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The null-aware index operator (?[]) conditionally accesses an element within a subscriptable object (such as a List or Map) only if that object is not null. If the receiver is null, the expression evaluates to null immediately; otherwise, it performs the standard index operation.

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Syntax

receiver?[key]

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Operational Semantics

When the expression e1?[e2] is executed:
  1. Receiver Evaluation: The expression e1 (the receiver) is evaluated.
  2. Null Check:
    • If e1 evaluates to null, the operation short-circuits. The index expression e2 is not evaluated, and the entire expression returns null.
    • If e1 is not null, e2 is evaluated, and the standard [] operator is invoked on e1 with the result of e2.

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Logical Equivalence

The operator is syntactic sugar that ensures the receiver is evaluated exactly once. It is logically equivalent to assigning the receiver to a temporary variable before checking for null: 
// Logical representation of: result = receiver?[index];
var tmp = receiver;
var result = (tmp == null) ? null : tmp[index];

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Type Implications

The return type of a null-aware index operation is always nullable. The operator “lifts” the element type of the collection into a nullable type. Given a receiver of type List<T>?:
  • Direct Access: list[0] results in a compile-time error because the receiver might be null.
  • Null-Aware Access: list?[0] is valid and returns type T?.
Even if the underlying collection holds non-nullable types (T), the result of the expression must be T? to account for the case where the receiver itself is null.

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Implementation Example

void main() {
  List<int>? nullableList = null;
  List<int> definedList = [10, 20, 30];

  // Scenario 1: Receiver is null
  // The expression short-circuits and returns null.
  int? result1 = nullableList?[0]; 
  assert(result1 == null);

  // Scenario 2: Receiver is not null
  // The standard index operator is called.
  int? result2 = definedList?[1];
  assert(result2 == 20);
  
  // Scenario 3: Short-circuiting side effects
  // Because nullableList is null, getIndex() is never called.
  int getIndex() {
    print("Calculating index...");
    return 0;
  }
  var result3 = nullableList?[getIndex()]; 
}
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