How do you define a constant that is computed at runtime in Dart?

In Dart, a constant that is computed at runtime cannot be declared using const, because const values must be known and evaluated at compile-time.
So, when we need a value that is determined only during program execution, we use final instead.

1. Using final for runtime constants #

final means the variable’s value can be set only once, but that value can be computed at runtime.

Example:

void main() {
  final currentTime = DateTime.now();
  print('Current time: $currentTime');
}

Here, DateTime.now() can only be evaluated at runtime, so using const would throw an error.
final ensures the value can’t be changed later, maintaining immutability after initialization.

2. Difference between const and final #

Example comparison:

const pi = 3.14;               // Known at compile time
final time = DateTime.now();   // Known only at runtime

3. Real-world example #

In one of my Flutter apps, I used a final variable to store the app’s session start time:

class AppSession {
  final startTime = DateTime.now();

  void printSessionStart() {
    print('Session started at: $startTime');
  }
}

This ensured the start time was computed only once when the session began and couldn’t be modified afterward — but it wasn’t a compile-time constant.

4. Challenge faced #

Initially, I mistakenly used const for values that depended on runtime operations like DateTime.now() or Platform.environment.
That caused compile-time errors like:

“Constant expressions must be compile-time constants.”

After understanding this, I switched to final, which fixed the issue.

5. Limitation #

The only limitation with final is that it’s runtime-only — meaning you can’t use it in a const context, like in annotations or as default parameter values.

Summary #

In Dart, if you need a constant value that’s computed during program execution, use final — not const.
const is for compile-time constants, while final is for runtime constants that you want to set once and never change.