How to set a combobox size to the maximum width of its content?

Question

I have this ComboBox

<ComboBox ItemsSource="{Binding Path=Foo.Bars}"/>

Can I set the size of the combobox to the width of its widest item?

For example, if the contents are:

John Doe
Jane Mary
Josh

The length would be equal to the length of Jane Mary.

Also, in this case the contents are not expected to change after initialization

Answer 1

What you can do is make a converter that would return the longest length of one your object property. You can implement the converter like this:

public class LongestListObjectToIntConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value is IEnumerable<FooBar>)
        {
            IEnumerable<FooBar> list = (IEnumerable<FooBar>)value;

            return list.Max(bar => bar.FullName.Length);
        }

        // Default value to return
        return 100;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

And then simply bind the Width property of your ComboBox by provinding the list as the Path binding and your converter as a value converter.

<Window.Resources>
    <conv:LongestListObjectToIntConverter x:Key=converter/>
</Windows.Resources>

    ...

<ComboBox ItemsSource="{Binding Path=Foo.Bars}" Width="{Binding Path=Foo.Bars, Converter={StaticResource converter}}"/>

This way, even if your collection changes and this changes are not notified, the ComboBox would resize depending on the longest word.

Another interesting idea would be to make a self binding on Width and grab the actual combobox in the converter to then inspect the displayed value, that would be even better I think.

The advantage of this solution is that uses no code-behind and is easily reusable. You can find more information on ValueConverters here: http://www.wpftutorial.net/ValueConverters.html

Answer 2

You can write converter like this

 public class ComboBoxToMaxItemWidthConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        double maxWidth = 0;
        ComboBox cb = (ComboBox)value;
        foreach (var item in cb.Items)
        {
            ComboBoxItem cbItem = (ComboBoxItem)cb.ItemContainerGenerator.ContainerFromItem(item);
            if (cbItem.ActualWidth > maxWidth)
                maxWidth = cbItem.ActualWidth;
        }
        return maxWidth;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

And use it

<ComboBox ItemsSource="{Binding Path=Foo.Bars}" Width={Binding RelativeSource={RelativeSource Self}, Converter={StaticResource comboBoxToMaxItemWidthConverter }/>

Hope this helps