Section 13.9 introduced nullable types and C# 6’s null-conditional operator (?.), which checks whether a reference is null before using it to call a method or access a property. C# 6 provides another null-conditional operator, ?[], for arrays and for collections that support the [] indexing operator.

Assume that a class Employee has a decimal Salary property and that an app defines a List<Employee> named employees. The statement

decimal? salary = employees?[0]?.Salary;

uses both null-conditional operators and executes as follows:

• First the ?[] operator determines whether employees is null. If so, the expression employees?[0]?.Salary short circuits—that is, it terminates immediately—and the expression evaluates to null. In the preceding statement, this is assigned to the nullable decimal variable salary. If employees is not null, employees?[0] accesses the element at position 0 of the List<Employee>.

• Element 0 could be null or a reference to an Employee object, so we use the ?. operator to check whether employees?[0] is null. If so, once again the entire expression evaluates to null, which is assigned to the nullable decimal variable salary; otherwise, the property Salary’s value is assigned to salary.

Note in the preceding statement that salary must be declared as a nullable type, because the expression employees?[0]?.Salary can return null or a decimal value.