Why .NET String is immutable? [duplicate]

Answer 1

Just to throw this in, an often forgotten view is of security, picture this scenario if strings were mutable:

string dir = "C:\SomePlainFolder";

//Kick off another thread
GetDirectoryContents(dir);

void GetDirectoryContents(string directory)
{
  if(HasAccess(directory) {
    //Here the other thread changed the string to "C:\AllYourPasswords\"
    return Contents(directory);
  }
  return null;
}

You see how it could be very, very bad if you were allowed to mutate strings once they were passed.