What's new in C# 12 | New Features of C# 12

The following are the new features that introduce in C# 12. Check out the new C# 12 preview features!

1.     Default value for Lambda Expressions (Optional parameters in Lambda Expressions)

2.     Primary Constructors

3.     Alias any type

4.     Interceptors

1) Default Value for Lambda Expressions:

In C# 12, you can provide default values for parameters on lambda expressions.


For example,

var IncrementValueBy = (int a, int b = 1) => a + b;

Console.WriteLine(IncrementValueBy(1)); // 2

Console.WriteLine(IncrementValueBy (1, 2)); // 3


2) Primary Constructors:

Primary constructor parameters are in scope for the entire body of the class.

In the Primary constructors, you can now create primary constructors in any class and struct.

In the Primary constructors, you are no longer restricted to record types.


For example,

Let's start to create Primary Constructors with a User class.

Here's how you would define a User class using a Primary Constructor,

    public class User(string name, int age)


        public string Name { get; init; } = name;

        public int Age { get; init; } = age;



In the above User class, we have defined two properties: Name, and Age. Also, we have used the init keyword to make the properties read-only and initialized them directly within the ‘Primary Constructors’ parameter list.


Here is how to would create a new User object using the Primary Constructor i.e.

var user = new User("Anil Singh", 35);


The User object is created with Name and Age which are used to initialize the corresponding properties.


Noted Point: The class 'User' doesn't have a constructor method with a parameter-less constructor. It will get a compiler error if we try to create an object using the default constructor.


Example 2,

All bank accounts have properties for the account number and an owner.

The following code initializes two properties in the primary constructor parameters:

    public class BankAccount(string accountID, string owner)


        public string AccountID { get; } = accountID;

        public string Owner { get; } = owner;

        public override string ToString() => $"Account ID: {AccountID}, Owner: {Owner}";



For example, the BankAccount class has specific requirements for the owner and accountID parameters: The owner must not be null or whitespace, and the accountID must be a string containing 10 digits. You can add this validation when you assign the corresponding properties:

    public class BankAccount(string accountID, string owner)


        public string AccountID { get; } = ValidAccountNumber(accountID) ? accountID : throw new ArgumentException("Invalid account number", nameof(accountID));

        public string Owner { get; } = string.IsNullOrWhiteSpace(owner) ? throw new ArgumentException("Owner name cannot be empty or null", nameof(owner)) : owner;

        public override string ToString() => $"Account ID: {AccountID}, Owner: {Owner}";

        public static bool ValidAccountNumber(string accountID) => accountID?.Length == 10 && accountID.All(c => char.IsDigit(c));


In the above example, we can see how we can validate the constructor parameters before assigning them to the properties. Also, we can use built-in methods, like String.IsNullOrWhiteSpace(String), or your own validation method, just like the above ValidAccountNumber method.


3) Alias any type:

Now, we can use the using alias directive to alias any type, not just named types. That means we can create semantic aliases for tuple types, array types, pointer types, or other unsafe types.


For example, we can define an alias for the tuple as given below,

    using point3D = (int x, int y, int z);


    public void AliasAnyType()


        point3D point3D = (2, 4, 6);

        Console.WriteLine($"{point3D.x},{point3D.y},{point3D.z}"); //2,4,6



In addition to this overview, you can also find detailed documentation in the What’s New in C# 12 article on Microsoft.


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