Johnathon Sullinger Johnathon Sullinger - 3 months ago 27
C# Question

WPF two-way binding not working

I have a data context (

UserPreferences
) assigned to my main window, and a textbox that binds two-way to a property within one of the data context's properties (
CollectionDevice
) within the context.

When the Window loads, the textbox's do not bind to the properties in my model. I verify within the debugger that the data context is set to the model object and the model's properties are properly assigned. All I get however are a series of textbox's with 0's in them.

When I enter the data into the textbox's, the data is updated in the model. The issue just happens when I load the data and apply it to the data context, the text box does not get updated.

When I save the model to the database, the proper data gets saved from the textbox. When I restore the model from the database, the proper data gets applied. When the model is applied to the data context within my constructor, the textbox's datacontext contains the correct data and it's properties are assigned as they should be. The issue is the UI does not reflect this.

XAML

<Window.DataContext>
<models:UserPreferences />
</Window.DataContext>

<!-- Wrap pannel used to store the manual settings for a collection device. -->
<StackPanel Name="OtherCollectionDevicePanel">
<StackPanel Orientation="Horizontal">
<TextBlock VerticalAlignment="Center" Margin="10, 10, 0, 0" Text="Baud Rate" />
<TextBox Name="BaudRateTextBox" Text="{Binding Path=SelectedCollectionDevice.BaudRate, Mode=TwoWay}" Margin="10, 10, 0, 0" MinWidth="80" ></TextBox>
</StackPanel>
<WrapPanel>
<TextBlock VerticalAlignment="Center" Margin="10, 10, 0, 0" Text="Com Port" />
<TextBox Text="{Binding Path=SelectedCollectionDevice.ComPort, Mode=TwoWay}" Margin="10, 10, 0, 0" MinWidth="80" ></TextBox>
</WrapPanel>
<WrapPanel>
<TextBlock VerticalAlignment="Center" Margin="10, 10, 0, 0" Text="Data Points" />
<TextBox Text="{Binding Path=SelectedCollectionDevice.DataPoints, Mode=TwoWay}" Margin="10, 10, 0, 0" MinWidth="80" ></TextBox>
</WrapPanel>
<WrapPanel Orientation="Horizontal">
<TextBlock VerticalAlignment="Center" Margin="10, 10, 0, 0" Text="WAAS" />
<CheckBox IsChecked="{Binding Path=SelectedCollectionDevice.WAAS, Mode=TwoWay}" Content="Enabled" Margin="20, 0, 0, 0" VerticalAlignment="Bottom"></CheckBox>
</WrapPanel>
</StackPanel>


Model <-- Datacontext.

/// <summary>
/// Provides a series of user preferences.
/// </summary>
[Serializable]
public class UserPreferences : INotifyPropertyChanged
{
private CollectionDevice selectedCollectionDevice;

public UserPreferences()
{
this.AvailableCollectionDevices = new List<CollectionDevice>();

var yuma1 = new CollectionDevice
{
BaudRate = 4800,
ComPort = 31,
DataPoints = 1,
Name = "Trimble Yuma 1",
WAAS = true
};

var yuma2 = new CollectionDevice
{
BaudRate = 4800,
ComPort = 3,
DataPoints = 1,
Name = "Trimble Yuma 2",
WAAS = true
};

var toughbook = new CollectionDevice
{
BaudRate = 4800,
ComPort = 3,
DataPoints = 1,
Name = "Panasonic Toughbook",
WAAS = true
};


var other = new CollectionDevice
{
BaudRate = 0,
ComPort = 0,
DataPoints = 0,
Name = "Other",
WAAS = false
};

this.AvailableCollectionDevices.Add(yuma1);
this.AvailableCollectionDevices.Add(yuma2);
this.AvailableCollectionDevices.Add(toughbook);
this.AvailableCollectionDevices.Add(other);

this.SelectedCollectionDevice = this.AvailableCollectionDevices.First();
}

/// <summary>
/// Gets or sets the GPS collection device.
/// </summary>
public CollectionDevice SelectedCollectionDevice
{
get
{
return selectedCollectionDevice;
}
set
{
selectedCollectionDevice = value;
this.OnPropertyChanged("SelectedCollectionDevice");
}
}

/// <summary>
/// Gets or sets a collection of devices that can be used for collecting GPS data.
/// </summary>
[Ignore]
[XmlIgnore]
public List<CollectionDevice> AvailableCollectionDevices { get; set; }

public event PropertyChangedEventHandler PropertyChanged;

/// <summary>
/// Notifies objects registered to receive this event that a property value has changed.
/// </summary>
/// <param name="propertyName">The name of the property that was changed.</param>
protected virtual void OnPropertyChanged(string propertyName)
{
if (this.PropertyChanged != null)
{
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}


CollectionDevice <-- Where text box binds to.

