could not add data to the ComboBox from parametarised constructor

catalina beaver 41

I'm trying to populate ComboBox. I'm able to add items from the default consctructor but not from the parameterised constructor.

 public  Dashboard()
        {
            this.InitializeComponent();
            MyCombobox1.Items.Add("item_constructor 1"); // this item is added successfully
        }

 public Dashboard(int count, string[] arr)
        {
            this.InitializeComponent();         
            local_arr = arr;
             for (int i = 0;i<count;i++)
             {
                 MyCombobox1.Items.Add(local_arr[i]); // not added..no error thrown                          
             }        
        }

 private async void button_Click(object sender, RoutedEventArgs e)
        {
          MyCombobox1.Items.Add("item_button_click"); // item added successfully
        }

The XAML code for the ComboBox..

<ComboBox
                Name="MyCombobox1"
                x:FieldModifier="public"
                Grid.Row="0"
                Grid.Column="1"
                Canvas.Left="-121"
                Canvas.Top="150"
                Margin="450,360,0,0"
                Background="#F1F1F5"
                FontSize="24"

                Header=" Choose file"
                PlaceholderText="select"
                RenderTransformOrigin="0.575,1.825"
                SelectionChanged="MyCombobox1_SelectionChanged"
                 >

            </ComboBox>

I'm not able to understand the mistake. Thank you.

Guido Franzke 2,196 Reputation points

2022-06-07T13:01:47.64+00:00

Show us the call to your function.
Did you check the values of count and arr with the debugger?
catalina beaver 41 Reputation points

2022-06-07T13:13:24.52+00:00
The call to the parameterized constructor happens in other class. like below

Dashboard d = new Dashboard(count, img_array);

The values in the variable count and arr are perfect. I have printed them by using

System.Diagnostics.Debug.WriteLine(arr[i]); System.Diagnostics.Debug.WriteLine(count);

in the parameterised constructor

1 answer

Karen Payne MVP 35,196 Reputation points

2022-06-07T13:38:34.523+00:00
Alternate, set the ItemSource

ExampleComboBox.ItemsSource = new List<string>() { "First", "Second" }; ExampleComboBox.SelectedIndex = 0;

Add item later

var list = (List<string>)ExampleComboBox.ItemsSource; list.Add("Last"); ExampleComboBox.SelectedIndex = list.Count - 1;
Please sign in to rate this answer.

1 person found this answer helpful.
catalina beaver 41 Reputation points

2022-06-08T13:12:12.17+00:00

I could only populate via a default constructor and a button click functions. My requirement is that the combobox (which is in class dashboard)should populate with data from other class(Main Page). This should be done as soon as I navigate to dashboard page from the Main page. The data domes to the dashboard from Main page via parameterized constructor.

Karen Payne MVP 35,196 Reputation points

2022-06-08T13:56:30.137+00:00

My example is conceptual, having valid list passed in from the constructor to your other class/window/page will work and this is basic programming not linked to a project type.

catalina beaver 41 Reputation points

2022-06-09T13:33:20.963+00:00

Understood. I have tried adding data to the combobox as you mentioned from the parameterized constructor. I think the data is added. However, I could not see that data on the combobox. I have debug messages everywhere, I see this works as I wanted however, I don't see that on the GUI.

namespace PROJECT { public sealed partial class MainPage : Page { public MainPage() { this.InitializeComponent(); } private async void login_Click(object sender, RoutedEventArgs e) { int count = 3; string[] data = some array input; Dashboard d = new Dashboard(count, data); } } }

