How to Find And Replace Text In A File With C#

Read all file content. Make a replacement with String.Replace. Write content back to file.

string text = File.ReadAllText("test.txt");
text = text.Replace("some text", "new value");
File.WriteAllText("test.txt", text);

You're going to have a hard time writing to the same file you're reading from. One quick way is to simply do this:

File.WriteAllText("test.txt", File.ReadAllText("test.txt").Replace("some text","some other text"));

You can lay that out better with

string str = File.ReadAllText("test.txt");
str = str.Replace("some text","some other text");
File.WriteAllText("test.txt", str);

You need to write all the lines you read into the output file, even if you don't change them.

Something like:

using (var input = File.OpenText("input.txt"))
using (var output = new StreamWriter("output.txt")) {
  string line;
  while (null != (line = input.ReadLine())) {
     // optionally modify line.
     output.WriteLine(line);
  }
}

If you want to perform this operation in place then the easiest way is to use a temporary output file and at the end replace the input file with the output.

File.Delete("input.txt");
File.Move("output.txt", "input.txt");

(Trying to perform update operations in the middle of text file is rather hard to get right because always having the replacement the same length is hard given most encodings are variable width.)

EDIT: Rather than two file operations to replace the original file, better to use File.Replace("input.txt", "output.txt", null). (See MSDN.)