throw

Learn to throw exceptions with throw.

throw

To throw your own error use throw. You can throw an Exception object or a string. For example throw Exception("negative number");. This is used to stop the program and warn the caller when a condition is not met.

The thrown value propagates up the call chain until a catch is found. If none is found the program ends. Typical use: validate a function parameter, throw if invalid. This lets the caller know and fix the issue. Throw is an expression, so it can be used in functions that return a value.

Conditional throw

Throw is most often used with an if. When the condition fails an error is thrown. For example an age check: if (age < 0) throw ArgumentError("age negative");. This is a guard at the start of a function.

Throw is an expression that returns a value; so it works with the ternary operator too. final result = cond ? value : throw Exception("Invalid");. This also works in a switch expression; _ => throw Exception("unknown"). Throw being an expression got stronger with Dart 3; it can now appear wherever a value is expected.

Exercise Your turn: validate a negative age

In main assign final age = -5;. Use an if to throw an ArgumentError when age < 0. Wrap the call in a try block and print the error with catch (e).

Show example solution

void main() {
  try {
    final age = -5;
    if (age < 0) throw ArgumentError("age negative");
  } catch (e) {
    print(e);
  }
}

Test yourself

The throw call

if (number < 0) {
  ____ Exception("negative number");
}

Show answer

if (number < 0) {
  throw Exception("negative number");
}

What do you do when a condition is not met?
```
if (age < 0) {
  ____ ArgumentError("age negative");
}
```
- throw
- return
- break
- continue
Show answer

return — throw raises; return gives a value, break exits a loop, continue skips a loop.
Match throw

throw

raises an error
Exception

general error class
ArgumentError

invalid parameter
if

tests a condition

Next lesson: finally

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