WPF CheckBox IsChecked Property binding issues

Question

Please help, I was trying to do this small example. My aim is to when I keep the checkbox ticked the app should show the the Ip address of the Host I enter. But The checkbox IsChecked property is never updated in the view model, Even it is been changed in the UI

My View `

    Title="MainWindow" Height="350" Width="525">
<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="*"/>
        <RowDefinition Height="Auto" />
    </Grid.RowDefinitions>
    <Grid.Background>
        <LinearGradientBrush>
            <LinearGradientBrush.GradientStops>
                <GradientStop Offset="0.00" Color="LavenderBlush" />
            </LinearGradientBrush.GradientStops>
        </LinearGradientBrush>
    </Grid.Background>
    <StackPanel Grid.Row="0" Margin="150,30,69,236" Grid.ColumnSpan="2">
        <TextBox x:Name="inputBox" Text="{Binding TxtHostName, Mode=TwoWay}"  Foreground="Azure" Background="YellowGreen" VerticalAlignment="Bottom" Height="45"/>

    </StackPanel>
    <Button Command="{Binding StartCommand }" Content="Get IP" HorizontalAlignment="Left" Margin="257,89,0,0" VerticalAlignment="Top" Width="75" RenderTransformOrigin="0.013,-0.273"/>
    <TextBlock Text="{Binding IpAddress}" Background="BlueViolet" Margin="150,153,69,104" Grid.ColumnSpan="2"  />
    <Button Content="Close" Command="{Binding CloseCommand}" HorizontalAlignment="Left" Margin="257,250,0,0" VerticalAlignment="Top" Width="75"/>
    <CheckBox Content="CheckBox" IsChecked="{Binding IsSelected, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" HorizontalAlignment="Left" Margin="150,111,0,0" VerticalAlignment="Top"/>
</Grid>

`

My ViewModel:

public class ViewModel:INotifyPropertyChanged
{
    #region INPC
    public void RaisePropertyChanged(string propName)
    {
        if (this.PropertyChanged != null)
        {
            this.PropertyChanged(this, new PropertyChangedEventArgs(propName));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;


    #endregion

    private string txtHostName;

    public string TxtHostName
    {
        get { return txtHostName; }
        set { txtHostName = value;
        RaisePropertyChanged("TxtHostName");
        }
    }

    private string ipAddress;

    public string IpAddress
    {
        get { return ipAddress; }
        set { ipAddress = value;
        RaisePropertyChanged("IpAddress");
        }
    }


    private bool checkbox;

    public bool CheckBox
    {
        get { return checkbox; }
        set { checkbox = value;
        RaisePropertyChanged("IsSelected");
        }
    }

    public event EventHandler RequestClose;

    protected void OnRequestClose()
    {
        if (RequestClose != null)
            RequestClose(this, EventArgs.Empty);
    }

    private RelayCommand _StartCommand;
    public ICommand StartCommand
    {
        get
        {
            if (this._StartCommand == null)
                this._StartCommand = new RelayCommand(StartClick);
            return this._StartCommand;
        }
    }

    private RelayCommand _CloseCommand;
    public ICommand CloseCommand
    {
        get
        {
            if(this._CloseCommand==null)
                this._CloseCommand=new RelayCommand(CloseClick);
            return this._CloseCommand;
        }
    }

    private void CloseClick(object obj)
    {
        OnRequestClose();
    }

    private void StartClick(object obj)
    {
        if (checkbox)
        {
            string HostName = TxtHostName;
            IPAddress[] ipaddress = Dns.GetHostAddresses(HostName);

            foreach (IPAddress ipaddr in ipaddress)
            {
                IpAddress = ipaddr.ToString();
            }

        }

        else
        {
            IpAddress = "Please tick the checkbox";
        }
    }
}

}

The RealyCommand is as it should be. The CheckBox Property value never changes weather I change it in the UI or not.

Answer 1

Your raising your property changed event against IsSelected, but your bindable property is called Checkbox, rename Checkbox to IsSelected and update your private variable to something like isSelected.

In this case Id rename the variable to IsChecked or ComboBoxIsChecked.

Answer 2

I'm not sure if there is a copy-and-paste error but your View Model property is called Checkbox while you are raising the property changed event using the label IsSelected.

This and the error in the binding might be your problem. Based on your View Model the binding should be:-

    <CheckBox Content="CheckBox" IsChecked="{Binding Checkbox, Mode=TwoWay}" HorizontalAlignment="Left" Margin="150,111,0,0" VerticalAlignment="Top"/>

Update: Recommendation if you are using C# 5.0 or above

To avoid typo's when creating setters and raising IPropertyNotifyChange events I would recommend using the CallerMemberName attribute as follows:-

  public void RaisePropertyChanged([CallerMemberName] string propName = "")
  {
      if (this.PropertyChanged != null)
      {
          this.PropertyChanged(this, new PropertyChangedEventArgs(propName));
      }
  }

Then your setter in your example becomes:-

  private bool checkbox;

  public bool CheckBox
  {
      get { return checkbox; }
      set { checkbox = value;
      RaisePropertyChanged();
    }
  }

Meaning as you refactor your View Model then the compiler will insert the name of the calling property to ensure the label in the INotifyProertyChanged event matches your property name without you having to remember to manually update it yourself.