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The is! operator is a binary type test operator used to assert that an object is not an instance of a specified type or any of its subtypes. It evaluates to a bool, returning true if the runtime type of the left-hand operand is incompatible with the type specified on the right-hand side.

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Syntax

expression is! Type
  • expression: The variable or expression whose runtime type is being evaluated.
  • Type: The data type (class, mixin, enum, or interface) being checked against.

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Operational Semantics

The operator compares the runtime type of the expression against Type adhering to Dart’s type hierarchy rules.
  1. Subtype Check: If the runtime type of expression is a subtype of Type (including Type itself), the operator evaluates to false.
  2. Incompatibility: If the runtime type of expression is not a subtype of Type, the operator evaluates to true.

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Logical Equivalence

The is! operator serves as syntactic sugar for the logical negation of the is operator. It improves readability by eliminating the need for parentheses around the type check. 
// The following expressions are semantically identical:
if (variable is! String) { ... }

if (!(variable is String)) { ... }

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Nullability Rules

The evaluation of is! differs based on the nullability of the target Type.
  • Non-nullable Target (T): null is not an instance of a non-nullable type. Therefore, null is! T evaluates to true.
  • Nullable Target (T?): null is a valid instance of a nullable type. Therefore, null is! T? evaluates to false.

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Evaluation Examples

The following code demonstrates the operator’s logic against inheritance hierarchies and null values: 
void main() {
  Object value = 123; // Runtime type is int
  var empty = null;

  // --- Type Hierarchy Checks 
  
  // true: int is not a String
  print(value is! String); 

  // false: int is an int
  print(value is! int);    

  // false: int is a subtype of num
  print(value is! num);    

  // --- Nullability Checks 

  // true: null is not a non-nullable int
  print(empty is! int);  

  // false: null is a valid nullable int (int?)
  print(empty is! int?); 
}
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