ComboBox SelectedIndexChanged event: how to get the previously selected index?

There is nothing built in, you will need to listen for this event and keep track in an instance variable.

Use -1 as an uninitialized "last index", so on first pass you set it but don't use it. Subsequent passes you use it and set it.

You could always do this using a derived ComboBox class of your own and override OnSelectedIndexChanged and expose a PreviousSelectedIndex property. This way, it wouldn't be tightly coupled to the form. Alternatively, as you can do this using events, its also eligible for implementation as an extender provider.

I guess you will have to store the current (that will become the previous later on) into a variable so that it is used like a cache or something like so.

private void cboTargetMode_SelectedIndexChanged(object sender, EventArgs e) {
    // need to get the previously selected index and do some handling here...
    // ... some handler code here ...

    // Assuming that the variable PreviousSelectedIndex is declared in the class with value -1.
    if (PreviousSelectedIndex < 0)
        PreviousSelectedIndex = cbo.TargetMode.SelectedIndex;
    else
        // Do some handling here...

    switch (cboTargetMode.SelectedIndex) {
        case 1:  // ..... some code here...
            break;
        case 2:  // ..... some code here...
            break;
        case 3:  // ..... some code here...
            break;
        default: // ..... some code here...
            break;
    }
}

Is this something you have already thought of?

Otherwise, perhaps working with the Control.Validating event? I just can't say whether this event occurs before or after the SelectedIndexChanged event. =(