So my situation is this: I want to be able to use MVVM with my WPF application using MongoDB. I am very new to MVVM (I know very little of it), but I've got some experience using .NET and WPF.
I have a namespace for recalling MongoDB collections, with the Model component stored there as a class called "User"
Model (in a separate namespace):
public class User
{
[BsonElement("_id")]
public ObjectId Id { get; set; }
public string name { get; set; }
// other methods listed here
public async static Task<List<User>> getUserList()
{
// allows me to get a list of users
var col = MongoDBServer<User>.openMongoDB("Users");
var filter = Builders<User>.Filter.Exists("name");
List<User> userList = await col.Find(filter).ToListAsync();
return userList;
}
}
I've created a very basic ViewModelBase (abstract ViewModelBase):
public abstract class ViewModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = this.PropertyChanged;
if(handler == null)
{
var e = new PropertyChangedEventArgs(propertyName);
handler(this, e);
}
}
}
And a derived class for handling the User Lists (ViewModel):
public class UserListViewModel : ViewModelBase
{
private User _user;
private ObservableCollection<User> _userList;
public User user
{
get { return _user; }
set
{
_user = value;
OnPropertyChanged("user");
}
}
public ObservableCollection<User> userList
{
get { return _userList; }
set
{
_userList = value;
OnPropertyChanged("userList");
}
}
public UserListViewModel()
{
user = new User();
this.userList = new ObservableCollection<User>();
// since MongoDB operations are asyncrhonous, the async method "getUserList()" is used to fill the observable collection
getUserList().Wait();
}
public async Task getUserList()
{
var UserListRaw = await User.getUserList();
this.userList = new ObservableCollection<User>(UserListRaw);
}
}
The view component is as a simple window with a listbox, as follows (View):
<Window x:Class="UserManagementMVVM.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:UserManagementMVVM"
mc:Ignorable="d"
Title="UsersWindow" Height="300" Width="300">
<Window.Resources>
<local:UserListViewModel x:Key="ViewModel"/>
<!-- Receiving error for this XAML block saying "Object reference not set to instance of an object -->
</Window.Resources>
<Grid DataContext="{Binding ViewModel}">
<ListBox Margin="5" ItemsSource="{Binding userList}"/>
</Grid>
</Window>
The App.Xaml and its codebehind are left untouched, as is the View's codebehind.
When I run the program, nothing shows up (ie: The Window starts, but the ListBox is empty even though there is data). I will soon add some button functionality that will perform atomic operations with MongoDB.
I've been trying for nearly 2 weeks to make my own MVVM program for this, with no success. Any assistance would be greatly appreciated.
You are not putting the getUserList() return value into a variable
I assume you mean to do the following
Task.Run(async ()=>this.userList = await getUserList());
this shall work you should think wether you want to wait for the task to finish or not, and than place a .Wait()
after it.
Your other issue might be the way you bind to the ViewModel in the context it should use StaticResource instead of binding
like This:
<Grid DataContext="{StaticResource ViewModel}">
<Window x:Class="UserManagementMVVM.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:UserManagementMVVM"
mc:Ignorable="d"
Title="UsersWindow" Height="300" Width="300">
<Window.DataContext>
<!--You have to set the DataContext -->
<local:UserListViewModel x:Key="ViewModel"/>
</Window.DataContext>
<Grid>
<ListBox Margin="5" ItemsSource="{Binding userList}"/>
</Grid>
</Window>
You have to set the DataContext right. i changed your xaml. but i prefer setting the DataContext for the Mainwindow in Codebehind or app.xaml.cs.
eg: app.xaml.cs
protected override void OnStartup(StartupEventArgs e)
{
var data = new MainWindowViewmodel();
this.MainWindow = new MainWindow(data);
this.MainWindow.Show();
}
all other DataContext for my views are done with DataTemplates within the ResourceDictionary
<DataTemplate DataType="{x:Type local:MyOtherViewmodel}">
<local::MyOtherViewmodelView />
</DataTemplate>
I want to give credit to both gilMishal and blindmeis for pointing me in the right direction. Both of your answers have helped. Here is my updated (and functional!) code:
App.xaml.cs has been modified as follows (Credit to blindmeis ):
public partial class App : Application
{
protected override void OnStartup(StartupEventArgs e)
{
base.OnStartup(e);
UsersWindow window = new UsersWindow();
var ViewModel = new UserListViewModel();
window.DataContext = ViewModel;
window.Show();
}
}
The ViewModel has been updated:
public class UserListViewModel : ViewModelBase
{
private User _user;
private ObservableCollection<string> _userList; // changed from "User" class to string
public User user
{
get { return _user; }
set
{
_user = value;
OnPropertyChanged("user");
}
}
public ObservableCollection<string> userList
{
get { return _userList; }
set
{
_userList = value;
OnPropertyChanged("userList");
}
}
public UserListViewModel()
{
userList = new ObservableCollection<string>();
Task.Run(async () => this.userList = await getUserList()); // Credit to gilMishal
}
public async Task<ObservableCollection<string>> getUserList()
{
var UserListRaw = await User.getUserList();
var userListOC = new ObservableCollection<string>();
foreach (var doc in UserListRaw) // extracting the "name" property from each "User" object
{
userListOC.Add(doc.name);
}
return userListOC;
}
}
And the view:
<Window x:Class="UserManagementMVVM.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:UserManagementMVVM"
mc:Ignorable="d"
Title="UsersWindow" Height="300" Width="300">
<Window.Resources>
<local:UserListViewModel x:Key="ViewModel"/>
</Window.Resources>
<Grid> <!-- data context removed from here, credit blindmeis -->
<ListBox Margin="5" ItemsSource="{Binding userList}"/>
</Grid>
</Window>
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