Dart null safety conditional statement?

Question

So I have a class Cache which has a method compare that returns a bool.

I have an instance of this class that is nullable.

Cache? recent;

I want to executed a piece of code when recent is not null and compare returns false

Without null safety I would have just done

if(recent!=null && !recent.compare()){ //code }

How to do the same with null safety enabled?

When I try above with null safety I get

The method 'compare' can't be unconditionally invoked because the receiver can be 'null'. Try making the call conditional (using '?.') or adding a null check to the target ('!')

When I try below

if(!recent?.compare()){ //code }

It gives me

A nullable expression can't be used as a condition. Try checking that the value isn't 'null' before using it as a condition.

See stackoverflow.com/q/65035574 for why this happens.

jamesdlin
– jamesdlin

2021-05-24 10:33:54 +00:00
Commented May 24, 2021 at 10:33 — jamesdlin
– jamesdlin, Commented May 24, 2021 at 10:33

CopsOnRoad · Accepted Answer · 2021-05-24 09:53:46Z

5

You can solve this problem either by

Using local variable (recommended)

var r = recent; // Local variable if (r != null && !r.compare()) { // ... }

Using Bang operator (!)
```
if (!recent!.compare()) { // ... } 
```

answered May 24, 2021 at 9:53

CopsOnRoad

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4 Comments

julemand101 Over a year ago

Or: if(!(recent?.compare() ?? true))

CopsOnRoad Over a year ago

@julemand101 Right! You can also do that. It's difficult to find out all the possible cases ;) But in that case, you need to provide a default value which may not be the best way in general.

coder_86 Over a year ago

julemand101 I understand your approach, wasn't able to think about that :(. @CopsOnRoad I don't understand why the local variable approach works, and also what is Bang operator. Thanks for your answer, I will read about these.

CopsOnRoad Over a year ago

@coder_86 Since you're using Dart null-safety, you should use one of the approaches I've mentioned or even julemand101's. For more detailed answer, you can take a look here

Abdulfattah · Accepted Answer · 2023-04-11 17:11:16Z

You can also declare the type as being dynamic recent; Which will match any 'data type' supplied at any given moment

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