Question

How to get a line number that contains a specified string in a text file?

Example text file contains:

Red
White
Yellow
Green

How to get “Yellow” line number? and can i write a string in a specified line, lets say i want to write a string in line 2?

Answer 1

To find a line in a text file, you need to read the lines from the start of the file until you find it:

string fileName = "file.txt";
string someString = "Yellow";

string[] lines = File.ReadAllLines(fileName);
int found = -1;
for (int i = 0; i < lines.Length; i++) {
  if (lines[i].Contains(someString)) {
    found = i;
    break;
  }
}

If you want to change a line in a file, you have to read the entire file and write it back with the changed line:

string[] lines = File.ReadAllLines(fileName);
lines[1] = "Black";
File.WriteAllLines(fileName, lines);