无法从其他表单设置组合框selectedIndex

Question

I am trying to set the selectedindex on another form (form2) based on the user selected index on the first form (form1).

I am getting the value with this code but it returns a negative number.

public int SelectedComboIndex
{
   get { return comboBox1.SelectedIndex; }
}

And I am trying to set the comboBox Index by

comboBox1.SelectedIndex = form1.SelectedComboIndex;

Can anyone point me in the right direction as to how to do this?

Edit: More code for where its calling the code on form1

Form1 form1 = null;
public Form2(Form1 parentForm1) : this()
{
    form1 = parentForm1;
}

Answer 1

A combobox returns a negative value (-1 normally) if no index is selected.

So I believe (I have not checked) that if you set a negative value for the SelectedIndex property, all you are going to do is clear the selection in your combobox.

Hope that helps.

Answer 2

Best practice is to usually leave any sort of UI alterations in the Load method of the form, that way the form has a chance to initialize properly and all bindings are setup before you actually make changes. The Constructors should only be used to set internal state.

private Form1 _parentForm;
public Form2(Form1 parentForm) : this()
{
    _parentForm = parentForm;
}

public Form2()
{
    InitializeComponents();
}

private void Form2_Load(object sender, EventArgs e)
{
    richTextBox1.Font = new Font("Times New Roman", 12f, FontStyle.Regular); 
    dropdown(); 
    if(_parentForm != null)
        comboBox1.SelectedIndex = _parentForm.SelectedComboIndex; 
    comboBox1.DropDownStyle = ComboBoxStyle.DropDownList; 
}

Try that out and see if it works. Just be sure to add the Load handler to the form properly (either through the designer or in code via this.Load += new EventHandler(Form2_Load)

Answer 3

Note: You should rename all your components to something more useful than controlType1, controlType2 etc. This is good for both you and us.

As other people said if form1.SelectedComboIndex returns -1 (because form1.comboBox1 has nothing selected) then the line comboBox1.SelectedIndex = form1.SelectedComboIndex will (correctly) set comboBox1 's value to nothing.

Additionally, just because there's text present in your ComboBox doesn't mean it's got a selected value. Make sure you combo box has actually selected a value (not just changed its text). You can force picking a value by setting the DropDownStyle to DropDownList . Both other styles allow user entry of custom values.

If you want users to be able to type, then consider setting AutoCompleteMode to SuggestAppend and AutoCompleteSource to ListItems . This makes it easier to correctly select values from the combobox rather than just change the text (accidentally leaving it null).