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Then your design is flawed. You could, however, make sure that both lists have the same amount of items and then loop from 0 to that amount:




int itemCount = listBox4.Items.Count;...

To explain what you're currently doing:



for (int i = 0; i < numericUpDown2.Value; i++) //loop from 0 to the current value of the NumericUpDown control
{
listBox4.SetSelected(i, true);...