/// <summary>
/// CollectionDevice model
/// </summary>
[Serializable]
public class CollectionDevice : INotifyPropertyChanged
{
/// <summary>
/// Gets or sets the COM port.
/// </summary>
private int comPort;

/// <summary>
/// Gets or sets a value indicating whether [waas].
/// </summary>
private bool waas;

/// <summary>
/// Gets or sets the data points.
/// </summary>
private int dataPoints;

/// <summary>
/// Gets or sets the baud rate.
/// </summary>
private int baudRate;

/// <summary>
/// Gets or sets the name.
/// </summary>
public string Name { get; set; }

/// <summary>
/// Gets or sets the COM port.
/// </summary>
public int ComPort
{
get
{
return this.comPort;
}

set
{
this.comPort= value;
this.OnPropertyChanged("ComPort");
}
}

/// <summary>
/// Gets or sets the COM port.
/// </summary>
public bool WAAS
{
get
{
return this.waas;
}

set
{
this.waas = value;
this.OnPropertyChanged("WAAS");
}
}

/// <summary>
/// Gets or sets the COM port.
/// </summary>
public int DataPoints
{
get
{
return this.dataPoints;
}

set
{
this.dataPoints = value;
this.OnPropertyChanged("DataPoints");
}
}

/// <summary>
/// Gets or sets the COM port.
/// </summary>
public int BaudRate
{
get
{
return this.baudRate;
}

set
{
this.baudRate = value;
this.OnPropertyChanged("BaudRate");
}
}

public event PropertyChangedEventHandler PropertyChanged;

/// <summary>
/// Notifies objects registered to receive this event that a property value has changed.
/// </summary>
/// <param name="propertyName">The name of the property that was changed.</param>
protected virtual void OnPropertyChanged(string propertyName)
{
if (this.PropertyChanged != null)
{
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}

public override string ToString()
{
return this.Name;
}
}


Can someone point me in the right direction? I assume the issue is my binding in XAML; I can't find it though. I need it to be two-way bound because the data can change at any time during the apps lifetime within the model (database is updated through syncs) and the UI needs to reflect those changes, yet the user can apply changes to the model via the UI.

Update 1

I tried to force the text box databind to be updated, but that did not work as well.

BindingExpression be = this.BaudRateTextBox.GetBindingExpression(TextBox.TextProperty);
be.UpdateSource();


I also tried setting the
UpdateSourceTrigger
to
PropertyChanged
and that did not seem to resolve the problem either.

<TextBox Name="BaudRateTextBox" Text="{Binding Path=SelectedCollectionDevice.BaudRate, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Margin="10, 10, 0, 0" MinWidth="80" ></TextBox>


Update 2

I tried to follow along with some documentation from Microsoft and it does not seem to fix the issue. The values still remain 0 when the window loads. The binding is not being updated after I restore the state of the object from the database. Binding is wired up though because as I enter data, the data context is updated. For some reason, it's acting like One-Way when I have it set to Two-Way.

Update 3

I tried to move the code into the window loaded event and out of the constructor but that did not appear to help. Something I found interesting is that the PropertyChanged event does not get fired during the deserialization process. I don't think it matters in this case because the object is fully restored properly and then I just assign it to the data context anyway. I moved the data context out of the XAML and into the WindowLoaded in order to test if the XAML was the problem. The result was the same.

private void WindowLoaded(object sender, RoutedEventArgs e)
{
// Restore our preferences state.
var preferences = new UserPreferenceCommands();
Models.UserPreferences viewModel = new Models.UserPreferences();

// Set up the event handler before we deserialize.
viewModel.PropertyChanged += viewModel_PropertyChanged;
preferences.LoadPreferencesCommand.Execute(viewModel);

// At this point, viewModel is a valid object. All properties are set correctly.
viewModel = preferences.Results;

// After this step, the UI still shows 0's in all of the text boxs. Even though the values are not zero.
this.DataContext = viewModel;
}

// NEVER gets fired from within the WindowLoaded event.
void viewModel_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
{
MessageBox.Show("Property changed!");
}

// This changes the model properties and is immediately reflected in the UI. Why does this not happen within the WindowLoaded event?
private void TestButtonClickEvent(object sender, RoutedEventArgs e)
{
var context = this.DataContext as Models.UserPreferences;
context.SelectedCollectionDevice.ComPort = 1536;
}


Update 4 - Problem identified

I have identified the problem, but still need a resolution. The whole point of data binding is so that I don't have to perform this manual assignment. Is there something wrong with my INotify implementations?

private void WindowLoaded(object sender, RoutedEventArgs e)
{
// Restore our preferences state.
var preferences = new UserPreferenceCommands();
Models.UserPreferences viewModel = new Models.UserPreferences();

// Set up the event handler before we deserialize.
viewModel.PropertyChanged += viewModel_PropertyChanged;
preferences.LoadPreferencesCommand.Execute(viewModel);

// At this point, viewModel is a valid object. All properties are set correctly.
viewModel = preferences.Results;

// After this step, the UI still shows 0's in all of the text boxs. Even though the values are not zero.
this.DataContext = viewModel;

// SOLUTION: - Setting the actual property causes the UI to be reflected when the window is initialized; setting the actual data context does not. Why? Also note that I set this property and my PropertyChanged event handler still does not fire.
((Models.UserPreferences) DataContext).SelectedCollectionDevice = viewModel.SelectedCollectionDevice;

}

Answer

Alright, I was able to determine the problem and get it resolved. It turned out to be a compilation of things causing this.

