C # question mark usage (non-expression)-single question mark, double question mark

Single question mark-used to assign null to the variable (INT type) when setting the initial value, rather than 0!

Double question mark-used to determine and assign values. First, judge whether the current variable is null. If yes, you can assign a new value. Otherwise, skip this step!

Public   Int ? Para;
// Public int para; if this line is not commented out and the previous line is commented out, an error will be reported below!
Public   Int PAR ()
{
Return   This . Para ??   0 ;
}