How to lock a file with C#?

You need to pass in a FileShare enumeration value of None to open on the FileStream constructor overloads:

fs = new FileStream(@"C:\Users\Juan Luis\Desktop\corte.txt", FileMode.Open, 
    FileAccess.ReadWrite, FileShare.None);

As per http://msdn.microsoft.com/en-us/library/system.io.fileshare(v=vs.71).aspx

FileStream s2 = new FileStream(name, FileMode.Open, FileAccess.Read, FileShare.None);