Question

And add these lines to a ListBox. (Windows Form)

I would use:

String file_name = @"x:\....";
int first_line = 24536;
int last_line = 25536;

for ( int i = first_line; i <= last_line; i++)
{
  this.listBox.Items.Add(File.ReadLines(this.file_name).ElementAt(i));
}

But this takes a lot of time to load 1000 lines.

How can I do this more efficiently (faster)?

Answer 1

Currently you’re calling ElementAt each time – which means reading the file from scratch for each new line.

Try this:

string[] lines = File.ReadLines(fileName)
                     .Skip(firstLine - 1) // Assuming 1-based line numbers
                     .Take(lastLine - firstLine)
                     .ToArray();
listBox.Items.AddRange(lines);

(Note that I’ve removed the unconventional underscores from your variable names.)