Ellipsis path by replacing path parts with some ellipsis const, ... in this particular case.

PathEllipsisConverter.cs

using System;
using System.IO;
using System.Linq;
using System.Windows.Data;

namespace Converters
{
    public class PathEllipsisConverter : IValueConverter
    {
        private const string EllipsisChars = "...";

        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            if (targetType != typeof(string))
            {
                throw new InvalidOperationException("The target must be a string");
            }

            // check that value is path
            try
            {
                Path.GetFullPath(value as string);
            }
            catch
            {
                throw new InvalidOperationException("The value must be a valid windows path");
            }

            // check that parameter is integer
            int maxLength;
            if (!int.TryParse(parameter.ToString(), out maxLength) || maxLength == 0)
            {
                throw new InvalidOperationException("The paramente must be a integer");
            }

            var path = value as string;

            if (path.Length <= maxLength)
            {
                return path;
            }

            var parts = path.Split(Path.DirectorySeparatorChar).ToList();

            var filenane = parts.Last();
            parts.RemoveAt(parts.Count - 1);

            var ellipsedValue = string.Empty;
            do
            {

                if (parts.Count > 0)
                {
                    parts.RemoveAt(parts.Count - 1);
                }

                ellipsedValue = string.Join(Path.DirectorySeparatorChar.ToString(),
                    parts.Concat(new string[] { EllipsisChars, filenane }));

            } while (ellipsedValue.Length > maxLength && parts.Count > 0);

            return ellipsedValue;
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            if (targetType != typeof(string))
            { 
                 throw new InvalidOperationException("The target must be a string");
            }

            return value;
        }
    }
}