Change SelectedValue of combobox before it's displayed

Question

The problem is strange, I have a combobox which is correctly filled with items through the DataSource properties (it binds to a bindingSource item, which contains a displaymember and a valuemember), this is a simple combobox which allows me to select suppliers by choosing between their names and not their ids. The problem is that when I change SelectedValue because I want an option set as the default one, I discovered that the List property is empty (I inherited the combobox class in another one), However when the combobox is shown, I have items inside it, so maybe the problem is related to WHEN data is really loaded from the datasource (maybe it's loaded only after displaying it?). When should I change my SelectedValue so?There isn't any "Shown" event in the control

Thanks for any suggestions

Update 1: I tested in your way but it's not this the case (the key is a long but even if I cast it it's not working), in fact the interesting thing is that I have a setter that allows me to set SelectedValue, even if doesn't throw exceptions, in the next MessageBox the Items.Count give me 0 as value, which is very strange, because when the combobox is shown on the form it has all values i wanted. Here is the code for my setter:

    public object InputValue
    {
        get
        {
            return SelectedValue;
        }
        set
        { 
            SelectedValue = ((long)value);
            MessageBox.Show(this.Items.Count.ToString());
        }
    }

See my comment on King King's answer: stackoverflow.com/questions/18343855/… — RLH

Bradley Smith Bradley Smith · Accepted Answer · 2010-11-02T06:17:16

You're not the only person who has experienced this problem, it's good to hear that it's not just me. What is not happening is something to do with type conversion - if you explicitly cast your SelectedValue to the exact type in the data source, you'll find that you are able to set the value successfully.

For example:

comboBox1.ValueMember = "Value";
comboBox1.DisplayMember = "Key";
object ds = new KeyValuePair<string, long>[] {
    new KeyValuePair<string,long>("a",0),
    new KeyValuePair<string,long>("b",1),
    new KeyValuePair<string,long>("c",2),
    new KeyValuePair<string,long>("d",3),
    new KeyValuePair<string,long>("e",4)
};
comboBox1.DataSource = ds;
comboBox1.SelectedValue = 3;        // this won't work
comboBox1.SelectedValue = (long)3;  // this will work

For whatever reason, even though '3' as a long is equal to '3' as an int, the ComboBox refuses to play ball unless you get the type precisely correct.

I can theorise that this has something to do with the fact that the WinForms ComboBox is a wrapper for the COM version, and different behaviour is exhibited after the native control has been created.

I hope this helps with your particular problem!

Change SelectedValue of combobox before it's displayed

3 Answers