Dart: Check Type T of generic function

July 18, 2023

I have a generic function. I’m trying to return a value based on the type T,
however, my function never seems to match any types (see code below):

int _getCollection<T>() {
    print(T); // the actual type
    print(T is ClockData); // ClockData is a class I created

    // always returns false when passing _getCollection<ClockData>()
    if (T is ClockData) { 
      return 1;
    } else if (T is UserData) {
      return 2;
    } else {
      throw Exception();
    }
  }

>Solution :

is operator will check if left operand is an instance of right operand. T is a type parameter and not an instance of ClockData. You can use == operator to compare those:

int _getCollection<T>() {
  if (T == ClockData) { 
    return 1;
  } else if (T == UserData) {
    return 2;
  } else {
    throw Exception();
  }
}

Please not this checks for identity and will not support subtypes. If you need to support type inheritance (passing subclasses of ClockData as T) you can use a trick like this:

int _getCollection<T>() {
  if (<T>[] is List<ClockData>) {
    return 1;
  } else if (<T>[] is List<UserData>) {
    return 2;
  } else {
    throw Exception();
  }
}