The above code is my mainpage class. Here I'm creating an object for the dashboard class and calling parameterized constructor. Below is my dashboard class code

catalina beaver 41 Reputation points

2022-06-09T13:33:33.267+00:00

namespace PROJECT { public Dashboard() { this.InitializeComponent(); } public Dashboard(int count, string[] arr) { this.InitializeComponent(); List<ComboData> ListData = new List<ComboData>(); int local_count = count; string[] local_arr = arr; for (int i = 0; i < local_coun; i++) { ListData.Add(new ComboData { Id = i + 1, Name = local_arr[i] }); } MyCombobox1.ItemsSource = ListData; MyCombobox1.DisplayMemberPath = "Name"; MyCombobox1.SelectedValuePath = "Id"; MyCombobox1.SelectedIndex = 0; } }

Karen Payne MVP 35,196 Reputation points

2022-06-09T16:04:11.397+00:00

Another conceptual example, note I don't set DisplayMemberPath or SelectedValuePath, ToString instead

namespace WpfAddItemsComboBox { /// <summary> /// Interaction logic for MainWindow.xaml /// </summary> public partial class MainWindow : Window { public MainWindow() { InitializeComponent(); ExampleComboBox.ItemsSource = new List<Category>() { new Category() {Id = 1, Name = "Meat"}, new Category() { Id = 2, Name = "Fruit" } }; ExampleComboBox.SelectedIndex = 0; } private void Button_Click(object sender, RoutedEventArgs e) { var list = (List<Category>)ExampleComboBox.ItemsSource; list.Add(new Category() { Id = 3, Name = "Fish" }); } } public class Category { public int Id { get; set; } public string Name { get; set; } public override string ToString() => Name; } }

catalina beaver 41 Reputation points

2022-06-10T09:13:31.75+00:00

Thank you. I dont have any issues to write data to the combobox. I understood your example. However, the problem is I can only see the data in combobox from default constructor and a button click method but not from the parameterized constructor.
I hope the error is due to calling this.InitializeComponent(); two times. Once in default constructor and once in parameterized constructor. I hope, if I use this.InitializeComponent() twice, I'll have two Combobox's. One may be the visible which is from default constructor. The other one (parameterized constructor) is the one that your code refers to.

If I dont use this.InitializeComponent(); in parameterized constructor , error show up as Object reference not set to an instance of an object.' Combobox was null.
I hope you understood the issue.
namespace PROJECT
{
public Dashboard()
{
this.InitializeComponent();
}
public Dashboard(int count, string[] arr)
{
this.InitializeComponent();
int local_count = count;
string[] local_arr = arr;
for (int i = 0; i < local_coun; i++)
{
ListData.Add(new ComboData { Id = i + 1, Name = local_arr[i] });
}
MyCombobox1.ItemsSource = ListData; // error shows up here
MyCombobox1.SelectedIndex = 0;
}
}

Nico Zhu (Shanghai Wicresoft Co,.Ltd.) 12,856 Reputation points

2022-06-24T06:32:01.477+00:00

I have made a sample with above code, But I could not reproduce your prolbem, you may check if you have add this usercontrol into current page content, and check if you have pass correct count value

var disboard = new Dashboard(3, new string[] { "item1", "item2", "item3" }); RootGrid.Children.Add(disboard);

Karen Payne MVP 35,196 Reputation points

2022-06-24T11:28:27.87+00:00

If I dont use this.InitializeComponent(); in parameterized constructor , error show up as Object reference not set to an instance of an object.' Combobox was null.

That is expected behavior, InitializeComponent must be called.

Nico Zhu (Shanghai Wicresoft Co,.Ltd.) 12,856 Reputation points

2022-06-27T08:11:44.51+00:00

That's right InitializeComponent method is used to Initialize xaml code and generate Combobox instance.

catalina beaver 41 Reputation points

2022-07-04T08:58:53.733+00:00

Using InitializeComponent() in default and parameterized constructor is causing the issue right. App instances are created twice. Hence I don't see the data what I get via parameterized constructor in the App/GUI. How to solve this issue?.

Nico Zhu (Shanghai Wicresoft Co,.Ltd.) 12,856 Reputation points

2022-07-06T10:10:33.857+00:00

You have no need keep two ctors, please try to remove non-paramter one.
Sign in to comment