Using asyn await in foreach loop

Question

Is it incorrect to use asyc await in a foreach loop? I get a WebException, The request was canceled.

The following is just a outline of the code I’m working on. Please let me know if this is not clear.

using (var client = new HttpClient()) { client.BaseAddress = new Uri("https://xxx.xxx.com/"); client.DefaultRequestHeaders.Accept.Clear(); client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json")); try { var response = await client.PostAsJsonAsync("customers/xxxx/xxx/documents", requestMessage); response.EnsureSuccessStatusCode(); var returnValue = response.Content.ReadAsAsync<ResponseMessage>(); var hasDocuments = returnValue.Result.SearchResults.Any(); if (hasDocuments) { // HTTP GET client.DefaultRequestHeaders.Accept.Clear(); client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("*/*")); returnValue.Result.SearchResults.ForEach(async d => { response = await client.GetAsync(d.Self + ".pdf"); }); } } catch (Exception ex) { throw; } }

Scott Chamberlain · Accepted Answer · 2015-06-01 21:18:42Z

You are not using async/await in a foreach loop, you are using async/await with List<T>.ForEach(Action<T>). Because ForEach only accepts Action<T> (void return type) your ForEach(async d => { ... } is being processed as a async void function because that is the only way for it to be passed in as a Action<T>. This makes ForEach not wait for the previous item to finish before moving to the next item and that is what causes your error.

Use a normal foreach instead of the List<T>.ForEach method and it should work fine.

 if (hasDocuments) { // HTTP GET client.DefaultRequestHeaders.Accept.Clear(); client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("*/*")); foreach (var d in returnValue.Result.SearchResults) { response = await client.GetAsync(d.Self + ".pdf"); }; }

The only time you are allowed to use async d => ... is when the function you are passing in to takes in a Func<T,Task> (See Task.Run as an example).

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