What I understood from your question is suppose if you made changes to the combo box value and if you try restarting or reloading the form you are getting the old value itself...If that is the issue then you have to save the new value in a file or you can save it in application settings. Then while loading for the first time read from wherever you save it and then display it.
If your question is like when you are changing the index you are getting wrong value then u r function need to be changed,
void dt1()
{
textBox1.Text = ar[0];
}
void dt2()
{
textBox1.Text = ar[1];
}
void dt3()
{
textBox1.Text = ar[2];
}