C# - Add HTML Id's To Tags In .aspx File?

So I'm writing an app that lets the user select a folder, it gets all the .aspx files in that folder, and lets the users check off which ones they want to add HTML ID's to.

Then they click start, and this runs

private void btnStart_Click(object sender, EventArgs e)
{
for (int i = 0; i < listFiles.CheckedItems.Count; i++)
{
}
}

It loops through all the selected file names. How do I open each of these .aspx files in the background, and go through them and add the id="thisItemId"