In C #?? Operator and? Operator

In C #?? And? How the operator is used and what it means:

For the conversion of nullable data to non-nullable type data

？？ The operator means to take the copy?? To the left, if the left is null, take the assignment?? On the right.

？ is System. Abbreviated form of nullable, int? Represents a nullable integral type

       例如：int? a = 1 或者 int? b=null。Nullable类型具有一个HasValue的bool类型只读属性，当Nullable类型实例的该属性为true时，则表示该实例是非空实例，包含一个已知值Value，

HasValue为 false 时，访问Value属性将导致System.InvalidOperationException。such as int I = NULL So the compilation will go wrong, but if it is an int? I =null can be executed in normal

In C #?? Operator and? Operator