C# Async Task returns value but value cannot be accessed to manipulate

Question

I have the following function Handle_Log_Clicked() which calls async Task CreateLogCall which makes an HTTP POST request using the repository.CREATE() method and returns HttpResponseMessage type as return value :

I have to use the response in Handle_Log_Clicked() method to retrieve Headers from the HttpResponseMessage response that I get from CreateLogCall ().

In the below code the response does not get set in HttpResponseMessage response .

When I am trying to take a substring of the response resulting in an error.

async void Handle_Log_Clicked(object sender, System.EventArgs e)
    {
        try
        {
            HttpResponseMessage response = await CreateLogCall();
            Debug.WriteLine("Response: ", response);
            var resultHeader = response.Headers.Location.ToString();
            string relatedCallID = resultHeader.Substring(resultHeader.IndexOf('('), resultHeader.Length -1 );

            await CreateLogCallProducts(relatedCallID);

            if (!response.IsSuccessStatusCode)
                await DisplayAlert("Failure", "The data could not be saved", "Ok");
            else
                await DisplayAlert("Success", "The call is logged successfully", "Ok");
        }
        catch (Exception err)
        {
            await DisplayAlert("Error", "Please fill all the details", "Ok");
            Debug.WriteLine(err.Message);
        }
    }

public async Task<HttpResponseMessage> CreateLogCall()
    {
        HttpResponseMessage result = null;
        try
        {
            if (isPrescriber)
            {
                Debug.WriteLine("inside prescriber calls");
                result = await repository.Create(GlobalVariables.AuthToken, "xxxx", logCall);
            }
            else
            {

                result = await repository.Create(GlobalVariables.AuthToken, "yyyyy", logCall);
            }

            return result;
        }
        catch (Exception e)
        {
            Debug.WriteLine(e.Message);
        }
        return result;
    }

Inside repository.cs

public async Task<HttpResponseMessage> Create(string accessToken, string entity, JObject data)
    {
            // The URL for the OData organization web service.
            string url = GlobalVariables.RootUrl + GlobalVariables.EndPoint + entity;

            var content = new StringContent(data.ToString(), Encoding.UTF8, "application/json");
            // Build and send the HTTP request.
            HttpClient httpClient = new HttpClient();
            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", accessToken);

            // Wait for the web service response.
            HttpResponseMessage response;
            response = await httpClient.PostAsync(url, content);
            Debug.WriteLine(response);
            return response;
    }

While putting the debugger I can see the response value being pass to Handle_Log_Clicked() but when the code reaches the substring method it throws error :

Index and length must refer to a location within the string. Parameter name: length

Also the code : Debug.WriteLine("Response: ", response); does not print the response but debugger shows value in it.

For substring you're giving an invalid second value. It's not the end of the substring, it's the length. So if ( is not the first character the length is more than there is in the string — Sami Kuhmonen, May 19 '17 at 18:21
that is also not the right syntax for Debug.WriteLine - you are passing a parameter list without specifying placeholders in your string — Jason, May 19 '17 at 18:31
@SamiKuhmonen rectified that I think that was the mistake for the error. But I still don't get why didn't `Debug.WriteLine("Response: ", response.Headers.Location.LocalPath.ToString()) ` @Jason (rectiifed the debug.writeline code) do not print the value in the log inside `Handle_Log_Clicked ()` but the same `Debug.WriteLine(response);` inside `Create()` prints the data. — souravlahoti, May 19 '17 at 18:47
your syntax is still wrong, try: Debug.WriteLine("Response: {0}",response); — Jason, May 19 '17 at 22:53
There are two ways your call to `Substring()` could go wrong: there is no `'('` character in the string, so the index value is wrong, or you try to get too many characters, making the length value wrong. The exception message clearly says the `length` is wrong, so it's the latter (but you should also prepare for the former). As noted in the first comment above, passing `Length - 1` is wrong; that would only be a valid value in the index value were `0` or `1`. More likely, you just want the remainder of the string, in which case you don't even need to pass the second parameter. — Peter Duniho, May 19 '17 at 23:00

C# Async Task returns value but value cannot be accessed to manipulate

0 Answers0