! is the Null-Forgiving Operator. To be specific it has two main effects:
it changes the type of the expression (in this case it modifies Entity) from a nullable type into a non-nullable type; (for example, object? becomes object)
it supresses nullability related warnings, which can hide other conversions
This seems to come up particularly with type parameters:
IEnumerable? maybeListOfMaybeItems = new object[] { 1, 2, 3 };
// inferred as IEnumerable
var listOfMaybeItems = maybeListOfMaybeItems!;
// no warning given, because ! supresses nullability warnings
IEnumerable
