It means that the value type in question is a nullable type . Nullable types are instances of the System. Nullable struct. A nullable type can represent the correct range of values for its underlying value type, plus an additional null value.22 Nis 2010
Read moreWhat does a question mark after a variable mean?
The question mark in JavaScript is commonly used as conditional operator — called ternary operator when used with a colon (:) and a question mark (?) — to assign a variable name conditionally .
Read moreWhat is the question mark in Dart?
Dart offers some handy operators for dealing with values that might be null. One is the ??= assignment operator, which assigns a value to a variable only if that variable is currently null: int a; // The initial value of a is null.
Read more