How to refactor async for parallel execute

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I Have await function, but I need to execute parallelly instead of one by one.

What I have tried:

```

C#

foreach( DataRow item in dt.Rows)
 {
   var response = await RestApIHelper.GetByLastDate(item["Url"].ToString(), 
       fetchAllFirstTime.token, Convert.ToDateTime(item["LastDate"]));
   var data = JsonConvert.DeserializeObject<List<createReservation>>(response);
              roomCreateOrUpdateViaAPI(data, "insert");
  }

```
RestAPIHelper.GetByLastDate
```

C#

public static async Task<string> GetByLastDate(string specificUrl, string token, DateTime date)
        {
 
          string fullUrl = baseUrl + specificUrl+ date;
          using (var requestMessage = new HttpRequestMessage(HttpMethod.Get, fullUrl))
                {
                    requestMessage.Headers.Add("access_token", token);
                    HttpResponseMessage res = await policy.ExecuteAsync(() => client.SendAsync(requestMessage));
                    using (HttpContent content = res.Content)
                    {
                        string data = await content.ReadAsStringAsync();
                        if (data != null)
                        {
                            return data;
                        }
                    }
                    return string.Empty;
                }
            }
        }

```

Posted 6-Sep-21 22:01pm

Member 14921707

Updated 7-Sep-21 0:21am

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Richard Deeming · Accepted Answer · 2021-09-07T00:21:00

Rather than awaiting each task, add them to a list and then use Task.WhenAll[^] to wait for them all to complete:

C#

public static async Task<List<createReservation>> GetByLastDate(string specificUrl, string token, DateTime date)
{
    string fullUrl = baseUrl + specificUrl+ date;
    using (var requestMessage = new HttpRequestMessage(HttpMethod.Get, fullUrl))
    {
        requestMessage.Headers.Add("access_token", token);
        using (HttpResponseMessage res = await policy.ExecuteAsync(() => client.SendAsync(requestMessage)))
        {
            string data = await res.Content.ReadAsStringAsync();
            if (string.IsNullOrEmpty(data)) return null;
            return JsonConvert.DeserializeObject<List<createReservation>>(response);
        }
   }
}

C#

var tasks = new List<Task<List<createReservation>>>(dt.Rows.Count);

foreach (DataRow item in dt.Rows)
{
    tasks.Add(RestApIHelper.GetByLastDate(item["Url"].ToString(), 
       fetchAllFirstTime.token, Convert.ToDateTime(item["LastDate"]));
}

IEnumerable<List<createReservation>> results = await Task.WhenAll(tasks);
foreach (List<createReservation> data in results)
{
    roomCreateOrUpdateViaAPI(data, "insert");
}