Data Binding
Remarks#
Possible Exceptions
System.ArrayTypeMismatchException: Attempted to access an element as a type incompatible with the array.
This exception can occur when attempting to bind a collection to a non-bindable property when XAML pre-compilation is enabled. A common example is attempting to bind to Picker.Items. See below.
System.ArgumentException: Object of type ‘Xamarin.Forms.Binding’ cannot be converted to type ‘System.String’.
This exception can occur when attempting to bind a collection to a non-bindable property when XAML pre-compilation is disabled. A common example is attempting to bind to Picker.Items. See below.
The Picker.Items Property Is Not Bindable
This code will cause an error:
<!-- BAD CODE: will cause an error -->
<Picker Items="{Binding MyViewModelItems}" SelectedIndex="0" />The exception may be one of the following:
System.ArrayTypeMismatchException: Attempted to access an element as a type incompatible with the array.
or
System.ArgumentException: Object of type ‘Xamarin.Forms.Binding’ cannot be converted to type ‘System.String’.
Specifically, the Items property is non-bindable. Solutions include creating your own custom control or using a third-party control, such as BindablePicker from FreshEssentials. After installing the FreshEssentials NuGet package in your project, the package’s BindablePicker control with a bindable ItemsSource property is available:
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="https://xamarin.com/schemas/2014/forms"
xmlns:x="https://schemas.microsoft.com/winfx/2009/xaml"
xmlns:fe="clr-namespace:FreshEssentials;assembly=FreshEssentials"
xmlns:my="clr-namespace:MyAssembly;assembly=MyAssembly"
x:Class="MyNamespace.MyPage">
<ContentPage.BindingContext>
<my:MyViewModel />
</ContentPage.BindingContext>
<ContentPage.Content>
<fe:BindablePicker ItemsSource="{Binding MyViewModelItems}" SelectedIndex="0" />
</ContentPage.Content>
</ContentPage>Basic Binding to ViewModel
EntryPage.xaml:
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="https://xamarin.com/schemas/2014/forms"
xmlns:x="https://schemas.microsoft.com/winfx/2009/xaml"
xmlns:vm="clr-namespace:MyAssembly.ViewModel;assembly=MyAssembly"
x:Class="MyAssembly.EntryPage">
<ContentPage.BindingContext>
<vm:MyViewModel />
</ContentPage.BindingContext>
<ContentPage.Content>
<StackLayout VerticalOptions="FillAndExpand"
HorizontalOptions="FillAndExpand"
Orientation="Vertical"
Spacing="15">
<Label Text="Name:" />
<Entry Text="{Binding Name}" />
<Label Text="Phone:" />
<Entry Text="{Binding Phone}" />
<Button Text="Save" Command="{Binding SaveCommand}" />
</StackLayout>
</ContentPage.Content>
</ContentPage>MyViewModel.cs:
using System;
using System.ComponentModel;
namespace MyAssembly.ViewModel
{
public class MyViewModel : INotifyPropertyChanged
{
private string _name = String.Empty;
private string _phone = String.Empty;
public string Name
{
get { return _name; }
set
{
if (_name != value)
{
_name = value;
OnPropertyChanged(nameof(Name));
}
}
}
public string Phone
{
get { return _phone; }
set
{
if (_phone != value)
{
_phone = value;
OnPropertyChanged(nameof(Phone));
}
}
}
public ICommand SaveCommand { get; private set; }
public MyViewModel()
{
SaveCommand = new Command(SaveCommandExecute);
}
private void SaveCommandExecute()
{
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
}