Nullable variables

Nullable<int> nullableInt = 10;
int? a = 10;
Nullable<double> nullableDouble = 3.14;
double? b = 3.14;
Nullable<bool> nullableBool = null;
bool? c = false;
Nullable<char> nullableChar = 'a';
char? d = 'd';
Nullable<int>[] arrayOfNullableInts = new int?[10];

int x = foo() ?? 100;
if foo's return value is nullable And
then if it is null , it will be replaced with 100