To read a text file one line at a time you can do like this:
using System.IO;
using (var reader = new StreamReader(fileName))
{
string line;
while ((line = reader.ReadLine()) != null)
{
// Do stuff with your line here, it will be called for each
// line of text in your file.
}
}
There are other ways as well. For example, if the file isn’t too big and you just want everything read to a single string, you can use File.ReadAllText()
myTextBox.Text = File.ReadAllText(fileName);