Unlike other languages where void is the absence of a type (and related value), void in Dart is a type, which is not
compatible with any other type, since it is conceptually a supertype of Object.
Therefore, in Dart it’s possible to write the following code:
void x = 3; // A variable of type void can be assigned a value of type int
void f(void x) => x; // A function that takes a void input argument and returns it as output
The variable x declared above, and the result of f(x) are effectively of type void. However, while
void variables can be declared, initialized and even computed, they cannot be used by any function that expects a non-void
type, by the very definition of void type.
print(x); // Error: This expression has type 'void' and can't be used.
print(f(x)); // Error: This expression has type 'void' and can't be used.
Therefore, assigning a value to a variable of type void is almost certainly a mistake.
For example, in the cases where the type is not known, the variable should have not been declared as void, but as
dynamic:
-
void variables can be assigned values of any type, but cannot be assigned to any non-void variable
-
dynamic variables, like void ones, can be assigned values of any type, and, unlike void variables, can
be assigned to any non-dynamic variable
Notice that, because void is a type, it can be used as a valid type parameter for a generic class, and that can lead to indirect or
"hidden" void assignments:
class Wrapper<T> {
T value;
Wrapper(this.value);
}
void main() {
Wrapper<void> w = Wrapper(3); // This is a "hidden" assignment to void
w.value = 3; // This is an explicit assignment to a field of type void
}
Another example of a "hidden" assignment to void is when assigning generic unconstrained type parameters in generic methods:
void f<T>(T x, T y) {
y = x; // This is a "hidden" assignment to void
T z;
z = x; // This too, is a "hidden" assignment to void
}
However, such scenarios are not reported by the rule because for any non-void type T, the assignment is valid. On the
other hand, calling the function f with anything other than void parameters will be reported as an issue:
f(42, 'a string'); // Both 1st and 2nd parameters are non compliant
