How to split Xaml Grid column 50/50 when the 2nd column's content is visible?

Question

With a grid that has 2 columns, how will I be able to split it 50/50 in width only when the content of the second column is visible?

<Grid>
   <Grid.ColumnDefinitions>
      <ColumnDefinition />
      <ColumnDefinition />
   </Grid.ColumnDefinitions>

   <Grid Grid.Column="0">
      <!-- Some content here -->
   </Grid>

   <Grid Grid.Column="1" Visibility="{Binding HasValue, Converter={StaticResource BooleanToVisibilityConverter}}">
      <!-- Some content here -->
   </Grid>    
</Grid>

If HasValue is true, the grid needs to be split 50/50, else, the first column must occupy the whole screen.

Answer 1

Create a converter BoolToGridLengthConverter and put it in your App.xaml as a static resource:

/// <summary>
/// Converts a boolean value to a grid length which is specified with a converter
/// parameter if the value is true, otherwise the grid lengthe will be zero.
/// <para>
/// If no parameter is specified the returned <see cref="Windows.UI.Xaml.GridLength"/> will be zero.
/// If the parameter cannot be parsed to a grid length then the returned <see cref="Windows.UI.Xaml.GridLength"/> will be zero.
/// If the value is a <see cref="bool"/> false, then the returned <see cref="Windows.UI.Xaml.GridLength"/> will be zero.
/// </para>
/// </summary>
public sealed class BoolToGridLengthConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, string language)
    {
        if (!(value is bool) || !(parameter is string))
        {
            return new GridLength(0);
        }

        if (!((bool)value))
        {
            return new GridLength(0);
        }

        var str = parameter as string;
        if (str.Equals("Auto"))
        {
            return new GridLength(0, GridUnitType.Auto);
        }

        if (str.Equals("*"))
        {
            return new GridLength(1, GridUnitType.Star);
        }

        if (str.EndsWith("*"))
        {
            var length = Double.Parse(str.Substring(0, str.Length - 1));
            return new GridLength(length, GridUnitType.Star);
        }

        var len = Double.Parse(str);
        return new GridLength(len);
    }

    public object ConvertBack(object value, Type targetType, object parameter, string language)
    {
        throw new NotImplementedException();
    }
}

Then you can use it in your xaml like this:

    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="{Binding HasValue, Converter={StaticResource BoolToGridLengthConverter}, ConverterParameter='*'}"/>
        </Grid.ColumnDefinitions>

        <Grid Grid.Column="0">
            <!--  Some content here  -->
        </Grid>

        <Grid Grid.Column="1" Visibility="{Binding HasValue, Converter={StaticResource BooleanToVisibilityConverter}}">
            <!--  Some content here  -->
        </Grid>
    </Grid>

The converter will use the ConverterParameter only if the boolean value is true, otherwise the column width will be set to zero. The converter parameter, can by Auto, or for example 0.5* or any fixed width for example 50.5 etc.