What is "as" in Dart language?

In this post we will take a look at "as" keyword in Dart with some example codes.

In Dart, the as keyword is used to perform type casting. Type casting is the process of converting a value of one data type to another data type. The as keyword is used to explicitly cast a value to a specified type.

Here’s an example of using the as keyword to cast an object to a specific type:

class Person {
  String name;
  int age;

  Person(this.name, this.age);
}

class Student extends Person {
  String school;

  Student(String name, int age, this.school) : super(name, age);
}

void main() {
  Person person = Student('Alice', 20, 'XYZ University');

  // Cast person to a Student object using the 'as' keyword
  Student student = person as Student;

  print(student.name);    // Output: Alice
  print(student.school);  // Output: XYZ University
}

In this example, we have a Person class and a Student class that extends Person. We create a Student object and assign it to a Person variable. We then use the as keyword to cast the Person object to a Student object.

If the value cannot be cast to the specified type, a TypeError is thrown at runtime. Here’s an example:

void main() {
  Person person = Person('Alice', 20);

  // Cast person to a Student object using the 'as' keyword
  Student student = person as Student;  // Throws a TypeError
}

In this example, we attempt to cast a Person object to a Student object using the as keyword. Since person is not an instance of Student, a TypeError is thrown at runtime.

The as keyword can also be used with nullable types. When casting a nullable type using the as keyword, if the value is null, the result is also null. Here’s an example:

void main() {
  Person? person = null;

  // Cast person to a Student object using the 'as' keyword
  Student? student = person as Student?;

  print(student);  // Output: null
}

In this example, we declare a nullable Person variable and assign it a value of null. We then attempt to cast it to a Student object using the as keyword. Since the value is null, the result is also null.

---

See Also