Reading and writing line by line

Whole-file convenience

string all = File.ReadAllText(path);          // one big string
string[] lines = File.ReadAllLines(path);     // split for you
File.WriteAllLines(path, new[] { "a", "b" }); // joins with newlines

ReadAllLines also strips line-ending differences: Windows CRLF or Unix LF, you get clean lines. For config files and small datasets, these helpers are the right tool.

Streaming with using

For anything potentially large — or when you process as you read — stream:

using (var reader = new StreamReader(path))
{
    string line;
    while ((line = reader.ReadLine()) != null)
    {
        Console.WriteLine(line);
    }
}

StreamReader.ReadLine() returns the next line or null at end-of-file. The using block guarantees the file handle closes even if the loop body throws — an open handle leaks a resource the OS grants in limited supply.