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Read the contents of a file into a string

string contents = System.IO.File.ReadAllText("filename.txt");

Process a file one line at a time

Open the source file to your solution and print each line in the file, prefixed by the line number, like:
1> First line of file
2> Second line of file
3> Third line of file
int counter = 0;

// If the file is large, you would want to buffer this instead of reading everything at once
foreach (string line in System.IO.File.ReadAllLines("filename.txt"))
Console.WriteLine("{0}> {1}", ++counter, line);

Write a string to a file

System.IO.File.WriteAllText("filename.txt", "Some text to write to the file");

Append to a file

System.IO.File.AppendAllText("filename.txt", "Some text to append to the file");

Process each file in a directory

foreach (string filename in System.IO.Directory.GetFiles(directory)) ProcessFile(filename);