C# Search txt file for missing text and if missing add text

Question

I have combined the content of three txt-files into one and now have to do the final touch. I have managed to search for a comma and changed it to a dot. So far so good. But...

Every now and then a line in that combined file reads "LENGTH " (Yes it is the word length followed by a space-character)

Sometimes that line is followed by a number... But... If that line is NOT followed by any number or other character at all it should be added the digit 0 (zero).

How do I solve this?

Answer 1

Read all lines of the file, raplace the wrong ones and write them back to the file:

string[] lines = File.ReadAllLines(filePath);
File.Delete(filePath);
File.WriteAllLines(filePath, lines.Select(l => l == "LENGTH " ? "LENGTH 0" : l));

Answer 2

You can simply stream the file line by line and replace the lines reading LENGTH with LENGTH 0 :

string source = ...; // your file's name
string target = ...; // your target file's name

File.WriteAllLines(target, File.ReadAllLines(source).Select(l => l == "LENGTH " ? "LENGTH 0" : l));