First, my model.

UserPreferences <-- MainWindow is data bound to this.

[Serializable]
public class UserPreferences : INotifyPropertyChanged
{
    private CollectionDevice selectedCollectionDevice;

    public UserPreferences()
    {
        this.AvailableCollectionDevices = new List<CollectionDevice>();

        var yuma1 = new CollectionDevice
        {
            BaudRate = 4800,
            ComPort = 31,
            DataPoints = 1,
            Name = "Trimble Yuma 1",
            WAAS = true
        };

        var yuma2 = new CollectionDevice
        {
            BaudRate = 4800,
            ComPort = 3,
            DataPoints = 1,
            Name = "Trimble Yuma 2",
            WAAS = true
        };

        var toughbook = new CollectionDevice
        {
            BaudRate = 4800,
            ComPort = 3,
            DataPoints = 1,
            Name = "Panasonic Toughbook",
            WAAS = true
        };


        var other = new CollectionDevice
        {
            BaudRate = 0,
            ComPort = 0,
            DataPoints = 0,
            Name = "Other",
            WAAS = false
        };

        this.AvailableCollectionDevices.Add(yuma1);
        this.AvailableCollectionDevices.Add(yuma2);
        this.AvailableCollectionDevices.Add(toughbook);
        this.AvailableCollectionDevices.Add(other);

        this.SelectedCollectionDevice = this.AvailableCollectionDevices.First();
    }

    /// <summary>
    /// Gets or sets the GPS collection device.
    /// </summary>
    public CollectionDevice SelectedCollectionDevice
    {
        get
        {
            return selectedCollectionDevice;
        }
        set
        {
            selectedCollectionDevice = value;

            if (selectedCollectionDevice.Name == "Other")
            {
                this.AvailableCollectionDevices[3] = value;
            }

            this.OnPropertyChanged("SelectedCollectionDevice");
        }
    }

    /// <summary>
    /// Gets or sets a collection of devices that can be used for collecting GPS data.
    /// </summary>
    [Ignore]
    [XmlIgnore]
    public List<CollectionDevice> AvailableCollectionDevices { get; set; }

    public event PropertyChangedEventHandler PropertyChanged;

    /// <summary>
    /// Notifies objects registered to receive this event that a property value has changed.
    /// </summary>
    /// <param name="propertyName">The name of the property that was changed.</param>
    protected virtual void OnPropertyChanged(string propertyName)
    {
        if (this.PropertyChanged != null)
        {
            this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

In the setter for the SelectedCollectionDevice I was not looking to see if the selected device was other. All of the other devices (yuma1, panasonic etc) have pre-determined property values that are never changed. When the user selects "Other" the textbox's are displayed and they can manually enter the data. The problem was that when the manually entered data was restored from the database during the window loading, I was not assigning the custom data in SelectedCollectionDevice to the corresponding object in the collection.

During window load, the Combobox.SelectedItem was set to the index of the SelectedCollectionDevice. The Combobox.ItemsSource was set to the AvailableCollectionDevices collection.

this.CollectionDevice.SelectedIndex = 
    viewModel.AvailableCollectionDevices.IndexOf(
        viewModel.AvailableCollectionDevices.FirstOrDefault(
            acd => acd.Name == viewModel.SelectedCollectionDevice.Name));

When the above code is executed, the combo box pulls the default object from its data source, which has all of the values set to zero. Within the combo box's SelectionChanged event I assigned the Data Context SelectedCollectionDevice to the zero'd out item associated with the combo box.

private void CollectionDeviceSelected(object sender, SelectionChangedEventArgs e)
{
    if (e.AddedItems.Count > 0 && e.AddedItems[0] is CollectionDevice)
    {
        // Assign the view models SelectedCollectionDevice to the device selected in the combo box.
        var device = e.AddedItems[0] as CollectionDevice;
        ((Models.UserPreferences)this.DataContext).SelectedCollectionDevice = device;

        // Check if Other is selected. If so, we have to present additional options.
        if (device.Name == "Other")
        {
            OtherCollectionDevicePanel.Visibility = Visibility.Visible;
        }
        else if (OtherCollectionDevicePanel.Visibility == Visibility.Visible)
        {
            OtherCollectionDevicePanel.Visibility = Visibility.Collapsed;
        }
    }
}

So long story short, I added the code above in the setter for the SelectedCollectionDevice to apply the value to the AvailableCollectionDevices List<>. This way, when the combo box has the "Other" value selected, it pulls the value from the collection with the correct data. During deserialization, I am just deserializing the SelectedCollectionDevice and not the List<> which is why the data was always being overwrote when the window first loaded.

This also explains why re-assigning the the data context SelectedCollectionDevice property with the local viewModel.SelectedCollectionDevice was working. I was replacing the zero'd out object associated with the combo box, which had set the data context during the SelectionChanged event. I am not able to set the DataContext in the XAML and remove the manual assignment.

Thanks for all of the help, it helped me narrow down my debugging until I finally resolved the issue. Much appreciated!

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