I did exactly the same thing, but still the same error is occurring: "The tag 'CenterToolTipConverter' does not exist in XML namespace 'clr-namespace:WpfApplication1;assembly=WpfApplication1'. Line 12 Position 10."

CenterToolTipConverter.cs

namespace WpfApplication1
{
    public class CenterToolTipConverter : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            if (values.FirstOrDefault(v => v == DependencyProperty.UnsetValue) != null)
            {
                return double.NaN;
            }
            double placementTargetWidth = (double)values[0];
            double toolTipWidth = (double)values[1];
            return (placementTargetWidth / 2.0) - (toolTipWidth / 2.0);
        }

        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
        {
            throw new NotSupportedException();
        }
    }
}

MainWindow.xaml

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:WpfApplication1;assembly=WpfApplication1"
    Title="MainWindow" Height="350" Width="525">

    <Window.Resources>
        <local:CenterToolTipConverter x:Key="myCenterToolTipConverter"/>
    </Window.Resources>
</Window>

What am I doing wrong? Thanks in advance!!!