WinForms comboBox_SelectedValueChanged在tabControl_SelectedIndexChanged上触发了三次

Question

I have a tabcontrol element with multiple tabs. On tab 2 (index 1) I have a combobox that manages a template selection. The template object itself holds an ID and a name

internal class Template
{
    public Template(double id, string name)
    {
        ID = id;
        Name = name;
    }

    public double ID { get; private set; }
    public string Name { get; private set; }
}

With this combobox I want to display the name and select the ID. I will provide a full working example.

My form

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        cmbx = new Cmbx(comboBox1); // pass in the combobox element
    }

    private Cmbx cmbx; // My combobox class

    private void tabControl1_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (tabControl1.SelectedIndex == 1) // tab 2 active?
        {
            cmbx.CreateTemplateSelection(myTemplates); // init combobox with the templates list
        }
    }

    private void comboBox1_SelectedValueChanged(object sender, EventArgs e)
    {
        // handle the new selected ID here
        MessageBox.Show($"Combobox Change Event - Selected ID {cmbx.SelectedTemplateID}");
    }
}

and my combobox class

class Cmbx
    {
        public Cmbx(ComboBox cmbx)
        {
            this.cmbx = cmbx;
        }

        private ComboBox cmbx; // GUI element
        private Template[] currentTemplates;

        public double SelectedTemplateID // Get the current selected value
        {
            get
            {
                Template templateInfo = cmbx.SelectedValue as Template;
                return templateInfo.ID;
            }
        }

        public void CreateTemplateSelection(Template[] templates) // populate the combobox
        {
            if (currentTemplates != null) // first time?
            {
                if (currentTemplates.SequenceEqual(templates)) // different data?
                {
                    return; // nothing changed -> return
                }
            }

            // different data

            cmbx.DataSource = null; // clear the combobox
            cmbx.Items.Clear();

            BindingSource bindingSource = new BindingSource();
            bindingSource.DataSource = templates;
            cmbx.DataSource = bindingSource.DataSource; // setup the data

            cmbx.DisplayMember = nameof(Template.Name); // display the template name
            cmbx.ValueMember = nameof(Template.ID); // select the template id

            bool valid = templates.Length > 0; // existing data?

            if (valid && cmbx.SelectedIndex != 0)
                cmbx.SelectedIndex = 0; // select the first item

            currentTemplates = templates; // assign the new templates to the current ones
        }
    }

When running the project and selecting the second tab the comboBox1_SelectedValueChanged fires 3 times.

On the first and second run cmbx.SelectedTemplateID returns me the selected template ID 1. This is correct.

The third run returns me a null pointer.

Why does it fire 3 times? And why does the third event call return a null pointer?

Answer 1

I will try to answer it on my own. I removed

cmbx.DataSource = null;
cmbx.Items.Clear();

and

cmbx.ValueMember = nameof(Template.ID);

Further I changed the binding order

BindingSource bindingSource = new BindingSource();
bindingSource.DataSource = templates;

cmbx.DisplayMember = nameof(CoBRATemplate.Name);

cmbx.DataSource = bindingSource.DataSource;

This works for me. But please feel free to provide a "better" answer.