searching dia.... search(di) cancelled searchterm changing to: diap searching diap.... search(diap) returned 323 records search(dia) returned 3230 records

as can see first search gets cancelled second 1 doesn't , gets returned after last search giving incorrect results user.

how can ensure previous tasks cancelled?

i think complicating solution little. need see if there existing operation in progress hasn't been cancelled , cancel it. perform new search. untested, think should it.

private cancellationtokensource cancellationtokensource;  private async task usersearch(string searchcriteria) {     debug.writeline("searching {0}....", searchcriteria);      try     {                if(cancellationtokensource != null &&             !cancellationtokensource.iscancellationrequested)         {             cancellationtokensource.cancel();         }          cancellationtokensource = new cancellationtokensource();          this.suggestions = await this.searchprovider.searchasync(searchcriteria, linkedcts.token);          debug.writeline("search({0}) returned {1} records", searchcriteria, results.count);      }     catch (operationcanceledexception)     {         debug.writeline("search({0}) cancelled", searchcriteria);     } }

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