动态更改comboBox中的项目

Question

I have 2 comboBox objects with same items specified via Design redactor.

If any specific item is selected in the first comboBox, it must be removed from the second.

What I currently do:

1) Added this

static ComboBox.ObjectCollection defaultCollection;

2) On Form_Load

defaultCollection = comboBoxRange1.Items;

3)

private void comboBoxRange1_SelectedIndexChanged(object sender, EventArgs e)
{
    comboBoxRange2.Enabled = true;
    ComboBox.ObjectCollection copyCollection = defaultCollection;
    copyCollection.RemoveAt(comboBoxRange1.SelectedIndex);
    comboBoxRange2.DataSource = copyCollection;
}

But after this I got selected item removed from both comboBoxes and defaultCollection becomes modified. How do I fix it? Do I need to make copies of collections or manually reassemble values on every change?

Answer 1

You are not creating a copy of the collection, you are simply creating a new variable that references the same collection as the first. It does not matter which variable you use to remove the item, you still have only one collection referenced by 2 variables and bound to 2 combo boxes.

Answer 2

ComboBox.ObjectCollection copyCollection = defaultCollection; doesn't do what you want it to do: it doesn't copy , but set a reference to defaultCollection .

Here are some more explanation on whether a variable is set by reference or set by value : if the data is a value type , basically struct and enumerations (say int , which is an alias of System.Int32 ), the value of the data will be copied when doing an assignment; if the data is a reference type , basically classes, interfaces and delegates, the reference to the object will be passed to the LHS when doing an assignment.

As for your question, yes, you need to make a copy to keep defaultCollection unmodified.