What does the exclamation mark mean before a function call?

Question

I was following a PR for Flutter and came across this code:

if (chunkCallback != null) {   chunkCallback!(0, 100); } 

What does the exclamation mark mean after chunkCallback? Nothing I search on Google works.

Answer 1

"!" is a new dart operator for conversion from a nullable to a non-nullable type. Read here and here about sound null safety.

Use it only if you are absolutely sure that the value will never be null and do not confuse it with the conditional property access operator.

Answer 2

chunkCallback is a nullable reference to a function.

If you are sure that chunkCallback can't be null at runtime you can "Cast away nullability" by adding ! to it to make compiler happy

typedef WebOnlyImageCodecChunkCallback = void Function(     int cumulativeBytesLoaded, int expectedTotalBytes);  ...  class Class1 {   final WebOnlyImageCodecChunkCallback? chunkCallback;    Class1(this.chunkCallback);    void test() {     if (chunkCallback == null) {       throw Exception("chunkCallback is null");     }      chunkCallback!.call(0, 100);   } } 

Esentially, ! in this case is a syntactic sugar for

(chunkCallback as WebOnlyImageCodecChunkCallback).call(0, 100);