Question

我正在使用RelayCommand构建我的第一个WPF程序并点击按钮以传递＆＃34;用户＆＃34; object作为编辑或创建用户的参数。现有用户传递给onClick罚款，但在新用户的空行上输入的信息始终具有NULL属性。但是，检查对象是否为NULL的测试始终指示对象不是。对象如何不是NULL但是在窗口中输入的内容是？任何人都可以发现新对象上的绑定有问题吗？非常感谢提前！

UserViewModel类：

public class UserViewModel : INotifyPropertyChanged
{
    private string _FirstName;
    public string FirstName
    {
        get { return _FirstName; }
        set { _FirstName = value; NotifyPropertyChanged("FirstName"); }
    }

    private string _LastName;
    public string LastName
    {
        get { return _LastName; }
        set { _LastName = value; NotifyPropertyChanged("LastName"); }
    }

    private string _EMail;
    public string EMail
    {
        get { return _EMail; }
        set { _EMail = value; NotifyPropertyChanged("EMail"); }
    }

    private int _UserID;
    public int UserID
    {
        get { return _UserID; }
        set { _UserID = value; NotifyPropertyChanged("UserID"); }
    }

    private string _Position;
    public string Position
    {
        get { return _Position; }
        set { _Position = value; NotifyPropertyChanged("Position"); }
    }

    private DateTime? _EndDate;
    public DateTime? EndDate
    {
        get { return _EndDate; }
        set { _EndDate = value; NotifyPropertyChanged("EndDate"); }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged(String info)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
    }
}

UsersViewModel类：

public partial class UsersViewModel : INotifyPropertyChanged
{
    public RelayCommand<object> editButton_Click_Command { get; set; }

    public UsersViewModel()
    {
        editButton_Click_Command = new RelayCommand<object>(OneditButton_Click_Command);
    }

    private ObservableCollection<UserViewModel> _Users;
    public ObservableCollection<UserViewModel> Users
    {
        get { return _Users; }
        set { _Users = value; NotifyPropertyChanged("Users"); }
    }

    private void OneditButton_Click_Command(object obj)
    {
        var associatedViewModel = obj as UserViewModel;
        string lastName = associatedViewModel.LastName; //Always NULL on new row entered in Window!!!
        if (associatedViewModel == null)
        {
            //Never reached
        }
        if (obj == null)
        {
            //Never reached
        } 
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged(String info)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
    }
}

RelayCommand：

public class RelayCommand<T> : ICommand
{
    #region Fields

    private readonly Action<T> _execute = null;
    private readonly Predicate<T> _canExecute = null;

    #endregion

    #region Constructors

    /// <summary>
    /// Creates a new command that can always execute.
    /// </summary>
    /// <param name="execute">The execution logic.</param>
    public RelayCommand(Action<T> execute)
        : this(execute, null)
    {
    }

    /// <summary>
    /// Creates a new command with conditional execution.
    /// </summary>
    /// <param name="execute">The execution logic.</param>
    /// <param name="canExecute">The execution status logic.</param>
    public RelayCommand(Action<T> execute, Predicate<T> canExecute)
    {
        if (execute == null)
            throw new ArgumentNullException("execute");

        _execute = execute;
        _canExecute = canExecute;
    }

    #endregion

    #region ICommand Members

    public bool CanExecute(object parameter)
    {
        return _canExecute == null ? true : _canExecute((T)parameter);
    }

    public event EventHandler CanExecuteChanged
    {
        add
        {
            if (_canExecute != null)
                CommandManager.RequerySuggested += value;
        }
        remove
        {
            if (_canExecute != null)
                CommandManager.RequerySuggested -= value;
        }
    }

    public void Execute(object parameter)
    {
        try
        {
            _execute((T)parameter);
        }
        catch
        {
            System.Windows.MessageBox.Show("Please enter values for the new entry.");
        }
    }

    #endregion
}

从MainWindow打开UsersWindow：

UsersViewModel Usersvm = new UsersViewModel();
Usersvm.Users = new ObservableCollection<UserViewModel>();
Usersvm.Users.Add(new UserViewModel() { FirstName = "John", LastName = "Doe", EMail = "JohnDoe@yahoo.com", EndDate = new DateTime(2016, 2, 1), Position = "Developer", UserID = 1 });
new UsersWindow(Usersvm).Show();

UsersWindow.xaml.cs：

public partial class UsersWindow : Window
{
    public UsersWindow(UsersViewModel uvm)
    {
        InitializeComponent();
        DataContext = uvm;
    }
}

最后是XAML：

<Window x:Name="Base_V"
    x:Class="DbEntities.UsersWindow"
    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:DbEntities"
    xmlns:ViewModels="clr-namespace:DbEntities"
    xmlns:staticData="clr-namespace:DbEntities"
    mc:Ignorable="d"
    Title="UsersWindow" Height="Auto" Width="900">
    <Window.Resources>
        <staticData:PositionsList x:Key="PositionsList" />
    </Window.Resources>
    <Window.DataContext>
        <ViewModels:UsersViewModel/>
    </Window.DataContext>
    <Grid>
        <DataGrid Name="DataGrid1" ItemsSource="{Binding Users}" HorizontalAlignment="Stretch" HorizontalContentAlignment="Stretch"
              ColumnWidth="*" AutoGenerateColumns="False">
            <DataGrid.Columns>
                <DataGridTemplateColumn>
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <Button Command="{Binding DataContext.editButton_Click_Command, ElementName=Base_V}" CommandParameter="{Binding}" >Edit</Button>
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                </DataGridTemplateColumn>
                <DataGridTextColumn Header="User ID" Binding="{Binding UserID}" IsReadOnly="True" />
                <DataGridTextColumn Header="Last Name" Binding="{Binding LastName}" />
                <DataGridTextColumn Header="First Name" Binding="{Binding FirstName}" />
                <DataGridTextColumn Header="E-Mail" Binding="{Binding EMail}" />
                <DataGridTemplateColumn>
                    <DataGridTemplateColumn.HeaderTemplate>
                        <DataTemplate>
                            <Label Content="Position" />
                        </DataTemplate>
                    </DataGridTemplateColumn.HeaderTemplate>
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <ComboBox ItemsSource="{StaticResource PositionsList}" SelectedItem="{Binding Position}" />
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                </DataGridTemplateColumn>
                <DataGridTextColumn Header="End Date" Binding="{Binding EndDate, StringFormat={}{0:MM/dd/yyyy}}" />
            </DataGrid.Columns>
        </DataGrid>
    </Grid>
</Window>

Answer 1

颠倒你正在做的事情的顺序。首先检查以确定obj是否为null。如果（且仅当） not null ，则将其转换（强制转换）为associatedViewModel。然后检查associatedViewModel是否为null。然后，如果（并且仅当）associatedViewModel not null ，您可以使用associatedViewModel.LastName。如果其中任何一个为空，则发出错误。

假设存在不应该为null的内容，请将代码减少到重新创建问题所需的最小值。

Answer 2

这里的答案是DataGrid始终将原始值发送到按钮单击。将用户引导到可以通过绑定的TextBox编辑单个对象的新页面解决了这个问题。

检查时WPF对象不为NULL，但属性为NULL

2 个答案: