Develop Reference

"Long" delay causes my view's button not to re-enable

I have a listview and a button in each column. When the user clicks a button, it triggers an asynchronous action in the viewmodel where I disable all buttons and do a big action. Once the action is completed, I re-enable them.
If the action takes too long, though, the buttons don't automatically get re-enabled, even though I'm setting the bound property to true and am notifying the view. If the user does ANY GUI action after the action is complete, the buttons will re-enable.
The other weird thing: If I do an await Task.Delay instead of doing Thread.Sleep (NB: I'm doing real work in the full application), it works correctly.
What's going on here?
I've simplified the code here by eliminating the model (all logic lives in the VM).
View-model code:
namespace WpfTestApp
{
public class viewmodel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
public RelayAsyncCommand<object> RunCommand { get; private set; }
private ObservableCollection<subVM> _subVMs;
public ObservableCollection<subVM> SubVMs
{
get => _subVMs; set
{
_subVMs = value;
NotifyPropertyChanged();
}
}
public viewmodel()
{
RunCommand = new RelayAsyncCommand<object>(OnRun);
SubVMs = new ObservableCollection<subVM>
{
new subVM("ItemA"),
new subVM("ItemB"),
};
}
private async void OnRun(object o)
{
subVM vm = o as subVM;
if (vm != null)
{
ChangeRunMode(false);
Thread.Sleep(500);
}
ChangeRunMode(true);
}
private void ChangeRunMode(bool on)
{
foreach (subVM vm in SubVMs)
{
vm.ButtonEnabled = on;
}
}
}
public class subVM : INotifyPropertyChanged
{
private string name = "";
public string Name
{
get => name;
set
{
if (value != name)
{
name = value;
}
}
}
public subVM(string name)
{
Name = name;
}
private bool tsk = true;
public bool ButtonEnabled
{
get => tsk;
set
{
if (tsk != value)
{
tsk = value;
NotifyPropertyChanged("ButtonEnabled");
}
}
}
public event PropertyChangedEventHandler PropertyChanged;
public void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
}
View XAML:
<Window x:Class="WpfTestApp.MainWindow"
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:WpfTestApp"
mc:Ignorable="d"
Title="MainWindow" Height="130" Width="350">
<Window.DataContext>
<local:viewmodel/>
</Window.DataContext>
<ListView Margin="5"
BorderBrush="DarkSlateGray" BorderThickness="1"
ScrollViewer.HorizontalScrollBarVisibility="Disabled"
ItemsSource="{Binding SubVMs}">
<ListView.View>
<GridView>
<GridViewColumn Header="Name"
Width="200" DisplayMemberBinding ="{Binding Name}"/>
<GridViewColumn>
<GridViewColumn.CellTemplate>
<DataTemplate>
<Button
Content="Load"
IsEnabled="{Binding ButtonEnabled, Mode=OneWay, UpdateSourceTrigger=PropertyChanged}"
Margin="0"
VerticalAlignment="Center"
Command="{Binding Path=DataContext.RunCommand, IsAsync=True, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ItemsControl}}}"
CommandParameter="{Binding}"
/>
</DataTemplate>
</GridViewColumn.CellTemplate>
</GridViewColumn>
</GridView>
</ListView.View>
</ListView>
</Window>
Things I've tried:
I tried await Task.Run on my actual problem, and it still happens.
RelayCommand and the Async version (I thought these were standard boilerplate, but here you go):
public class RelayAsyncCommand<T> : RelayCommand<T>
{
private bool isExecuting = false;
public event EventHandler Started;
public event EventHandler Ended;
public bool IsExecuting
{
get { return this.isExecuting; }
}
public RelayAsyncCommand(Action<T> execute, Predicate<T> canExecute)
: base(execute, canExecute)
{
}
public RelayAsyncCommand(Action<T> execute)
: base(execute)
{
}
public override Boolean CanExecute(Object parameter)
{
return ((base.CanExecute(parameter)) && (!this.isExecuting));
}
public override void Execute(object parameter)
{
try
{
this.isExecuting = true;
if (this.Started != null)
{
this.Started(this, EventArgs.Empty);
}
Task task = Task.Factory.StartNew(() =>
{
this._execute((T)parameter);
});
task.ContinueWith(t =>
{
this.OnRunWorkerCompleted(EventArgs.Empty);
}, TaskScheduler.FromCurrentSynchronizationContext());
}
catch (Exception ex)
{
this.OnRunWorkerCompleted(new RunWorkerCompletedEventArgs(null, ex, true));
}
}
private void OnRunWorkerCompleted(EventArgs e)
{
this.isExecuting = false;
if (this.Ended != null)
{
this.Ended(this, e);
}
}
}
public class RelayCommand<T> : ICommand
{
#region Fields
readonly protected Action<T> _execute;
readonly protected Predicate<T> _canExecute;
#endregion // Fields
#region Constructors
public RelayCommand(Action<T> execute)
: this(execute, null)
{
}
public RelayCommand(Action<T> execute, Predicate<T> canExecute)
{
if (execute == null)
throw new ArgumentNullException("execute");
_execute = execute;
_canExecute = canExecute;
}
#endregion // Constructors
#region ICommand Members
[DebuggerStepThrough]
public virtual bool CanExecute(object parameter)
{
return _canExecute == null ? true : _canExecute((T)parameter);
}
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
public virtual void Execute(object parameter)
{
_execute((T)parameter);
}
#endregion // ICommand Members
}

inside the OnRun method you are actually locking the UI thread, preventing any UI update/refresh; you should await for the long operation (and that's exactly what you noticed doing await Task.Delay):
private async void OnRun(object o)
{
subVM vm = o as subVM;
if (vm != null)
{
ChangeRunMode(false);
await Task.Run(() =>
{
//put here your long operation as per your example
for (int i = 0; i < 500; i++)
{
for (int k = 0; k < 100000; k++) ;
}
});
}
ChangeRunMode(true);
}
indeed if you take a closer look to your window, you'll see that currently everything is freezed during the long operation execution, not just only the buttons being disabled.

I figured out my problem:
The button deactivating isn't coming from my binding when I'm running asynchronously. It's coming from the RelayAsyncCommand's CanExecute. CanExecute here returns false while the task is running, but we don't trigger a requery ever when it's done.
Easily fixed by adding a private set to the IsExecuting property, which calls the invalidate/requery function on change (just like the standard notifypropertychanged pattern). For posterity, here's the full fixed RelayAsyncCommand:
public class RelayAsyncCommand<T> : RelayCommand<T>
{
private bool _isExecuting = false;
public event EventHandler Started;
public event EventHandler Ended;
public bool IsExecuting
{
get { return _isExecuting; }
private set
{
if (value != _isExecuting)
{
_isExecuting = value;
CommandManager.InvalidateRequerySuggested();
}
}
}
public RelayAsyncCommand(Action<T> execute, Predicate<T> canExecute)
: base(execute, canExecute)
{
}
public RelayAsyncCommand(Action<T> execute)
: base(execute)
{
}
public override bool CanExecute(object parameter)
{
return ((base.CanExecute(parameter)) && (!IsExecuting));
}
public override void Execute(object parameter)
{
try
{
IsExecuting = true;
Started?.Invoke(this, EventArgs.Empty);
Task task = Task.Factory.StartNew(() =>
{
_execute((T)parameter);
});
task.ContinueWith(t =>
{
OnRunWorkerCompleted(EventArgs.Empty);
}, TaskScheduler.FromCurrentSynchronizationContext());
}
catch (Exception ex)
{
OnRunWorkerCompleted(new RunWorkerCompletedEventArgs(null, ex, true));
}
}
private void OnRunWorkerCompleted(EventArgs e)
{
IsExecuting = false;
Ended?.Invoke(this, e);
}
}
Thanks Peter for making me actually look at (what I thought was) boilerplate code.

Binding ContentControl to the ApplicationViewModel that will determine which user control to view?

I am very new to WPF and relatively new to C# programming (programming in general), and I'm trying to develop a WPF application.
I have tried to go through several posts similar to this, but I can't seem to find the answer of why this is not working.
So, I'm having a hard time understanding the MVVM architecture, how and what it requires to switch between multiple user controls binded to a single <ContentControl />.
From what I understand and read so far, is that I have to bind the view model like this:
<ContentControl Content="{Binding ApplicationViewModel}"/>
So here is what I want to a achieve:
An ApplicationWindow.xaml with sidebar menu on the left side that will be shown at all times when the application is running, and a <ContentControl/> on the remaining space. Buttons shown on the sidebar menu will be:
Main (will show MainView.xaml User Control, should be the default User Control)
Settings (will show SettingsView.xaml User Control)
Exit (will close the application)
I understand that I need to bind the buttons to ICommand commands, and I understand the concept of a RelayCommand.cs class.
So let's jump into the simplified code of my idea and figure out what I need to understand and what I may have misunderstood in the process.
What MainView.xaml and SettingsView.xaml contain are not important right now, as I'm just trying to figure out how to show them in my application.
Here's the ApplicationWindow.xaml:
<Window x:Class="WpfApp1.ApplicationWindow"
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:WpfApp1"
xmlns:v="clr-namespace:WpfApp1.View"
xmlns:vm="clr-namespace:WpfApp1.ViewModel"
mc:Ignorable="d"
Title="ApplicationWindow" Height="1080" Width="1920"
WindowStyle="None" WindowState="Maximized">
<Window.Resources>
<DataTemplate DataType="{x:Type vm:MainViewModel}">
<v:MainView/>
</DataTemplate>
<DataTemplate DataType="{x:Type vm:SettingsViewModel}">
<v:SettingsView/>
</DataTemplate>
</Window.Resources>
<DockPanel>
<!--Menu bar on the left-->
<Border DockPanel.Dock="Left">
<StackPanel Orientation="Vertical" Background="Gray" Width="120">
<Button Content="Main" Command="{Binding ShowMainCommand}"/>
<Button Content="Settings" Command="{Binding ShowSettingsCommand}"/>
<Button Content="Exit" Command="{Binding ExitApplicationCommand}"/>
</StackPanel>
</Border>
<!--The content control that view the current view-->
<ContentControl Content="{Binding ApplicationViewModel}"/>
</DockPanel>
</Window>
Note: DataContext is set to ApplicationViewModel.cs in App.xaml.cs by overriding the OnStartup() method.
public partial class App : Application
{
protected override void OnStartup(StartupEventArgs e)
{
base.OnStartup(e);
ApplicationWindow app = new ApplicationWindow
{
DataContext = new ApplicationViewModel()
};
app.Show();
}
}
Here's the ApplicationViewModel.cs:
public class ApplicationViewModel : ViewModelBase
{
#region Fields
private List<ViewModelBase> _viewModels;
private ViewModelBase _currentViewModel;
private ICommand _showMainCommand;
private ICommand _showSettingsCommand;
private ICommand _exitApplicationCommmand;
#endregion
#region Constructor
public ApplicationViewModel()
{
ViewModels = new List<ViewModelBase>
{
new MainViewModel(),
new SettingsViewModel()
};
CurrentViewModel = ViewModels[0];
}
#endregion
#region Public Properties
public List<ViewModelBase> ViewModels
{
get
{
return _viewModels;
}
set
{
if (_viewModels != value)
{
_viewModels = value;
OnPropertyChanged(nameof(ViewModels));
}
}
}
public ViewModelBase CurrentViewModel
{
get
{
return _currentViewModel;
}
set
{
if(_currentViewModel != value)
{
_currentViewModel = value;
OnPropertyChanged(nameof(CurrentViewModel));
}
}
}
#endregion
#region Commands
public ICommand ShowMainCommand
{
get
{
if(_showMainCommand == null)
{
_showMainCommand = new RelayCommand(action => ShowMain());
}
return _showMainCommand;
}
}
public ICommand ShowSettingsCommand
{
get
{
if (_showSettingsCommand == null)
{
_showSettingsCommand = new RelayCommand(action => ShowSettings());
}
return _showSettingsCommand;
}
}
public ICommand ExitApplicationCommand
{
get
{
if (_exitApplicationCommmand == null)
{
_exitApplicationCommmand = new RelayCommand(action => ExitApplication());
}
return _exitApplicationCommmand;
}
}
#endregion
#region Private Methods
private void ShowMain()
{
CurrentViewModel = ViewModels[0];
}
private void ShowSettings()
{
CurrentViewModel = ViewModels[1];
}
private void ExitApplication()
{
MessageBoxResult result = MessageBox.Show("Are you sure you want to exit?", "Exit", MessageBoxButton.YesNo);
if (result == MessageBoxResult.Yes)
{
System.Windows.Application.Current.Shutdown();
}
}
#endregion
}
So, from what I understand, the ApplicationWindow.xaml should be able to determine which view to show out from what the CurrentViewModel is set to.
For the sake of information (or miss-information), here are ViewModelBase.cs:
public class ViewModelBase : INotifyPropertyChanged
{
#region INotifyPropertyChanged
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
#endregion
}
And RelayCommand.cs:
public class RelayCommand : ICommand
{
#region Fields
private readonly Action<object> _execute;
private readonly Predicate<object> _canExecute;
#endregion
#region Constructors
public RelayCommand(Action<object> execute, Predicate<object> canExecute)
{
if (execute == null)
throw new ArgumentNullException("execute");
_execute = execute;
_canExecute = canExecute;
}
public RelayCommand(Action<object> execute) : this(execute, null)
{
}
#endregion
#region ICommand
public bool CanExecute(object parameters)
{
return _canExecute == null ? true : _canExecute(parameters);
}
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
public void Execute(object parameters)
{
_execute(parameters);
}
#endregion
}
I hope my thought process on this was clear to you, and that one of you smart programmers out there can help solving this, and help me understand why this isn't turning out as I want it to.
In case of what I'm trying to do is harder than Elon Musk's project on making life multiplanetary, feel free to explain why and suggest me a better way to

Your Content control binding should be pointed at the actual property you change when switching ViewModels
<ContentControl Content="{Binding CurrentViewModel}"/>

I want to enable the a particular column of my dataGrid to edit on button click

I want to disable(close for editing) the 3rd column of my dataGrid(using MVVM-WPF) but want to enable to edit again on click of button outside the Grid.How can I achieve that?
The Challenge I am facing here is how to retrieve dataGrid's property on button click, I am using command pattern out here .This is how my ViewModel looks now, what I am supposed to write in those method to make certain column editable:
public class TicketOverViewViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private TicketDataService ticketDataService;
private ObservableCollection<Ticket> tickets;
public ObservableCollection<Ticket> Tickets
{
get
{
return tickets;
}
set
{
tickets = value;
RaisePropertyChanged("Tickets");
}
}
private void RaisePropertyChanged(string propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
public TicketOverViewViewModel()
{
ticketDataService = new TicketDataService();
LoadData();
LoadCommands();
}
private void LoadData()
{
tickets = ticketDataService.GetAllTickets().ToObservableCollection();
}
public ICommand EditCommand { get; set; }
private void LoadCommands()
{
EditCommand = new CustomCommand(EditTicket, CanEditTicket);
}
private void EditTicket(object obj)
{
//TODO
}
private bool CanEditTicket(object obj)
{
return true;
}
}
This is how my Customcommand looks:
public class CustomCommand : ICommand
{
private Action<object> execute;
private Predicate<object> canExecute;
public CustomCommand(Action<object> execute, Predicate<object> canExecute)
{
this.execute = execute;
this.canExecute = canExecute;
}
public bool CanExecute(object parameter)
{
bool b = canExecute == null ? true : canExecute(parameter);
return b;
}
public event EventHandler CanExecuteChanged
{
add
{
CommandManager.RequerySuggested += value;
}
remove
{
CommandManager.RequerySuggested -= value;
}
}
public void Execute(object parameter)
{
execute(parameter);
}
}
My View:
<Window x:Class="DataGridMVVM.View.TicketOverView"
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:DataGridMVVM.View"
mc:Ignorable="d"
Title="Ticket Overview" Height="348.936" Width="421.277"
DataContext= "{Binding Source={StaticResource mainViewModelLocator}, Path=TicketOverViewViewModel }">
<Grid>
<DataGrid x:Name="dataGrid" HorizontalAlignment="Left" VerticalAlignment="Top"
AutoGenerateColumns="True" ItemsSource="{Binding Tickets}" IsReadOnly="True"
CanUserResizeColumns="True" Height="100" Width="400" />
<Button x:Name="button" Content="Edit" Command="{Binding EditCommand}"
HorizontalAlignment="Left" Margin="239,183,0,0" VerticalAlignment="Top" Width="75" />
</Grid>
</Window>

Swap around the read only attributes on each cell. So start like this
dataGridView1.Rows[row.index].Cells[column.index].ReadOnly= true;
then on button click
dataGridView1.Rows[row.index].Cells[column.index].ReadOnly= false;
you can set the whole row or column like this
dataGridView1.Rows[row.index].ReadOnly= false;
dataGridView1.Columns[columns.index].ReadOnly= false;
or the entire grid
dataGridView1.ReadOnly = false;

Binding ICommand to button?

I am new to the MVVM pattern and things are coming to me ever so slowly, I want to be able to click a button on my form and then it dynamically create a textbox at runtime. I have a 'Add Title' and also 'Add Question' which both add textboxes but at different locations, you can add as many questions under one title. I have Created a class called Standard in this class it holds:
public class Standard
{
string _title;
ObservableCollection<string> _questions;
public event PropertyChangedEventHandler PropertyChanged;
#region NofiftyPropChnage
protected void NotifyOfPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
protected void NotifyOfPropertyChanged<TProperty>(Expression<Func<TProperty>> property)
{
NotifyOfPropertyChanged(property.GetMemberInfo().Name);
}
#endregion
#region Properties
public string Title
{
get { return _title; }
set
{
_title = value;
NotifyOfPropertyChanged(() => Title);
}
}
public ObservableCollection<string> Questions
{
get { return _questions; }
set
{
_questions = value;
NotifyOfPropertyChanged(() => Questions);
}
}
#endregion
}
This class holds a Title property and also a list of Questions property because you can add Questions under a Title.
I also have a ViewModel class which holds:
class ViewModel :INotifyPropertyChanged
{
#region NotifyPropertyChange
public event PropertyChangedEventHandler PropertyChanged;
protected void NotifyOfPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
protected void NotifyOfPropertyChanged<TProperty>(Expression<Func<TProperty>> property)
{
NotifyOfPropertyChanged(property.GetMemberInfo().Name);
}
#endregion
private ObservableCollection<Standard> _standardCollection;
public ObservableCollection<Standard> StandardCollection
{
get
{
return _standardCollection;
}
set
{
_standardCollection = value;
NotifyOfPropertyChanged(() => StandardCollection);
}
}
}
This class holds a list of standards, a standard is when you click save with the text boxes and information in the text boxes done. It saves as a Standard
Finally my XAML code:
<Grid>
<button Content="Add Title"/>
<button Content="Add Question"/>
<StackPanel>
<ItemsControl ItemsSource="{Binding StandardCollection}">
<ItemsControl.ItemTemplate>
<DataTemplate DataType="{x:Type local:Standard}">
<Grid>
<TextBox Text="{Binding Title}"/>
<ItemsControl ItemsSource="{Binding Questions}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<TextBox Text="{Binding Questions}"/>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
</Grid>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
</StackPanel>
</Grid>
Everything runs and there are no errors but when I click 'Add Title' or 'Add Question' no textbox appears, any help?

Ok, I'll have another shot at this one. I've stripped out the Title part and just concentrated on the Questions in order to keep this as a minimal example. First you'll need a base class that implements INotifyPropertyChanged for your view models:
public abstract class ObservableObject : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged<T>(Expression<Func<T>> propertyExpresion)
{
var property = (MemberExpression)propertyExpresion.Body;
this.OnPropertyChanged(property.Member.Name);
}
protected void OnPropertyChanged(string propertyName)
{
if (this.PropertyChanged != null)
{
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
}
Next you'll need a class that implements ICommand for your buttons to bind to which causes handlers to get called when those buttons are pressed:
// by Josh Smith, http://msdn.microsoft.com/en-us/magazine/dd419663.aspx
public class RelayCommand : ICommand
{
#region Fields
readonly Action<object> _execute;
readonly Predicate<object> _canExecute;
#endregion // Fields
#region Constructors
public RelayCommand(Action<object> execute)
: this(execute, null)
{
}
public RelayCommand(Action<object> execute, Predicate<object> canExecute)
{
if (execute == null)
throw new ArgumentNullException("execute");
_execute = execute;
_canExecute = canExecute;
}
#endregion // Constructors
#region ICommand Members
public bool CanExecute(object parameter)
{
return _canExecute == null ? true : _canExecute(parameter);
}
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
public void Execute(object parameter)
{
_execute(parameter);
}
#endregion // ICommand Members
}
Those two classes were written by others, if you add MVVM Lite project to your project you'll get them provided for you.
Next we need to create a view model with an ObservableCollection of Questions and a handler that gets called when the user presses the button:
public class MyViewModel : ObservableObject
{
public ICommand AddQuestionCommand {get; private set;}
ObservableCollection<string> _questions = new ObservableCollection<string>();
public ObservableCollection<string> Questions
{
get { return _questions; }
set
{
_questions = value;
OnPropertyChanged(() => Questions);
}
}
public MyViewModel()
{
this.AddQuestionCommand = new RelayCommand(new Action<object>((o) => OnAddQuestion()));
}
private void OnAddQuestion()
{
this.Questions.Add("new item");
}
}
Obviously you'll need to create an instance of this and set it as your window's DataContext. When the command gets triggerd the handler gets called and it in turn adds a new string to the collection. The XAML now needs to bind a button to that command and use the Questions collection to create a list of TextBlocks that display them all:
<StackPanel>
<Button Content="Add Question" Command="{Binding AddQuestionCommand}" HorizontalAlignment="Left"/>
<ItemsControl ItemsSource="{Binding Questions}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<TextBox Text="{Binding .}" Width="200" HorizontalAlignment="Left"/>
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
</StackPanel>
Hopefully this should give you a starting point. If I've missed something or you need clarification on anything then pls post a follow-up and I'll do my best.

Standard needs to implement the INotifyPropertyChanged interface. Generally you shouldn't do this more than once though, just declare one base class that implements that stuff and inherit all your view models from that. Also if you use package manager to add MVVM Lite to your project then you'll get a lot of this stuff provided for you.

I have no idea why these other guys are banging on about the INotifyPropertyChanged interface, as that has so very little to do with ICommand, although it does appear that you have tried to use it without adding it to the Standard class definition.
Either way, it sounds to me like you need to use the RelayCommand, or similar. This is a class that extends the ICommand interface... you can think of it as a delegate command. Instead of defining a separate class for each command, you can simply define the command logic and the canExecute handler inline. Here is a simplified example:
public ICommand SaveCommand
{
get { return new RelayCommand(execute => Save(), canExecute => CanSave()); }
}
...
<Button Content="Save" Command="{Binding SaveCommand}" />
You can find an implementation of it in the RelayCommand.cs page on GitHub and a description of it in the Commands, RelayCommands and EventToCommand page on MDSN Magazine.

You will need to change your code heavily to make it work. Do the following:
Step 1. Add Class RelayCommand:
public class RelayCommand : ICommand
{
public Func<bool> CanExecute { get; set; }
public Action Execute { get; set; }
public RelayCommand()
{
}
public RelayCommand(Action execute)
{
Execute = execute;
}
#region ICommand Members
bool ICommand.CanExecute(object parameter)
{
if (this.CanExecute == null)
{
return true;
}
else
{
return this.CanExecute();
}
}
event EventHandler ICommand.CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
void ICommand.Execute(object parameter)
{
this.Execute();
}
#endregion
}
Step 2. Add Commands in ViewModel
public ICommand AddTitle { get; private set; }
public ICommand AddQuestion { get; private set; }
public ViewModel()
{
_standardCollection = new ObservableCollection<Standard>();
AddTitle = new RelayCommand(OnAddTitle);
AddQuestion = new RelayCommand(OnAddQuestion);
}
void OnAddTitle()
{
_standardCollection.Add(new Standard());
}
void OnAddQuestion()
{
_standardCollection.Last().Questions.Add(new Question("Some Question"));
}
Step 3. Bind buttons
<Button Content="Add Title" Command="{Binding AddTitle}"/>
<Button Content="Add Question" Command="{Binding AddQuestion}"/>
You will also have to fix you layount in XAML.
Since the user can change the question text, you should create a separate class Question.

Try implementing INotifyPropertyChanged on class Standard.
public class Standard : INotifyPropertyChanged
{
string _title;
ObservableCollection<string> _questions;
public event PropertyChangedEventHandler PropertyChanged;
#region NofiftyPropChnage
protected void NotifyOfPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
protected void NotifyOfPropertyChanged<TProperty>(Expression<Func<TProperty>> property)
{
NotifyOfPropertyChanged(property.GetMemberInfo().Name);
}
#endregion
#region Properties
public string Title
{
get { return _title; }
set
{
_title = value;
NotifyOfPropertyChanged(() => Title);
}
}
public ObservableCollection<string> Questions
{
get { return _questions; }
set
{
_questions = value;
NotifyOfPropertyChanged(() => Questions);
}
}
#endregion
}

How do I pass a variable as a CommandParameter

I'm trying to send a variable from the ViewModel as a parameter to a command. The command looks like this:
public class EditPersonCommand : ICommand
{
private bool _CanExecute = false;
public bool CanExecute(object parameter)
{
PersonModel p = parameter as PersonModel;
CanExecuteProperty = (p != null) && (p.Age > 0);
return CanExecuteProperty;
}
public event EventHandler CanExecuteChanged;
public void Execute(object parameter) { }
private bool CanExecuteProperty
{
get { return _CanExecute; }
set
{
if (_CanExecute != value)
{
_CanExecute = value;
EventHandler can_execute = CanExecuteChanged;
if (can_execute != null)
{
can_execute.Invoke(this, EventArgs.Empty);
}
}
}
}
}
The ViewModel looks like this:
public class PersonViewModel : ViewModelBase
{
private PersonModel _PersonModel;
private EditPersonCommand _EditPersonCommand;
///<remarks>
/// must use the parameterless constructor to satisfy <Window.Resources>
///</remarks>
public PersonViewModel()
: this(new PersonModel())
{
}
public PersonViewModel(PersonModel personModel)
{
_PersonModel = personModel;
}
public ICommand EditPersonCommand
{
get
{
if (_EditPersonCommand == null)
{
_EditPersonCommand = new EditPersonCommand();
}
return _EditPersonCommand;
}
}
}
The xaml looks like this:
<Button Content="Edit" HorizontalAlignment="Right" Height="20" Width="80"
Command="{Binding EditPersonCommand}"
CommandParameter="{Binding _PersonModel}" />
I've tried creating a property in the ViewModel instead of using the private local variable name, but that didnt work either. The object parameter always shows null in the call to CanExecute and the button is never enabled. If I change the CommandParameter value to Hello, then I receive Hello in the call to CanExecute, so I'm not sure why the variable doesnt work. Any help would be appreciated.
Update: I've also tried making a public property to the model (which I dont really want to expose the model, but just tried it to see if it works, but it doesnt).
// Added this to the ViewModel
public PersonModel PersonModelProp
{
get
{
return _PersonModel;
}
set
{
_PersonModel = value;
OnPropertyChanged("PersonModelProp");
}
}
And changed the xaml to this:
<Button Content="Edit" HorizontalAlignment="Right" Height="20" Width="80"
Command="{Binding EditPersonCommand}"
CommandParameter="{Binding PersonModelProp}" />
But still no luck. The ViewModel does implement INotifyPropertyChanged

Is the CommandParameter always null or are you only checking the first time it is being executed?
It appears that the order in which you declare your properties matters in this case since setting the Command property causes the CanExecute to fire immediately before the CommandParameter has been set.
Try moving the CommandParameter property before the Command property:
<Button Content="Edit" HorizontalAlignment="Right" Height="20" Width="80"
CommandParameter="{Binding PersonModelProp}"
Command="{Binding EditPersonCommand}" />
Also, see here and here.
Edit
To ensure that your events are being raised properly you should raise the CanExecuteChanged event when the PersonModelProp value changes.
The Command:
public class EditPersonCommand : ICommand
{
public bool CanExecute(object parameter)
{
PersonModel p = parameter as PersonModel;
return p != null && p.Age > 0;
}
public event EventHandler CanExecuteChanged;
public void Execute(object parameter)
{
//command implementation
}
public void RaiseCanExecuteChanged()
{
var handler = CanExecuteChanged;
if(handler != null)
{
handler(this, EventArgs.Empty);
}
}
}
And the view model:
public class PersonViewModel : ViewModelBase
{
private PersonModel _PersonModel;
private EditPersonCommand _EditPersonCommand;
///<remarks>
/// must use the parameterless constructor to satisfy <Window.Resources>
///</remarks>
public PersonViewModel()
: this(new PersonModel())
{
_EditPersonCommand = new EditPersonCommand();
}
public PersonViewModel(PersonModel personModel)
{
_PersonModel = personModel;
}
public ICommand EditPersonCommand
{
get
{
return _EditPersonCommand;
}
}
public PersonModel PersonModelProp
{
get
{
return _PersonModel;
}
set
{
_PersonModel = value;
OnPropertyChanged("PersonModelProp");
EditPersonCommand.RaiseCanExecuteChanged();
}
}
}

Two points to the answer:
First, as #akton mentioned, you can only bind to public properties. It doesn't have to be a DependencyProperty though.
Second, which took me some tome to figure out, is that you have to set the binding for the CommandParameter before the Command property. i.e.
<Button Content="Edit" HorizontalAlignment="Right" Height="20" Width="80"
CommandParameter="{Binding PersonModelProp}"
Command="{Binding EditPersonCommand}" />
Hope this helps :)

_PersonModel is private and so cannot be accessed. Create a public property that exposes it and bind to that in the CommandParameter. Remember to make the property a dependency property (technically not required but it helps) and the ViewModel should implement INotifyProperty changed and fire the PropertyChanged event so the binding is updated.

I think you have a problem in your EditPersonCommand (it not fired ok).I check it with relayCommand and it work!
This is the code:
ViewModel:
public class PersonViewModel : ViewModelBase
{
private PersonModel _PersonModel;
private ICommand _EditPersonCommand;
///<remarks>
/// must use the parameterless constructor to satisfy <Window.Resources>
///</remarks>
public PersonViewModel()
: this(new PersonModel())
{
}
public PersonViewModel(PersonModel personModel)
{
PersonModelProp = personModel;
}
public ICommand EditPersonCommand
{
get
{
if (_EditPersonCommand == null)
{
_EditPersonCommand = new RelayCommand(ExecuteEditPerson,CanExecuteEditPerson);
}
return _EditPersonCommand;
}
}
private bool CanExecuteEditPerson(object parameter)
{
PersonModel p = parameter as PersonModel;
return (p != null) && (p.Age > 0);
}
private void ExecuteEditPerson(object o)
{
}
public PersonModel PersonModelProp
{
get
{
return _PersonModel;
}
set
{
_PersonModel = value;
NotifyPropertyChanged("PersonModelProp");
}
}
}
And this RelayCommand (Fire events ok!)
public class RelayCommand : ICommand
{
#region Constants and Fields
private readonly Predicate<object> canExecute;
private readonly Action<object> execute;
#endregion
#region Constructors and Destructors
public RelayCommand(Action<object> execute)
: this(execute, null)
{
}
public RelayCommand(Action<object> execute, Predicate<object> canExecute)
{
if (execute == null)
{
throw new ArgumentNullException("execute");
}
this.execute = execute;
this.canExecute = canExecute;
}
#endregion
#region Events
public event EventHandler CanExecuteChanged
{
add
{
CommandManager.RequerySuggested += value;
}
remove
{
CommandManager.RequerySuggested -= value;
}
}
#endregion
#region Implemented Interfaces
#region ICommand
[DebuggerStepThrough]
public bool CanExecute(object parameter)
{
return this.canExecute == null || this.canExecute(parameter);
}
public void Execute(object parameter)
{
this.execute(parameter);
}
#endregion
#endregion
}
Xmal:
<Button Content="Edit" HorizontalAlignment="Right" Height="20" Width="80"
CommandParameter="{Binding PersonModelProp}"
Command="{Binding EditPersonCommand}" />