Nullable types supported by c#2.0 or later

The single question mark---is used to set the initial value of the variable, assigning the variable (int type) to NULL instead of 0!
Double question mark---used to judge and assign a value, first determine whether the current variable is null, if it is possible to assign a new value, otherwise skip!

public int? Para

public int para; If you do not comment out this line, and the previous line commented out, the following will be an error!

public int par ()

{

Return This.para?? 0;

}

Nullable types supported by c#2.0 or later