在WPF中利用ICommand - 命令返回null

Question

我正在研究WPF中的示例ViewModel，並且我正在將視圖中的3個文本框綁定到Id,FirstName和LastName。然後，我在xaml.cs文件中有一個按鈕單擊事件，該文件執行更新示例數據庫表的方法。我正試圖圍繞MVVM和WPF進行包裝。我想取消按鈕單擊事件並將其綁定到Command。我正在使用Linq to Sql進行數據庫連接。

當我調試代碼時，一切都返回null。我在正確的道路上嗎？

謝謝！

這裏是VM的部分（我沒有要發佈的所有代碼。）

public class People 
{ 
    public Guid Id {get; set;} 
    public string FirstName {get; set;} 
    public string LastName {get; set;} 
} 

private People mPeople = null; 
private Guid mId = Guid.empty; 

public People people 
{ 
    get {return mPeople;} 
    set {mPeople = value; OnPropertyChanged("people");} 
} 


public overide void Refresh()  
{  
    Using (DatabaseDataContext dc = new DatabaseDataContext())  
    { 
     mId = CurrentUser.Identity; 

     var query = from person in dc.PeopleTables 
      where person.PId = mId 
      select new People() 
      { 
       Id = person.PId, 
        FirstName = person.FirstName, 
       LastName = person.LastName 
      }; 

     mPeople = query.First(); 
    } 
} 

在這裏的虛擬機就是我用命令

private RelayCommand mUserUpdate = null; 

public ICommand UserUpdate 
{ 
    get 
    { 
     if(mUserUpdate == null) mUserUpdate = new RelayCommand(p => UpdateUser(p)); 
     return mUserUpdate; 
    } 
} 

private void UpdateUser(object parameter) 
{ 
    People pe = parameter as People; 
    UpdateUser(pe.Id, pe.FirstName, pe.LastName); 
} 

public static void UpdateUser(Guid Id, string FirstName, string LastName) 
{ 
    DatabaseDataContext DC = new DatabaseDatacontext(); 

    var User = (from c in DC.GetTable<PeopleTable>() 
       where c.PId = mId 
      select c).SingleOrDefault(); 

    if (User == null 
    { 
     //Do my inserts here 
    } 
    else 
    { 
     try 
     { 
      User.Id = Id; 
     User.FirstName = FirstName; 
     User.LastName = LastName; 

     DC.SubmitChanges(); 
     } 
     catch 
     { 
      throw ex; 
     } 
    } 
} 

想這是我的單擊事件我用

private void btnSave_Click(object sender, RoutedEventArgs e) 
{ 
    UserViewModel.UpdateUser(txtId.Text, txtFirstName.Text, txtLastName.Text); 
} 

---

是啊，我拿出click事件，並與

<Button Command="{Binding Path=UserUpdate}"/> 

我綁定文本框這樣

<TextBox Width="250" Text="{Binding Path=Id}"/> 
</StackPanel> 
<StackPanel Orientation="Horizontal" Margin="0,5,0,0"> 
<Label Content="First Name:" Margin="0,0,4,0"/> 
<TextBox Width="250" Text="{Binding Path=FirstName}"/> 
</StackPanel> 
<StackPanel Orientation="Horizontal" Margin="0,5,0,0"> 
<Label Content="Last Name:" Margin="35,0,4,0"/> 
<TextBox Width="250" Text="{Binding Path=LastName}"/> 

Answer 1

在UpdateUser兩個方法

Private void UpdateUser(object obj) 
{ 
    UpdateUser(Id, FirstName, LastName); 
} 

Answer 2

命令旨在replace按鈕的Click事件處理程序取代了它。

您應該刪除的事件處理程序，並做

<Button Command="{Binding UserUpdate}"/> 

Answer 3

您可能需要設置的命令參數：

<Button Command="{Binding UserUpdate}" CommandParameter="{Binding SelectedUser}"/> 

如果「SelectedUser」來自你的虛擬機，而不是當前的UI你不不需要參數。只需獲取要在虛擬機中修改的用戶實例的引用即可。

Answer 4

因爲它代表着你的代碼不工作，因爲你期待一個Person類型的參數被傳遞的命令，這是不可能的（我認爲）。

您的視圖模型需要看起來像這樣：

public class PersonViewModel : *INotifyPropertyChanged base class here* 
{ 
    public PersonViewModel() 
    { 
     UpdatePersonCommand = new RelayCommand(UpdateUser); 
    } 

    public Guid Id {get{ return _id;} { set _id = Id; RaisePropertyChanged("Id"); } 
    public string FirstName { ... same } 
    public string LastName { ... same } 

    public ICommand UpdatePersonCommand {get; private set;} 

    private void UpdateUser() 
    { 
     UpdateUser(Id, FirstName, LastName); 
    } 

    private void UpdateUser(Guid Id, string FirstName, string LastName) 
    { 
     DatabaseDataContext DC = new DatabaseDatacontext(); 
     ... 
    } 

}