What is 'long?' data type?

I am going over some code written by another developer and am not sure what long? means:

protected string AccountToLogin(long? id)
{
   string loginName = "";
   if (id.HasValue)
   {
      try
      {....

long is the same as Int64

long data type

The ? means it is nullable

A nullable type can represent the normal range of values for its underlying value type, plus an additional null value

Nullable Types

Nullable example:

int? num = null;
if (num.HasValue == true)
{
    System.Console.WriteLine("num = " + num.Value);
}
else
{
    System.Console.WriteLine("num = Null");
}

This allows you to actually check for a null value instead of trying to assign an arbitrary value to something to check to see if something failed.

I actually wrote a blog post about this here.

long is an Int64, the ? makes it nullable.