public class ProductNotifyDataErrorInfo :
     ProductBase,
     INotifyDataErrorInfo
{

    // validate before setting the value
    protected override void SetValue(string p, object value)
    {
        // set property value
        base.SetValue(p, value);

        // validate
        var valid = IsValid(p, value);

        // item-level validation
        if (Price <= Cost)
        {
            AddError(string.Empty, "Price must be > Cost");
        }
        else
        {
            RemoveError(string.Empty);
        }
    }
    bool IsValid(string propName, object value)
    {
        // assume all is OK
        bool valid = true;
        RemoveError(propName);

        // property-level validation
        switch (propName)
        {
            case "Price":
                {
                    var p = value as double?;
                    if (p <= 0)
                    {
                        AddError(propName, "Price must be > 0");
                        valid = false;
                    }
                }
                break;
            case "Cost":
                {
                    var c = value as double?;
                    if (c <= 0)
                    {
                        AddError(propName, "Cost must be > 0");
                        valid = false;
                    }
                }
                break;
        }
        return valid;
    }

    // error dictionary
    Dictionary<String, List<String>> _errors = new Dictionary<string, List<string>>();
    void AddError(string propName, string error)
    {
        if (!_errors.ContainsKey(propName))
        {
            _errors[propName] = new List<string>();
        }

        if (!_errors[propName].Contains(error))
        {
            _errors[propName].Insert(0, error);
            RaiseErrorsChanged(propName);
        }
    }
    void RemoveError(string propName)
    {
        if (_errors.ContainsKey(propName))
        {
            _errors.Remove(propName);
            RaiseErrorsChanged(propName);
        }
    }
    void RaiseErrorsChanged(string propName)
    {
        if (ErrorsChanged != null)
        {
            ErrorsChanged(this, new DataErrorsChangedEventArgs(propName));
        }
    }

    #region ** INotifyDataErrorInfo
    public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;
    IEnumerable INotifyDataErrorInfo.GetErrors(string propName)
    {
        if (propName == null)
        {
            propName = string.Empty;
        }
        if (!_errors.ContainsKey(propName))
        {
            return null;
        }
        return _errors[propName];
    }
    bool INotifyDataErrorInfo.HasErrors
    {
        get { return _errors.Count > 0; }
    }