Question

I try to get a Button to Enable if all textbox have a value but it doesn´t work and i can´t find out why.

NullToBoolConverter.cs

public class NullToBoolConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        TypeConverter converter = TypeDescriptor.GetConverter(typeof(bool));
        bool nullResult = false;
        if (parameter != null)
            nullResult = (bool)converter.ConvertFrom(parameter);
        if (value == null)
            return nullResult;
        else
            return !nullResult;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return Binding.DoNothing;
    }
}

XAML-Ressourcedict.

<ex:NullToBoolConverter x:Key="NullToBoolConverter"/>

<Style x:Key="okButtonStyle" TargetType="{x:Type Button}">
    <Setter Property="IsEnabled" Value="False" />
    <Style.Triggers>
        <MultiDataTrigger>
            <MultiDataTrigger.Conditions>
                <Condition Binding="{Binding Hostadress, Converter={StaticResource NullToBoolConverter}, UpdateSourceTrigger=PropertyChanged}" Value="True" />
                <Condition Binding="{Binding UserBox, Converter={StaticResource NullToBoolConverter}, UpdateSourceTrigger=PropertyChanged}" Value="True"  />
                <Condition Binding="{Binding PasswordTextBox, Converter={StaticResource NullToBoolConverter}, UpdateSourceTrigger=PropertyChanged}" Value="True" />
                <Condition Binding="{Binding PortBox, Converter={StaticResource NullToBoolConverter}, UpdateSourceTrigger=PropertyChanged}" Value="True" />
            </MultiDataTrigger.Conditions>
            <Setter Property="IsEnabled" Value="True" />
        </MultiDataTrigger>
    </Style.Triggers>
</Style>

Answer 1

I tried your code and I your only problem is that you are only comparing the value to null and not to string.Empty. Try this

public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
    TypeConverter converter = TypeDescriptor.GetConverter(typeof(bool));
    bool nullResult = false;
    if (parameter != null)
        nullResult = (bool)converter.ConvertFrom(parameter);
    if (value == null || value.ToString() == string.Empty)
        return nullResult;
    else
        return !nullResult;
}