Xamarin Forms中的ListView绑定到数据库错误

Question

I am building an app using a small SQLLite based database. This worked well on Windows Phone. Now I am trying to get it to work with Xamarin Forms (Portable). The database building and filling of the tables is working. Now I am trying to show elements of an table in ListView, but I am having problems getting the items of my Table Notes in the ListView. My Table Notes is set up and is filled with two Items:

public class Notes
{
//The Id property is marked as the Primary Key
[SQLite.PrimaryKey, SQLite.AutoIncrement]
public int Id { get; set; }
public string Note { get; set; }
public DateTimeOffset NoteDate { get; set; }
public TimeSpan NoteStart { get; set; }
public Notes()
{
}
public Notes(string remark, DateTimeOffset notedate, TimeSpan noteStart)
{
Note = remark;
NoteDate = notedate;
NoteStart = noteStart;
}
}
db.Insert(new Notes("Test", DateTimeOffset.Now.AddDays(0), TimeSpan.Parse("7:45")));
db.Insert(new Notes("Test", DateTimeOffset.Now.AddDays(1), TimeSpan.Parse("7:45")));

I am calling the Items with:

public IEnumerable<Notes> GetItems()
{
{
return (from i in db.Table<Notes>() select i).ToList();
}
}

I have a XAML based Contentpage:

<?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="Todo.Views.DatePage">
<ListView x:Name="Listnotes">
<ListView.ItemTemplate>
<DataTemplate>
<Label Text="{Binding Note}" />
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</ContentPage>

The Binding of the ListView is performed in C#:

NoteHelper nh = new NoteHelper();
private IEnumerable<Notes> notelist = new List<Notes>();

notelist = nh.GetItems((DateTimeOffset.Now));
Listnotes.ItemsSource = notelist;

When I build the solution on my Windows Phone I am getting the following error statements. What am I doing wrong?

System.NullReferenceException: Object reference not set to an instance of an object.

at Xamarin.Forms.Platform.WinRT.CellControl.SetCell(Object newContext)
at Xamarin.Forms.Platform.WinRT.CellControl.MeasureOverride(Size availableSize)
at Windows.UI.Xaml.UIElement.Measure(Size availableSize)
at Xamarin.Forms.Platform.WinRT.VisualElementRenderer`2.MeasureOverride(Size availableSize)
at Windows.UI.Xaml.UIElement.Measure(Size availableSize)
at Xamarin.Forms.Platform.WinRT.VisualElementR

Answer 1

System.NullReferenceException: Object reference not set to an instance of an object. at Xamarin.Forms.Platform.WinRT.CellControl.SetCell(Object newContext)

Your DataTemplate needs to define the Cell ( TextCell , ViewCell ) that will be rendered otherwise you will receive an runtime exception when it tries to render the the first data bound item.

So surround your Label with at least ViewCell \\ ViewCell.View ,

Example:

  <DataTemplate>
    <ViewCell>
      <ViewCell.View>
        <StackLayout Orientation="Horizontal">
            Label Text="{Binding Note}" />
          </StackLayout>
      </ViewCell.View>
    </ViewCell>
  </DataTemplate>