如何使用数据绑定来处理null类？

Question

我正在开发一个几乎是应用程序表单的WPF应用程序。它有多个文本字段。这是我使用MVVM模型的第一个应用程序，所以我确信我错过了一些东西。我有很多文本框，所以我展示的代码将集中在四个文本字段，第四个字段假设总计与其他三个字段一样。但是我的数据绑定没有执行。我无法弄清楚为什么当我在文本框中键入数字txbFamO时，不会执行FamilyO get语句。我没有正确绑定吗？申请人是否没有正确初始化？

这是我的XAML：

<UserControl x:Class="ApplicationForm.Views.ApplicationView" 
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:ApplicationForm"
    xmlns:model="clr-namespace:ApplicationForm.Model"
    xmlns:views="clr-namespace:ApplicationForm.Views"
    xmlns:viewModel="clr-namespace:ApplicationForm.ViewModel"
    mc:Ignorable="d"
    Height="1100" Width="800" Background="#FFDAFDF2">

<Grid x:Name="grdAppForm">
    <Grid x:Name="grdAppFormGrid" DataContext="{Binding Applicant}"
          Height="881" Width="700" Margin="0,0,0,0">
        <Label x:Name="lblFamilySize" Content="Family Size:" Height="28" Width="102" HorizontalAlignment="Left" VerticalAlignment="Top" Margin="372,71,0,0" FontFamily="Arial" FontWeight="Bold" FontSize="16" />
        <Label x:Name="lblFamO" Content="O:" Height="28" Width="27" HorizontalAlignment="Left" VerticalAlignment="Top" Margin="471,71,0,0" FontFamily="Arial" FontWeight="Bold" FontSize="16"/>
        <TextBox x:Name="txbFamO" Text="{Binding FamilyO, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Height="22" Width="20" HorizontalAlignment="Left" 
                 VerticalAlignment="Top" Margin="496,71,0,0" FontFamily="Arial" FontSize="16" 
                 FontWeight="Bold" BorderThickness="0,0,0,3" BorderBrush="Black" VerticalContentAlignment="Bottom"/>
        <Label x:Name="lblFamA" Content="A:" Height="28" Width="25" HorizontalAlignment="Left" VerticalAlignment="Top" Margin="516,71,0,0" FontFamily="Arial" FontWeight="Bold" FontSize="16"/>
        <TextBox x:Name="tbxFamA" Text="{Binding FamilyA, Mode=OneWay, UpdateSourceTrigger=PropertyChanged}" Height="22" Width="20" HorizontalAlignment="Left" 
                 VerticalAlignment="Top" Margin="541,71,0,0" FontFamily="Arial" FontSize="16" 
                 FontWeight="Bold" BorderThickness="0,0,0,3" BorderBrush="Black" VerticalContentAlignment="Bottom"/>
        <Label x:Name="lblFamC" Content="C:" Height="28" Width="25" HorizontalAlignment="Left" VerticalAlignment="Top" Margin="562,71,0,0" FontFamily="Arial" FontWeight="Bold" FontSize="16"/>
        <TextBox x:Name="txtFamC" Text="{Binding FamilyC, Mode=OneWay, UpdateSourceTrigger=PropertyChanged}" 
                 Height="22" Width="20" HorizontalAlignment="Left" VerticalAlignment="Top" Margin="587,71,0,0" FontFamily="Arial" FontSize="16" FontWeight="Bold" BorderThickness="0,0,0,3" BorderBrush="Black" VerticalContentAlignment="Bottom"/>
        <Label x:Name="lblEqual" Content="=" Height="28" Width="20" HorizontalAlignment="Left" VerticalAlignment="Top" 
               Margin="611,71,0,0" FontFamily="Arial" FontWeight="Bold" FontSize="16"/>
        <TextBox x:Name="tbxFamTot" Text="{Binding FamilyTotal, 
                Mode=OneWay, UpdateSourceTrigger=PropertyChanged}" IsReadOnly="True" Height="22" Width="30" 
                HorizontalAlignment="Left" VerticalAlignment="Top" Margin="634,71,0,0" FontFamily="Arial" 
                FontWeight="Bold" FontSize="16" BorderBrush="Black" BorderThickness="0,0,0,3" IsTabStop="False" 
                 VerticalContentAlignment="Bottom"/>
    </Grid>
</Grid>

我的型号代码：

namespace ApplicationForm.Model
{
public class ApplicationModel
{
}


public class Applicant : INotifyPropertyChanged
{
    private string familyO;
    private string familyA;
    private string familyC;

    public string FamilyO
    {
        get { return familyO; }

        set
        {
            if (familyO != value)
            {
                familyO = value;
                RaisePropertyChanged("FamilyO");
                RaisePropertyChanged("FamilyTotal");
            }
        }
    }

    public string FamilyA
    {
        get { return familyA; }

        set
        {
            if (familyA != value)
            {
                familyA = value;
                RaisePropertyChanged("FamilyA");
                RaisePropertyChanged("FamilyTotal");
            }
        }
    }

    public string FamilyC
    {
        get { return familyC; }

        set
        {
            if (familyC != value)
            {
                familyC = value;
                RaisePropertyChanged("FamilyC");
                RaisePropertyChanged("FamilyTotal");
            }
        }
    }

    public string FamilyTotal
    {
        get
        {
            return (Convert.ToInt16(familyO) + Convert.ToInt16(familyA) + Convert.ToInt16(familyC)).ToString();
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void RaisePropertyChanged(string property)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(property));
        }
    }
}

}

我的ViewModel代码：

namespace ApplicationForm.ViewModel
{
class ApplicationViewModel : INotifyPropertyChanged
{
    // If modifying these scopes, delete your previously saved credentials
    // at ~/.credentials/sheets.googleapis.com-dotnet-quickstart.json
    static string[] Scopes = { SheetsService.Scope.Spreadsheets };
    static string ApplicationName = "NB Food Pantry Application";


    public RelayCommand LoadApplicantCommand
    {
        get;
        set;
    }

    public RelayCommand ClearDataCommand
    {
        get;
        set;
    }

    public RelayCommand PrintApplicantCommand
    {
        get;
        set;
    }

    public RelayCommand CloseCommand
    {
        get;
        set;
    }

    public Applicant applicants;





    public ApplicationViewModel()
    {
        applicants = new Applicant();
        PrintApplicantCommand = new RelayCommand(PrintApplicant);
        ClearDataCommand = new RelayCommand(ClearApplicantData);
        CloseCommand = new RelayCommand(CloseApp);
    }

    public ObservableCollection<Applicant> Applicants
    {
        get;
        set;
    }
}

}