[Proposal] IsNull / IsNotNull Converters

IsNull IsNotNull Converters

• Proposed
• Prototype
• Implementation
  • iOS Support
  • Android Support
  • macOS Support
  • Windows Support
• Unit Tests
• Sample
• Documentation: Done

Link to Discussion

Discussion

Summary

The IsNullConverter is a converter that allows users to convert an incoming binding to a bool value. This value represents if the incoming binding value is null.

The IsNotNullConverter is a converter that allows users to convert an incoming binding to a bool value. This value represents if the incoming binding value is not null.

Motivation

As mentioned in the discussion this was the behaviour of another converter, so these converters would just reimplement the old behaviour. It’s been quite common in the projects I’ve worked on to be able to check any object type if it’s null or not.

Detailed Design

IsNotNullConverter.shared.cs

/// <summary>
/// Converts the incoming value to a <see cref="bool"/> indicating whether or not the value is not null.
/// </summary>
public class IsNotNullConverter : BaseConverter<object?, bool>
{
	/// <summary>
	/// Converts the incoming object to a <see cref="bool"/> indicating whether or not the value is not null.
	/// </summary>
	/// <param name="value">The value to convert.</param>
	/// <returns>A <see cref="bool"/> indicating if the incoming value is not null</returns>
	public bool ConvertFrom(object? value) =>  value is not null;
}

IsNullConverter.shared.cs

/// <summary>
/// Converts the incoming value to a <see cref="bool"/> indicating whether or not the value is null.
/// </summary>
public class IsNullConverter : BaseConverter<object?, bool>
{
	/// <summary>
	/// Converts the incoming object to a <see cref="bool"/> indicating whether or not the value is not null.
	/// </summary>
	/// <param name="value">The value to convert.</param>
	/// <returns>A <see cref="bool"/> indicating if the incoming value is not null</returns>
	public bool ConvertFrom(object? value) =>  value is null;
}

Usage Syntax

XAML Usage

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:xct="http://xamarin.com/schemas/2020/toolkit"
             x:Class="MyLittleApp.MainPage">

    <ContentPage.Resources>
        <ResourceDictionary>
            <xct: IsNullConverter x:Key="IsNullConverter" />
        </ResourceDictionary>
    </ContentPage.Resources>

    <StackLayout IsVisible="{Binding MyObject, Converter={StaticResource IsNullConverter}}">

    </StackLayout>
</ContentPage>

C# Usage

class MyPage : ContentPage
{
  public MyPage()
  {
    Content = new StackLayout().Bind(StackLayout.IsVisibleProperty, nameof(ViewModel.MyObject), converter: new IsNullConverter())
  }
}

Drawbacks

None

Alternatives

None

[IeuanWalker]

Cool no prob, i’ll wait

[IeuanWalker]

@brminnick awesome, ye will do