Problem with Xamarin app while calling an API

Question

I'm a beginner with C# and whole .NET (just wanted to mention at the beginning). So I'm learning Xamarin and APIs.

I've got the problem with fetching some data from API (https://jsonplaceholder.typicode.com/posts) to display inside the app. Once I execute the app, it gives me an error:

System.InvalidCastException: 'Specified cast is not valid'.

I also don't see where is it failing (after error, exception window is blank - like not showing where it stops).

using ExerciseApp.Models;
using Newtonsoft.Json;
using System.Collections.Generic;
using System.Net.Http;
using Xamarin.Forms;

namespace ExerciseApp
{
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();

            GetPosts();
        }

        private async void GetPosts()
        {
            HttpClient client = new HttpClient();
            var response = await client.GetStringAsync("https://jsonplaceholder.typicode.com/posts");
            var posts = JsonConvert.DeserializeObject<List<Posts>>(response);

            PostsListView.ItemsSource = posts;
        }
    }
}

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="ExerciseApp.MainPage">

    <StackLayout BackgroundColor="White">
        <Frame BackgroundColor="#581845" Padding="10" CornerRadius="5" Margin="25">
            <Label Text="ʕ•ᴥ•ʔ" HorizontalTextAlignment="Center" TextColor="White" FontSize="30"/>
        </Frame>
        <Label Text="Welcome to Exercise app with RESTAPI and NHibernate" TextColor="#581845" FontSize="Title" Padding="10" HorizontalTextAlignment="Center"/>
        <Label FontSize="20" Padding="20" HorizontalTextAlignment="Center" TextColor="#581845">
            <Label.FormattedText>
                <FormattedString>
                    <FormattedString.Spans>
                        <Span Text="Here we will load some data"/>
                    </FormattedString.Spans>
                </FormattedString>
            </Label.FormattedText>
        </Label>
        <ListView x:Name="PostsListView">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <StackLayout Orientation="Horizontal">
                        <Label Text="{Binding id}" TextColor="#581845"></Label>
                        <Label Text="{Binding title}" TextColor="#581845"></Label>
                    </StackLayout>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </StackLayout>
</ContentPage>

namespace ExerciseApp.Models
{
    public class Posts
    {
        public int userId { get; set; }
        public int id { get; set; }
        public string title { get; set; }
        public string body { get; set; }
    }
}

if the exception doesn't show you where it occurs, check the stack trace, and if that doesn't help then you need to set a breakpoint in the debugger and step through each line of code until you hit the exception — Jason
– Jason, Commented Mar 21, 2021 at 16:18
@Jason I went through all the breakpoints now - It didn't stop at certain point. It just showed after executing everything (so after function). I think this is why it's not showing any info in error handler. — hajpson
– hajpson, Commented Mar 21, 2021 at 16:24
GetPosts is async, so try calling it from OnAppearing using await instead of the constructor — Jason
– Jason, Commented Mar 21, 2021 at 16:35
@Jason sorry but I'm quite new into it and I have no idea how to do it. — hajpson
– hajpson, Commented Mar 21, 2021 at 16:41

Jason · Accepted Answer · 2021-03-21 18:02:35Z

1

ListView must use a Cell type in their template. Adding a ViewCell into your XAML will fix this problem

    <ListView x:Name="PostsListView">
        <ListView.ItemTemplate>
            <DataTemplate>
              <ViewCell>
                <StackLayout Orientation="Horizontal">
                    <Label Text="{Binding id}" TextColor="#581845"></Label>
                    <Label Text="{Binding title}" TextColor="#581845"></Label>
                </StackLayout>
              </ViewCell>
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>

alternately, you can use a CollectionView which allows you to create templates without the restriction of using a Cell

answered Mar 21, 2021 at 18:02

Jason

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1 Comment

hajpson Over a year ago

And You are the boss! Works like a charm. Thank You very much!

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