Display list of strings in WPF window in C#

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SelectedFiles contain an array of string, and I want to display it on Listbox.

when I debugg this code.

<TextBlock Text="{Binding Path=SelectedFiles}" Foreground="Yellow">

when I hover on

SelectedFiles

it shows the array but not reflected on UI.no change for UI

What I have tried:

xaml

  <pre><Grid Grid.Row="1">
            <ListBox ItemsSource="{Binding Path=SelectedFiles}" x:Name="listbox"   FontSize="15" Width="780" Margin="15,103,15,224" >
                <ListBox.ItemTemplate>
                    <DataTemplate>
                        <StackPanel>
                            <TextBlock Text="{Binding Path=SelectedFiles}" Foreground="Yellow">
                            </TextBlock>
                        </StackPanel>
                    </DataTemplate>
                </ListBox.ItemTemplate>
            </ListBox>
        </Grid>

xaml.cs

try
{
listbox.DataContext = SelectedFiles;

}

Posted 11-Jan-21 2:52am

john321a

Updated 12-Jan-21 6:28am

v2

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john321a 11-Jan-21 10:50am

have updated the code.plz take a look

3 solutions

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OriginalGriff · Answer 1 · 2021-01-11T03:04:00

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterday's shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

Chris Copeland · Answer 2 · 2021-01-11T03:08:00

Solution 2

C#

hello(selectedFiles);
InitializeComponent();

You're calling hello(selectedFiles); before the UI has been initialized by InitializeComponent();. Try moving the hello(..) below that and see whether it works.

Posted 11-Jan-21 3:08am

Chris Copeland

Comments

john321a 11-Jan-21 9:27am

no change

Chris Copeland 11-Jan-21 11:51am

You've changed the code since this solution was suggested. You're using binding now, but where have you added the SelectedFiles property? Is it on the control/page itself If so, does it use the DependencyProperty model which WPF expects?

Also your code example shows you setting the DataContext of the ListBox to the list itself, and then trying to read a child property called SelectedFiles. Instead, you might want to change your ItemsSource to just "{Binding}"

Richard Deeming · Answer 3 · 2021-01-12T06:28:00

You'll need a custom value converter to bind a list or array of strings to a textbox.

For example:

C#

public class JoinStringConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var list = value as IEnumerable<string>();
        return list is null ? null : string.Join(Environment.NewLine, list);
    }
    
    public object ConvertBack(object value, Type targetTypes, object parameter, CultureInfo culture)
    {
        string valueString = value as string;
        return valueString is null ? null : valueString.Split(new[] { Environment.NewLine });
    }
}

However, It would probably be simpler to join the strings in the view-model, and bind to the joined string property:

C#

private string[] _selectedFiles;

public string[] SelectedFiles
{
    get { return _selectedFiles ?? Array.Empty<string>; }
    set 
    {
        _selectedFiles = value;
        OnPropertyChanged(nameof(SelectedFiles));
        OnPropertyChanged(nameof(SelectedFilesString));
    }
}

public string SelectedFilesString
{
    get { return _selectedFiles == null ? null : string.Join(Environment.NewLine, _selectedFiles); }
    set { SelectedFiles = value == null ? null : value.Split(new[] { Environment.NewLine }); }
}

XAML

<TextBlock Text="{Binding Path=SelectedFilesString}" Foreground="Yellow" />

Display list of strings in WPF window in C#

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