ArrayList myList = new ArrayList();
int firstNumber=int.Parse(txt1.Text);
int secondNumber = int.Parse(txt2.Text);
for (int i = firstNumber; i <= secondNumber; i++) {
    if (i%2==0) {
        myList.Add(i);
        lstbox1.Items.Add(myList);
    }
}

ArrayList myList = new ArrayList();
int firstNumber=int.Parse(txt1.Text);
int secondNumber = int.Parse(txt2.Text);
for (int i = firstNumber; i <= secondNumber; i++) {
    if (i%2==0) {
        myList.Add(i);
        lstbox1.Items.Add(myList[i]);
    }
}