WPF databinding binding error notification

Question

OK, working on WPF(using MVVM) and came across a question, want some input. I have a simple class

like below(assume I have IDataErrorInfo implemented):

public class SimpleClassViewModel
{
  DataModel Model {get;set;}
  public int Fee {get { return Model.Fee;} set { Model.Fee = value;}}
}

I then try to bind to it in xaml:

<TextBox Text={Binding Fee, ValidatesOnDataErrors=true}/>

when a user then clears out the text, a databinding error occurs because it cant convert string.empty to int. Well, Fee is a required field, but because the databinding won't convert back I can't provide error information because my class isn't updated. So am I required to then do the following?

public class SimpleClassViewModel
{
  DataModel Model {get;set;}
  int? _Fee;
  public int? Fee 
  {
   get { return _Fee;} 
   set { _Fee = value;if (value.HasValue) { Model.Fee = value;}
  }
}

Answer 1

This can be done using a ValueConverter:

using System.Windows.Data;

namespace MyNameSpace
{
    class IntToStringConverter:IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return ((int) value).ToString();
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            int result;
            var succes = int.TryParse((string) value,out result);
            return succes ? result : 0;
        }
    }
}

You reference it in the XAML thus:

<Window xmlns:local="clr-namespace:MyNameSpace">
   <Window.Resources>
      <local:IntToStringConverter x:Key="IntConverter"/>
   </Window.Resources>
   <TextBox Text={Binding Fee, ValidatesOnDataErrors=true,
            Converter={StaticResource IntConverter}}/>
</Window>

Answer 2

You can also take advantage of the fact you're doing MVVM and change the type of the Fee property to string. After all, your VM should provide a model that supports the view, and the view allows users to enter a string. Then you can provide a separate property that exposes the parsed fee as an int. That way your conversion logic is right there in the Fee property, making it easier to reuse, debug, and maintain.