I have a project where I am using System.Net.Http.HttpClient. I am trying to centralize all calls to my Web APIs so that I have common error handing etc. I created the following class in my project.
using ModernHttpClient;
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;
namespace WebAPIHelper
{
class WebAPICaller
{
public async Task<string> CallWebService(string ps_URI)
{
HttpClient lobj_HTTPClient = null;
HttpResponseMessage lobj_HTTPResponse = null;
string ls_Response = "";
//We assume the internet is available.
try
{
//Get the Days of the Week
lobj_HTTPClient = new HttpClient(new NativeMessageHandler());
lobj_HTTPClient.BaseAddress = new Uri(App.APIPrefix);
lobj_HTTPClient.DefaultRequestHeaders.Accept.Clear();
lobj_HTTPClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
lobj_HTTPResponse = await lobj_HTTPClient.GetAsync(ps_URI);
if (!lobj_HTTPResponse.IsSuccessStatusCode)
{
Debug.WriteLine(lobj_HTTPResponse.ReasonPhrase);
}
else
{
ls_Response = await lobj_HTTPResponse.Content.ReadAsStringAsync();
}
}
catch (Exception ex)
{
Debug.WriteLine(ex.Message);
}
finally
{
if (lobj_HTTPClient != null)
lobj_HTTPClient.Dispose();
if (lobj_HTTPResponse != null)
{
lobj_HTTPResponse.Dispose();
}
}
return ls_Response;
}
}
}
I call the function from an instance object I created in my ViewModel class for languages as follows:
using ModernHttpClient;
using Newtonsoft.Json;
using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Diagnostics;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Runtime.CompilerServices;
using System.Threading.Tasks;
namespace WebAPIHelper
{
public class VM_Languages : INotifyPropertyChanged
{
/// <summary>
/// A collection for CGSLanguage objects.
/// </summary>
public ObservableCollection<GBSLanguage_ForList> Items_ForList { get; private set; }
const string ic_LanguagesAPIUrl = @"/languages/true";
/// <summary>
/// Constructor for the Languages view model.
/// </summary>
public VM_Languages()
{
this.Items_ForList = new ObservableCollection<GBSLanguage_ForList>();
}
/// <summary>
/// Indicates of the view model data has been loaded
/// </summary>
public bool IsDataLoaded
{
get;
private set;
}
/// <summary>
/// Creates and adds a the countries to the collection.
/// </summary>
public async Task LoadData()
{
HttpClient lobj_HTTPClient = null;
HttpResponseMessage lobj_HTTPResponse = null;
string ls_Response = "";
try
{
IsDataLoaded = false;
string ls_WorkLanguageURI = ic_LanguagesAPIUrl;
//Get the Days of the Week
lobj_HTTPClient = new HttpClient(new NativeMessageHandler());
lobj_HTTPClient.BaseAddress = new Uri(App.APIPrefix);
lobj_HTTPClient.DefaultRequestHeaders.Accept.Clear();
lobj_HTTPClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
**//This call will not work
//WebAPICaller lobj_APICaller = new WebAPICaller();
//ls_Response = lobj_APICaller.CallWebService(ls_WorkLanguageURI).Result;
//This call will work
lobj_HTTPResponse = await lobj_HTTPClient.GetAsync(ls_WorkLanguageURI);**
if (lobj_HTTPResponse.IsSuccessStatusCode)
{
if (this.Items_ForList != null)
this.Items_ForList.Clear();
ls_Response = await lobj_HTTPResponse.Content.ReadAsStringAsync();
Items_ForList = JsonConvert.DeserializeObject<ObservableCollection<GBSLanguage_ForList>>(ls_Response);
}
}
catch (Exception ex)
{
Debug.WriteLine(ex.Message);
}
finally
{
this.IsDataLoaded = true;
NotifyPropertyChanged("GBSLanguages_ForList");
}
}
/// <summary>
/// Notifies subscribers that a property has changed.
/// </summary>
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(String propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (null != handler)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
}
I have all my ViewModels in a static class so I only ever get one instance of them as follows:
namespace WebAPIHelper
{
public static class ViewModelObjects
{
private static VM_Languages iobj_Languages;
public static VM_Languages Languages
{
get
{
if (iobj_Languages == null)
iobj_Languages = new VM_Languages();
return iobj_Languages;
}
}
}
}
In my on appearing code of my main page I have the following call to retrieve the data from the WebAPI
protected override async void OnAppearing()
{
Device.BeginInvokeOnMainThread(() =>
{
if (!ViewModelObjects.Languages.IsDataLoaded)
ViewModelObjects.Languages.LoadData();
});
//If the DOW and Language data are not loaded yet - wait
while (!ViewModelObjects.Languages.IsDataLoaded)
{
await Task.Delay(1000);
}
}
The problem is when I attempt to use my WebAPICaller class, it appears to crash on the line. I never get a return from it. No exceptions are ever raised and my program never continues.
lobj_HTTPResponse = await lobj_HTTPClient.GetAsync(ps_URI);
If I make what I believe to be the exact same call from my ViewModel, it works. (I have both the call to the WebAPICaller class as well as a direct call to GetAsync in the View Model so you can test it out by commenting and uncommenting.)
Any idea as to what I am doing wrong?
Link to full sample project: https://1drv.ms/u/s!Ams6cZUzaeQy3M8uGAuaGggMt0Fi-A
