undefined_instance_member_write

The field '{0}' of the type '{1}' is final, so it can't be assigned to.

The member '{0}' is declared in '{1}', but private names are visible only in their own library.

The setter '{0}' isn't defined for the type '{1}'.

The {0} '{1}' of the type '{2}' can't be assigned to.

There's a getter '{0}' for the type '{1}', but no setter.

The analyzer produces this diagnostic when an instance member is assigned, but neither the static type of the receiver nor any applicable extension has an accessible setter or non-final field with that name.

The following code produces this diagnostic because C doesn't have a setter named z:

class C {
  int x = 0;
}
​
void f(C c) {
  c.z = 0;
}

The following code produces this diagnostic because C has only a getter named x:

class C {
  int get x => 0;
}
​
void f(C c) {
  c.x = 0;
}

The following code produces this diagnostic because the field x is final:

class C {
  final int x = 0;
}
​
void f(C c) {
  c.x = 0;
}

If the setter you intend to use has a different name, then change the name to match:

class C {
  int x = 0;
}
​
void f(C c) {
  c.x = 0;
}

If the setter is declared in an extension that isn't imported, then import the library that declares the extension.

If the type has only a getter, then add a setter with the same name:

class C {
  int get x => 0;
​
  set x(int _) {}
}
​
void f(C c) {
  c.x = 0;
}

If the field is final, then make it non-final:

class C {
  int x = 0;
}
​
void f(C c) {
  c.x = 0;
}

Otherwise, remove the assignment.