Cascade Notation #

Here is a neat bit of syntax in Dart that can save some typing. If you access a field or method on an object with .. instead of . then it will return the object instead of the value from the field/method. The .. is known as cascade notation, and it allows you to chain multiple operations on the same object.

It is best illustrated with an example. Say you have a counter class, and you want to create an instance, increment the value 3 times then print the result.

Dart

class Counter {
  var _value = 0;
  get count => _value;

  int increment() => _value++;
}

main() {
  var counter = Counter();
  counter.increment();
  counter.increment();
  counter.increment();
  print(counter.count);
}

JavaScript

class Counter {
  #value = 0;
  get count() {
    return this.#value;
  }
  increment() {
    this.#value++;
  }
}

var counter = new Counter();
counter.increment();
counter.increment();
counter.increment();
console.log(counter.count);

TypeScript

class Counter {
  private _value: number = 0;
  get count(): number {
    return this._value;
  }
  increment(): void {
    this._value++;
  }
}

var counter = new Counter();
counter.increment();
counter.increment();
counter.increment();
console.log(counter.count);

public class Counter {
  public int Count { get; private set; } = 0;
  public void Increment() {
    Count++;
  }
}

var counter = new Counter();
counter.Increment();
counter.Increment();
counter.Increment();

Console.WriteLine(counter.Count);

Java

public class Test {
  public static class Counter {
    private int _count = 0;
    public int getCount() {
      return _count;
    }
    public void increment() {
      _count++;
    }
  }

  public static void main(String[] args) {
    var counter = new Counter();
    counter.increment();
    counter.increment();
    counter.increment();

    System.out.println(counter.getCount());
  }
}

For the increment and print part, you would probably do this (or write a loop):

var counter = Counter();
counter.increment();
counter.increment();
counter.increment();
print(counter.value);

Using cascade notation, you don’t have to keep repeating counter. You can just write it like:

print(
  Counter()
    ..increment()
    ..increment()
    ..increment()
);

The .. can also be used to set values for fields.

Dart

class Contact {
    String? name;
    String? email;
    String? phone;
}

var contact = Contact()
    ..name = "Joe Doe"
    ..email = "joe@example.com"
    ..phone = "12345678";

public class Contact {
    public string Name { get; set; }
    public string Email { get; set; }
    public string Phone { get; set; }
}

var contact = new Contact() {
    Name = "Joe Doe",
    Email = "joe@example.com",
    Phone = "12345678",
};

Property initialization in C# can only be used when instantiating the class. If you want to update the fields on an existing instance then you would have to do something like the TypeScript example.

TypeScript

class Contact {
  public name?: string;
  public email?: string;
  public phone?: string;
}

var contact = new Contact();
contact.name = "Joe Doe"
contact.email = "joe@example.com"
contact.phone = "12345678";

However, making all fields final and using named parameters should be preferred when possible.

Dart

class Contact {
    final String name;
    final String email;
    final String phone;

    Contact({
        required this.name,
        required this.email,
        required this.phone,
    });
}

var contact = Contact(
    name: "Joe Doe",
    email: "joe@example.com",
    phone: "12345678",
);

Official documentation