Null Safety in Dart

Null Safety

Null safety is a feature in the Dart programming language that helps developers to avoid null errors. This feature is called Sound Null Safety in dart. This allows developers to catch null errors at edit time.

Advantage Of Null Safety

• Write safe code.
• Reduce the chances of application crashes.
• Easy to find and fix bugs in code.

Example 1: Using Null In Variables

In the example below, the variable age is a int type. If you pass a null value to this variable, it will give an error instantly.

void main() { 
   int age = null; // give error
}

Error: Compilation failed.

Problem With Null

Programmers do have a lot of difficulties while handling null values. They forget that there are null values, so the program breaks. In real world null mostly acts as time bomb for programmers, which is ready to break the program.

Non-Nullable By Default

In Dart, variables and fields are non-nullable by default, which means that they cannot have a value null unless you explicitly allow it.

int productid = 20; // non-nullable
int productid = null; // give error

How To Declare Null Value

With dart sound null Safety, you cannot provide a null value by default. If you are 100% sure to use it, then you can use ? operator after the type declaration.

// Declaring a nullable variable by using ?
String? name;

This declares a variable name, which can be null or a string.

How To Assign Values To Nullable Variables

You can assign a value to nullable variables just like any other variable. However, you can also assign null to them.

void main(){
// Declaring a nullable variable by using ?
String? name;
// Assigning John to name
name = "John";
// Assigning null to name
name = null;
}

How To Use Nullable Variables

You can use nullable variables in many ways. Some of them are shown below:

• You can use if statement to check whether the variable is null or not.
• You can use ! operator, which returns null if the variable is null.
• You can use ?? operator to assign a default value if the variable is null.

void main(){
// Declaring a nullable variable by using ?
String? name;
// Assigning John to name
name = "John";
// Assigning null to name
name = null;
// Checking if name is null using if statement
if(name == null){
print("Name is null");
}
// Using ?? operator to assign a default value
String name1 = name ?? "Stranger";
print(name1);
// Using ! operator to return null if name is null
String name2 = name!;
print(name2);
}

Name is null
Stranger
Uncaught TypeError: Cannot read properties of null (reading 'toString')Error: TypeError: Cannot read properties of null (reading 'toString')

Example 2: Define List Of Nullable Items

You can also store null in list values. In this example, the items is a list of nullable integers. It can contain null values as well as integers.

void main() {
  // list of nullable ints
  List<int?> items = [1, 2, null, 4];
  print(items);
}

[1, 2, null, 4]

Example 3: Null Safety In Dart Functions

In this example, the function printAddress has a parameter address which is a String type. If you pass a null value to this function, it will give a edit-time error.

void printAddress(String address) {
  print(address);
}

void main() {
  printAddress(null); // give error
}

Error: Compilation failed.

Example 4: Define Function With Nullable Parameter

If you are 100% sure, then you can use ? for the type declaration. In this example, the function printAddress has a parameter address, which is a String? type. You can pass both null and string values to this function.

// address is a nullable string
void printAddress(String? address) {
  print(address);
}
void main() {
  // Passing null to printAddress
  printAddress(null); // Works
}

null

Example 5: Null Safety In Dart Classes

In the example, the class Person has a parameter name, which is a String type. If you pass a null value to this class, it will give a compile-time error.

class Person {
  String name;
  Person(this.name);
}

void main() {
  Person person = Person(null); // give error
}

Error: Compilation failed.

Example 6: Define Null To Class Property

In this example, the class Person has a parameter name, which is a String? type. You can pass both null and string values to this class. To define a nullable property in a class, you can use the ? operator after the type.

class Person {
  String? name;
  Person(this.name);
}

void main() {
  Person person = Person(null); // Works
}

null

Example 7: Working With Nullable Class Properties

In the example below, the Profile class has two nullable properties: name and bio. The printProfile method prints the name and bio of the profile. If the name or bio is null, it prints a default value instead.

  class Profile {
  String? name;
  String? bio;

  Profile(this.name, this.bio);

  void printProfile() {
    print("Name: ${name ?? "Unknown"}");
    print("Bio: ${bio ?? "None provided"}");
  }
}

void main() {
  // Create a profile with a name and bio
  Profile profile1 = Profile("John", "Software engineer and avid reader");
  profile1.printProfile();

  // Create a profile with only a name
  Profile profile2 = Profile("Jane", null);
  profile2.printProfile();

  // Create a profile with only a bio
  Profile profile3 = Profile(null, "Loves to travel and try new foods");
  profile3.printProfile();

  // Create a profile with no name or bio
  Profile profile4 = Profile(null, null);
  profile4.printProfile();
}

Name: John
Bio: Software engineer and avid reader
Name: Jane
Bio: None provided
Name: Unknown
Bio: Loves to travel and try new foods
Name: Unknown
Bio: None provided

Important Point In Dart Null Safety

• Null means no value.
• Common error in programming is caused due to null.
• Dart 2.12 introduced sound null Safety to solve null problems.
• Non-nullable type is confirmed never to be null.

Video

Watch our video on the null safety in Dart.