if (bindingGroup.CommitEdit())
     SaveEverything();

object OnUpdateSourceExceptionFilter(object bindExpression, Exception exception)
{
    BindingExpression expr = (bindExpression as BindingExpression);
    if (expr.DataItem is IReceivesBindingErrorInformation)
    {
        ((IReceivesBindingErrorInformation)expr.DataItem).ReceiveBindingErrorInformation(expr.ParentBinding.Path.Path, exception);
    }

    // check for FormatException and produce a nicer error
    // ...
 }

HashSet<string> bindingErrors = new HashSet<string>();
void IReceivesBindingErrorInformation.ReceiveBindingErrorInformation(string path, Exception exception)
{
    bindingErrors.Add(path);
}

protected ObservableCollection<string> errors = new ObservableCollection<string>();

public virtual ObservableCollection<string> Errors
{
    get { return errors; }
}

protected ObservableCollection<string> externalErrors = new ObservableCollection<string>();

public ObservableCollection<string> ExternalErrors
{
    get { return externalErrors; }
}

public virtual bool HasError
{
    get { return Errors != null && Errors.Count > 0; }
}

public override ObservableCollection<string> Errors
{
    get
    {
        errors = new ObservableCollection<string>();
        errors.AddUniqueIfNotEmpty(this["Name"]);
        errors.AddUniqueIfNotEmpty(this["EmailAddresses"]);
        errors.AddUniqueIfNotEmpty(this["SomeOtherProperty"]);
        errors.AddRange(ExternalErrors);
        return errors;
    }
}

public override string this[string propertyName]
{
    get
    {
        string error = string.Empty;
        if (propertyName == "Name" && Name.IsNullOrEmpty()) error = "You must enter the Name field.";
        else if (propertyName == "EmailAddresses" && EmailAddresses.Count == 0) error = "You must enter at least one e-mail address into the Email address(es) field.";
        else if (propertyName == "SomeOtherProperty" && SomeOtherProperty.IsNullOrEmpty()) error = "You must enter the SomeOtherProperty field.";
        return error;
    }
}

private void ValidateUniqueName(Genre genre)
{
    string errorMessage = "The genre name must be unique";
    if (!IsGenreNameUnique(genre))
    {
        if (!genre.ExternalErrors.Contains(errorMessage)) genre.ExternalErrors.Add(errorMessage);
    }
    else genre.ExternalErrors.Remove(errorMessage);
}

private bool IsGenreNameUnique(Genre genre)
{
    return Genres.Where(d => d.Name != string.Empty && d.Name == genre.Name).Count() == 1;
}

public object FooObject { get; set; } // Implement INotifyPropertyChanged

public int Foo
{
    get { return FooObject.GetType() == typeof(int) ? int.Parse(FooObject) : -1; }
}

public override string this[string propertyName]
{
    get
    {
        string error = string.Empty;
        if (propertyName == "FooObject" && FooObject.GetType() != typeof(int))
            error = "Please enter a whole number for the Foo field.";
        ...
        return error;
    }
}

container.AddHandler(Validation.ErrorEvent, Container_Error);

...

void Container_Error(object sender, ValidationErrorEventArgs e) {
    ...
}

public struct Failable<T>
{
    public T Value { get; private set; }
    public string Text { get; private set; }
    public bool IsValid { get; private set; }

    public Failable(T value)
    {
        Value = value;

        try
        {
            var converter = TypeDescriptor.GetConverter(typeof(T));
            Text = converter.ConvertToString(value);
            IsValid = true;
        }
        catch
        {
            Text = String.Empty;
            IsValid = false;
        }
    }

    public Failable(string text)
    {
        Text = text;

        try
        {
            var converter = TypeDescriptor.GetConverter(typeof(T));
            Value = (T)converter.ConvertFromString(text);
            IsValid = true;
        }
        catch
        {
            Value = default(T);
            IsValid = false;
        }
    }
}

public class StringToFailableConverter<T> : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value.GetType() != typeof(Failable<T>))
            throw new InvalidOperationException("Invalid value type.");

        if (targetType != typeof(string))
            throw new InvalidOperationException("Invalid target type.");

        var rawValue = (Failable<T>)value;
        return rawValue.Text;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value.GetType() != typeof(string))
            throw new InvalidOperationException("Invalid value type.");

        if (targetType != typeof(Failable<T>))
            throw new InvalidOperationException("Invalid target type.");

        return new Failable<T>(value as string);
    }
}

public static class Failable
{
    public static StringToFailableConverter<Int32> Int32Converter { get; private set; }
    public static StringToFailableConverter<double> DoubleConverter { get; private set; }

    static Failable()
    {
        Int32Converter = new StringToFailableConverter<Int32>();
        DoubleConverter = new StringToFailableConverter<Double>();
    }
}

public Failable<int> NumberValue
{
    //Custom logic along with validation
    //using IsValid property
}

<TextBox Text="{Binding NumberValue,Converter={x:Static local:Failable.Int32Converter}}"/>

public abstract class ViewModelBase : INotifyPropertyChanged
{
   public event PropertyChangedEventHandler PropertyChanged;

   public void RaisePropertyChanged([CallerMemberName] string propertyName = null)
   {
      if (PropertyChanged != null)
         PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
   }
}

public class VmSomeEntity : ViewModelBase, INotifyDataErrorInfo
{
    //This one is part of INotifyDataErrorInfo interface which I will not use,
    //perhaps in more complicated scenarios it could be used to let some other VM know validation changed.
    public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;

    //will hold the errors found in validation.
    public Dictionary<string, string> ValidationErrors = new Dictionary<string, string>();

    //the actual value - notice it is 'int' and not 'string'..
    private int storageCapacityInBytes;

    //this is just to keep things sane - otherwise the view will not be able to send whatever the user throw at it.
    //we want to consume what the user throw at us and validate it - right? :)
    private string storageCapacityInBytesWrapper;

    //This is a property to be served by the View.. important to understand the tactic used inside!
    public string StorageCapacityInBytes
    {
       get { return storageCapacityInBytesWrapper ?? storageCapacityInBytes.ToString(); }
       set
       {
          int result;
          var isValid = int.TryParse(value, out result);
          if (isValid)
          {
             storageCapacityInBytes = result;
             storageCapacityInBytesWrapper = null;
             RaisePropertyChanged();
          }
          else
             storageCapacityInBytesWrapper = value;

          HandleValidationError(isValid, "StorageCapacityInBytes", "Not a number.");
       }
    }

    //Manager for the dictionary
    private void HandleValidationError(bool isValid, string propertyName, string validationErrorDescription)
    {
        if (!string.IsNullOrEmpty(propertyName))
        {
            if (isValid)
            {
                if (ValidationErrors.ContainsKey(propertyName))
                    ValidationErrors.Remove(propertyName);
            }
            else
            {
                if (!ValidationErrors.ContainsKey(propertyName))
                    ValidationErrors.Add(propertyName, validationErrorDescription);
                else
                    ValidationErrors[propertyName] = validationErrorDescription;
            }
        }
    }

    // this is another part of the interface - will be called automatically
    public IEnumerable GetErrors(string propertyName)
    {
        return ValidationErrors.ContainsKey(propertyName)
            ? ValidationErrors[propertyName]
            : null;
    }

    // same here, another part of the interface - will be called automatically
    public bool HasErrors
    {
        get
        {
            return ValidationErrors.Count > 0;
        }
    }
}