how to StringFormat number in datagrid wpf

Here is my Demo of ValueConverter in C#:
The cs code is:

public partial class MainWindow : Window  
    {  
        public MainWindow()  
        {  
            InitializeComponent();  
            ObservableCollection<Member> memberData = new ObservableCollection<Member>()  
            {  
                new Member(){DataNumber = "150.1200"},  
                new Member(){DataNumber = "150.13699"},  
                new Member(){DataNumber = "150.110"},  
                new Member(){DataNumber = "12.000"},  
                new Member(){DataNumber = "61.333"},  
                new Member(){DataNumber = "72.100"}  
            };  
            dataGrid.DataContext = memberData;  
        }  
    }  
    public class Member  
    {  
        public string DataNumber { get; set; }         
    }  
    public class Converter : IValueConverter  
    {  
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)  
        {  
            string strNum = System.Convert.ToDouble(value.ToString()).ToString("0.000");  
            double dNum = System.Convert.ToDouble(strNum);  
            int intNum = System.Convert.ToInt32(dNum);  
            if(intNum==dNum)  
            {  
                return intNum;  
            }else  
            {  
                return dNum.ToString("0.000");  
            }  
              
        }  
  
        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)  
        {  
            throw new NotImplementedException();  
        }  
    }  

The Xaml code is:

The result is: