This is a practice implementation of 7GUIs. It uses WPF and ReactiveUI as a base.

This is a trivial implementation. ViewModel contains a command that adds 1 to a bindable property every time it's called. The command is bound to the <Button> in the UI.

There are several approaches here.

1. No viewmodel needed. The Fahrenheit TextBox can be relative-bound to to the Celsius TextBox.Text through a converter with Convert() (that does C->F conversion) and ConvertBack() (F->C).

2. (current implementation) ViewModel stores a Celsius and Fahrenheit bindable properties. Celsius setter updates the Fahrenheit private field and raises PropertyChanged(Fahrenheit).

Setting the private field is preferred to using the public setter directly: Fahrenheit = value * 9.... Calling the Fahrenheit setter would trigger an update to Celsius, triggering a set to Fahrenheit, and so on, leading to an infinite loop.

Having decimal? Celsius/Fahrenheit properties serves several purposes. If the user hasn't entered anything, both TextBoxes would be empty instead of showing 0.0. If the user enters non-numerics, WPF's binding conversion throws an exception due to different types (string in UI, decimal? in VM).

3. ReactiveUI commands and calculated properties. The viewmodel would look like this:

CalculateCelsiusCommand = ReactiveCommand.Create<decimal, decimal?>(CalculateCelsius);
_celsius = CalculateCelsiusCommand.ToProperty(this, x => x.Celsius);

private static decimal? CalculateCelsius(decimal value)
{
    return (value - 32) * (5m / 9m);
}

The view would invoke the respective commands on KeyUp and would be bound one-way from the Celsius/Fahrenheit source properties.

Celsius.Events().KeyUp
        .Select(ev => ((TextBox)ev.Source).Text)
        .Where(t => decimal.TryParse(t, out _))
        .Select(decimal.Parse)
        .InvokeCommand(ViewModel, vm => vm.CalculateFahrenheitCommand);

<TextBox Grid.Column="0" Text="{Binding Celsius, Mode=OneWay, UpdateSourceTrigger=PropertyChanged}" x:Name="Celsius" />