I have a scenario in dart where I have a generic type T, which may be <void> at runtime. I need to test if a dynamic variable I get back from a service is of that generic type.
However the following test using the is operator fails when T is void, and I’m struggling to understand why. Can anyone explain this, and suggest a way I can test for this?
void main() {
test<void>();
}
void test<T>() {
dynamic foo = 42;
print('$foo is void: ${foo is T}');
}
42 is void: true
>Solution :
In Dart, the is operator checks whether a given object is an instance of a specific type, or a subtype of that type. However, since void is a special type in Dart, it cannot be used as a type parameter for a generic class or function. This means that the type T cannot be replaced with void at runtime.
If you want to test whether a dynamic variable is of a generic type, you can use the Type class instead. Here’s an example:
void main() {
test<void>();
}
void test<T>() {
dynamic foo = 42;
print('$foo is of type $T: ${foo.runtimeType == T}');
}
In this code, I’m using runtimeType to get the runtime type of the foo variable, and comparing it to the type T using the == operator. This will return true if the runtime type of foo is the same as T, and false otherwise. Note that this approach will work with any type, including void.