Question

string value1 = File.ReadAllText("C:\\file.txt");
string value2 = File.ReadAllText(@"C:\file.txt");

In the above statements when is the difference of using @”C:\file.txt” and C:\file.txt

Answer 1

string value1 = "C:\file.txt";
string value2 = @"C:\file.txt";

the string for value1 will contain a formfeed character where the \f is, while the second one will keep the backslash and the f. (This becomes very clear if you try to output them in a console application with Console.Write…)

The correct way for the value1 version would be "C:\\file.txt"

(The value2 version uses whats called, as Dmitry said, a verbatim string)