Anywhere you are doing the StreamWriter stuff, change it to the following.
Also, since you're utilizing the using() block, there is no need to close the file, the using block of the streamwriter will close it for you.
private void hrs_combobx_SelectedIndexChanged(object sender, EventArgs e)
{
if (File.Exists(@"C:\Users\admin\Documents\combobox1.txt"))
{
using (StreamWriter sw = new StreamWriter(new FileStream(@"C:\Users\admin\Documents\combobox1.txt", FileMode.Append)))
{
sw.WriteLine(hrs_combobx.SelectedItem);
}
}
}