the null coalescing operator is used for set the value when object is null. if object have some value that nothing change and still have their default value they have.

 string str = "i am string";
            string message = str ?? "it is null";

the message have same value as str variable because str not null. if str is null that message have value “it is null”; as declared in statement.

coalescing operator does not work on nullable operator such as int?