I have a list of urls . I run OpenreadAsyn(url1) to OpenReadAsync(urlN) .. How do I wait for all of them to complete

I have a result list of length N. I have a URL list of N URLs. I need to perform below operation.

            List<Result> results=new List<Result>;             //Urls is a list of URLs.              public void FillResults()             {                  foreach( var urlx in Urls)                  {                         GetResponse(urlx, (response) =>                         {                             if (response != null && response.StatusCode==200)                             {                                 Result result=new Result;                                 result=response.SomeValue;                                 result.url=urlx;------>It is important that SomeValue corresponds to urlx and not other say,urly                                 results.Add(result);                               }                         });                 }             }               //wait till List results is completely filled               private void GetResponse(Uri uri, Action<MyResponse> callback)                     {                         using(var m_webClient = new WebClient())                         {                         m_webClient.OpenReadCompleted += (o, a) =>                         {                             if (callback != null)                             {                                 DataContractJsonSerializer ser = new DataContractJsonSerializer(MyResponse));                                 callback(ser.ReadObject(a.Result) MyResponse);                              }                         };                         m_webClient.OpenReadAsync(uri);                      }                 }                  public class MyResponse:Response                 {                 } 

// Wait for the results list to be filled completely

I want every request to happen asynchronously which is happening but want to wait untill results list is completely filled to move forward.