Entry binding to int? in Xamarin Forms

Question

I have a simple entry in Xamarin Forms:

<Entry Text="{Binding Number, Mode=TwoWay}" Placeholder="Number" Keyboard="Numeric" />

In ViewModel there is property:

    private int? _number;
    public int? Number
    {
        get { return _number; }
        set
        {
            if (SetProperty(ref _number, value))
            {
                OnPropertyChange(nameof(Number));
            }
        }
    }

I enter number in Entry and press button, but in button clicked procedure - Number is still null. What am I doing wrong?

Solution 1

Entry accept a string. If you would like to bind a int property you should use a IValueConverter but I think the best solution is to use a String property than convert the value from String to Int

public string StrNumber{
   get {
        if (Number == null) 
            return "";
        else
            return Number.ToString();
   }

   set {
       try {
           Number = int.Parse(value);
       }
       catch{
           Number = null;
       }
   }
}

Solution 2

You can bind an int to an Entry but you can't bind a nullable int. You can either add another property which converts the number to a string, or you can easily create a value converter like this...

class NullableIntConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var nullable = value as int?;
        var result = string.Empty;

        if (nullable.HasValue)
        {
            result = nullable.Value.ToString();
        }

        return result;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var stringValue = value as string;
        int intValue;
        int? result = null;

        if (int.TryParse(stringValue, out intValue))
        {
            result = new Nullable<int>(intValue);
        }

        return result;
    }

... and use it in your page like this...

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

    <ContentPage.Resources>
        <ResourceDictionary>
            <local:NullableIntConverter x:Key="NullableIntConverter" />
        </ResourceDictionary>
    </ContentPage.Resources>

    <StackLayout>
        <Entry Text="{Binding Number, Mode=TwoWay, Converter={StaticResource NullableIntConverter}}" Placeholder="Number" Keyboard="Numeric" />
        <Label Text="{Binding Number, Converter={StaticResource NullableIntConverter}}" />
    </StackLayout>
</ContentPage>

Comments

[Jeroen Corteville]

Is there a special reason that you are using the nullable int contstructor instead of doing result = intValue?

[Arvis]

You can't convert to negative numbers, because conversation happens on every character input and minus sign get converted to null.