isAssignableTo abstract method

Checks if this type is assignable to another type.

Parameters:

• other: The target type to check assignability to

Returns:

• true if this type can be assigned to other
• false otherwise

Assignability Rules

This method implements Dart's type assignability rules:

• Subclass to superclass: ✅ Allowed
• Superclass to subclass: ❌ Not allowed
• Class to implemented interface: ✅ Allowed
• Interface to implementing class: ❌ Not allowed
• Identical types: ✅ Allowed
• Unrelated types: ❌ Not allowed

Type Assignability Table:

DART TYPE ASSIGNABILITY TABLE
────────────────────────────────────────────────────────────────────────────
From (A) → To (B)                   A.isAssignableTo(B)   Valid?   Notes
────────────────────────────────────────────────────────────────────────────
String    → Object                 ✅ true               ✅      String extends Object
Object    → String                 ❌ false              ❌      Superclass to subclass not allowed
int       → num                    ✅ true               ✅      int is a subtype of num
num       → int                    ❌ false              ❌      Can't assign broader to narrower
List<int> → List<int>              ✅ true               ✅      Identical type
List<S>   → List<T>                ❌ false              ❌      Dart generics are invariant
List<int> → List<dynamic>          ❌ false              ❌      Invariant generics
B         → A (B extends A)        ✅ true               ✅      Subclass to superclass: OK
C         → A (no relation)        ❌ false              ❌      Unrelated types
Class     → Interface (implements) ✅ true               ✅      Implements interface
Class     → Mixin (with mixin)     ✅ true               ✅      Class includes mixin
anything  → dynamic                ✅ true               ✅      Everything is assignable to dynamic
dynamic   → anything               ✅ true (unchecked)   ✅      Allowed but unsafe
anything  → Never                  ❌ false              ❌      Can't assign anything to Never
Never     → anything               ✅ true               ✅      Never fits anywhere
────────────────────────────────────────────────────────────────────────────

RULE OF THUMB:
A.isAssignableTo(B) → Can you do: B b = A();
✓ Subclass → Superclass: OK
✗ Superclass → Subclass: Not OK
✓ Class → Interface it implements: OK
✗ Interface → Class: Not OK
✓ Identical types: OK
✗ Unrelated types: Not OK

Example

final stringClass = Class.forType<String>();
final objectClass = Class.forType<Object>();

print(stringClass.isAssignableTo(objectClass)); // true - String → Object
print(objectClass.isAssignableTo(stringClass)); // false - Object → String

Relationship to isAssignableFrom

This is the inverse of isAssignableFrom:

• A.isAssignableTo(B) ≡ B.isAssignableFrom(A)
• Use isAssignableTo when asking "Can I assign this to that?"
• Use isAssignableFrom when asking "Can that be assigned to this?"