Type Promotion in Dart

Type Promotion In Dart

Type promotion in dart means that dart automatically converts a value of one type to another type. Dart does this when it knows that the value is of a specific type.

How Type Promotion Works In Dart?

Types Promotion in Dart works in the following ways:

• Promoting from general types to specific subtypes.
• Promoting from nullable types to non-nullable types.

Example 1: Promoting From General Types To Specific Subtypes

In this example, the variable name is declared as an Object. The Object class doesn’t have a .length property. Variable name gets promoted from Object to String so that you can access the .length property of the String class.

void main(){
Object name = "Pratik";
// print(name.length) will not work because Dart doesn't know that name is a String

if(name is String) {
// name promoted from Object to String
  print("The length of name is ${name.length}");
}
}

The length of name is 6

Example 2: Type Promotion In Dart

In this example, the variable result is declared as a String. In both if and else blocks, the variable result is assigned a value of type String. Therefore, the variable result is automatically promoted to a non-nullable type String.

void main(){
// result is a String
String result;
// result is promoted to a non-nullable type String
if(DateTime.now().hour < 12) {
  result = "Good Morning";
} else {
  result = "Good Afternoon";
}
// display the result
print("Result is $result");
print("Length of result is ${result.length}");
}

Result is Good Afternoon
Length of result is 15

Example 3: Type Promotion With Nullable To Non-Nullable Type

In Dart, you can also throw an exception if the variable is null. In this example, method printLength, takes a String type parameter. If the parameter is null, then it will throw an exception.

// method to print the length of the text
void printLength(String? text){
    if(text == null) {
        throw Exception("The text is null");
    }
    print("Length of text is ${text.length}");
}
// main method
void main() {
    printLength("Hello");
}

Length of text is 5

Example 4: Type Promotion With Nullable Type To Non-Nullable Type

In this example, the variable value contains a value of type String or null. The variable value is promoted to a non-nullable type String in the if block. If the variable value is null, then the else block is executed.

// importing dart:math library
import 'dart:math';
// creating a class DataProvider
class DataProvider{
    // creating a method stringorNull
    String? get stringorNull => Random().nextBool() ? "Hello" : null;

    // creating a method myMethod
    void myMethod(){
        String? value = stringorNull;
        // checking if value String or not
        if(value is String){
            print("The length of value is ${value.length}");
        }else{
            print("The value is not string.");
        }

    }
}
// main method
void main() {
    DataProvider().myMethod();
}

The length of value is 5

Video

Watch our video on the type promotion in Dart.