How to validate data in WPF and set default value?

Question

I'm trying to perform a validation property. We have a nullable property called:

    public int? Number
    {
        get { return _number; }
        set
        {
            if (_number != value)
            {
                _number = value;
                RaisePropertyChanged("Number");
            }
        }
    }

And this property is bound to a textbox. I only want to validate this two escenarios:

• Imagine that the user left the textbox empty (textbox.text=""), so Number property must receive null value (instead "" ).
• And if the user inputs "2b", Number property must have a null value (because is an error), but the textbox must still say "2b".

I guess IDataNotifyError and ValidationRules is not working for this. How can I resolve these situations?

EDIT: I'm using also a ValidationRule to show a custom message when the user inputs an incorrect format. But when this happens, does not fire the property to null. And if a put true in that error, it fired, but does not show any error message.

                 <TextBox.Text>
                    <Binding Path="Number" UpdateSourceTrigger="PropertyChanged" ValidatesOnDataErrors="True" ValidatesOnExceptions="True" NotifyOnValidationError="True" Converter="{x:Static c:IntConverter.Default}" >
                        <Binding.ValidationRules>
                                <r:NumericValidation />
                        </Binding.ValidationRules>
                    </Binding>
                </TextBox.Text>

The validation rule

public class NumericValidation : ValidationRule
{
    public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
    {
        int? response;
        bool noIllegalChars = TryParseStruct<int>(value.ToString(), out response);

        if (noIllegalChars == false)
        {
            return new ValidationResult(false, "Input is not in a correct format.");
        }
        else
        {
            return new ValidationResult(true, null);
        }
    }

    ...
}

Answer 1

Try following converter:

public class IntConverter : IValueConverter
{
    public object Convert(object value,
                          Type targetType,
                          object parameter,
                          CultureInfo culture)
    {
        if (value is int?)
        {
            int? intValue = (int?)value;
            if (intValue.HasValue)
            {
                return intValue.Value.ToString();
            }
        }

        return Binding.DoNothing;
    }

    public object ConvertBack(object value,
                              Type targetType,
                              object parameter,
                              CultureInfo culture)
    {
        if(value is string)
        {
            int number;
            if (Int32.TryParse((string)value, out number))
            {
                return number;
            }
        }

        return null;
    }
}

Answer 2

You can use converter and manipulate the values before they are assigned to underlying property.

Answer 3

Use @Amit's converter and then to get the error use IDataErrorInfo with the following code:

public string this[string columnName]
{
    get
    {
        if (columnName == "Number")
        {
            if (Number == null) return "Invalid number";
        }

        return null;
    }
}

Or you could do it with your ValidationRule but you would need to change its ValidationStep property. Your validation is currently being fired at the default time RawProposedValue, i.e. before the converter is kicking in.