问题
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
{....
回答1:
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.
回答2:
long is an Int64, the ? makes it nullable.
回答3:
long? is a 64-bit, nullable integer.
To clarify, nullable means it can be null or an integer number ( 0, 1, etc.).
回答4:
"long?" is a nullable 64-bit signed integer. It's equivalent to Nullable<Int64>.
回答5:
long? is a nullable type. This means that the id parameter can have a long value or be set to null. Have a look at the HasValue and Value properties of this parameter.
回答6:
It's a nullable type declaration.
来源：https://stackoverflow.com/questions/3893143/what-is-long-data-type