The error message “the argument type ‘bool?’ can’t be assigned to the parameter type ‘bool'” is encountered when trying to assign a nullable boolean value (bool?) to a non-nullable boolean variable (bool). In Dart, the type ‘bool?’ represents a boolean value that can be either true, false, or null. On the other hand, the type ‘bool’ can only be true or false without the possibility of being null.
To resolve this error, you need to ensure that you are assigning a non-null boolean value to the boolean variable or parameter. Here are a few examples to illustrate the issue and its solution:
Example 1: Assigning a Nullable Boolean Value to a Non-Nullable Boolean Variable
bool? nullableBool = true;
bool nonNullableBool = nullableBool; // This will cause the error
In the above example, ‘nullableBool’ is a nullable boolean variable that holds the value true. The error occurs when attempting to assign the nullable boolean value to the non-nullable boolean variable ‘nonNullableBool’. To fix this error, you can either assign a non-nullable boolean value or handle the nullable value appropriately.
Example 2: Fixing the Error by Handling Nullable Value
bool? nullableBool = true;
bool nonNullableBool = nullableBool ?? false; // Assign a default value if nullableBool is null
In this example, we use the null-aware operator ‘??’ to assign a default value (false) to the non-nullable boolean variable ‘nonNullableBool’ when ‘nullableBool’ is null. This ensures that ‘nonNullableBool’ always holds a non-null boolean value.
Example 3: Fixing the Error by Asserting Non-Nullability
bool? nullableBool = true;
bool nonNullableBool = nullableBool!; // Assert that nullableBool is not null
In this example, we use the null assertion operator ‘!’ to assert that ‘nullableBool’ is not null. This is appropriate when you are confident that the value is not null, and it helps avoid the error. However, be cautious while using this operator, as it can lead to runtime null errors if the value is actually null.