Simple Combobox in Winforms - Firing twice (Use mouse to expand combobox and select a item using keyboard)

Question

Consider a very basic form with a simple combo box

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        this.comboBox1.Items.Add("test1");
        this.comboBox1.Items.Add("test2");
        this.comboBox1.Items.Add("test3");
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        this.comboBox1.SelectedIndexChanged += (o, args) => 
        {
            MessageBox.Show("Combo box changed!");
        };
    }
}

I have even changed the eventhandler to the below code. (Based on the linked question. Still the same problem)

this.comboBox1.SelectedValueChanged += (o, args) => //or even `Textchanged` event too
            {

                MessageBox.Show("Combo box changed!");
            };

Expand the dropdown using mouse and select any item using keyboard.

The combo-box fires twice (The message box appears twice)

Any ideas why?

Use `Debug.Print` instead of message boxes for debug purposes. — i486, Mar 13 '15 at 09:12
possible duplicate of [Catching Unselecting All in ListView -- SelectedIndexChanged Firing Twice](http://stackoverflow.com/questions/5740756/catching-unselecting-all-in-listview-selectedindexchanged-firing-twice) — Sayse, Mar 13 '15 at 09:13
Do you have any other code that initialized this event? i.e., in the designer.cs — , Mar 13 '15 at 09:14
@allentranks: Nope. This is the only code that I have written. The designer is untouched — now he who must not be named., Mar 13 '15 at 09:14
@Sayse: Actually the linked question doesn't seem to solve my issue. Even after using `ItemSelectionChanged` the event still fires twice — now he who must not be named., Mar 13 '15 at 09:18
Replace that `MessageBox.Show` with `Console.WriteLine` and check. It fires only once. `MessageBox` is doing something that causes it to be raised twice. — Sriram Sakthivel, Mar 13 '15 at 09:21
@nowhewhomustnotbenamed. - Searching for "SelectedValueChanged fires twice" returns lots of results.. Can I ask what it is that you are actually doing with the combobox? — Sayse, Mar 13 '15 at 09:22
@SriramSakthivel definitely fires twice, click on expander, type 'test3', hit 'enter', two events fire — paul, Mar 13 '15 at 09:22
@paul Read my comment again. With messagebox, it fires twice but if you add `Console.WriteLine`, It will be fired only once. — Sriram Sakthivel, Mar 13 '15 at 09:24
@Sayse: Pretty simple. Just wanna call a function (which inturn updates a text on a label). But problem is whenever I expand the combobox using a mouse and choose an item using keyboard, it fires twice — now he who must not be named., Mar 13 '15 at 09:24
@SriramSakthivel: Sorry. But how to check the items written in `console.writeline` ? Winforms doesn't have a console.writeline window ? — now he who must not be named., Mar 13 '15 at 09:25
You need to look at the output in output window of visual studio, or change the project to console application; console will appear. — Sriram Sakthivel, Mar 13 '15 at 09:26
@SriramSakthivel not on my pc, `Debug.Print`, `Console.WriteLine`, `MessageBox` all show twice — paul, Mar 13 '15 at 09:26
@SriramSakthivel: I did that.. As Paul says, they are fired twice — now he who must not be named., Mar 13 '15 at 09:27
@nowhewhomustnotbenamed. - Have you tried using the `TextChanged` event instead? — Sayse, Mar 13 '15 at 09:27
@Sayse: Still firing twice.. `Expand using mouse. Choose an item using keyboard`.. fires twice even with `TextChanged` — now he who must not be named., Mar 13 '15 at 09:28
@SriramSakthivel: Hey sriram, do you know if this is a bug ? Or am I doing wrong somewhere ? — now he who must not be named., Mar 13 '15 at 09:33
Trust me it fires only once for me. Windows 8, .Net 4.0, Vs 2012. Your mileage may vary. — Sriram Sakthivel, Mar 13 '15 at 09:34
@SriramSakthivel once if you click, once if you use the keyboard up down arrows, twice if you type 'test3' and hit the 'enter' key — paul, Mar 13 '15 at 09:36
@paul: Exactly. the same issue. Do you think this is an issue with `winforms` ? — now he who must not be named., Mar 13 '15 at 09:37
@SriramSakthivel: Make sure `you expand the dropdown with a mouse` and `using up/down keys in keyboard, select an item` . Then the item is fired twice — now he who must not be named., Mar 13 '15 at 09:38
Done everything you said. With MessageBox, it is called twice as I already said. But when I replace it with Console.WriteLine, it is called only once. Not sure what's going on with you. I tried as you said "typed test3 and press enter" it fired only once. — Sriram Sakthivel, Mar 13 '15 at 09:44
@SriramSakthivel: In real time code, it is not a message box. I am performing a complex logic within a method that is fired on `SelectedIndexChanged`. So, this method is called twice. And this is why i am worried — now he who must not be named., Mar 13 '15 at 09:49

score 0 · Accepted Answer · answered Mar 17 '15 at 12:07

Seems like this is a bug with the underlying framework.

So, I managed to get a work around of the issue.

Here's how:

Have a private variable (preferably) which can store the value of the selected item in the combo-box
Next time, compare it with the current selection in the combo-box. If they are same, just return.

Simple Combobox in Winforms - Firing twice (Use mouse to expand combobox and select a item using keyboard)

1 Answers1