C# 9 With Expressions

A with expression allows for "non-destructive mutation", designed to produce a copy of the receiver expression with modifications in assignments.

• A with expression is not permitted as a statement.
• A valid with expression has a receiver with a non-void type.
• The receiver type must be a record.
• On the right-hand side of the with expression is a member_initializer_list with a sequence of assignments to an identifier, which must be an accessible instance field or property of the receiver's type.

Let's suppose you have a record with many properties, and you want to create a new record with only one or a few changes.

1. You would have to write a constructor taking each property from the existing record
2. then you will specify that one change or few changes. You can use the with keyword and specify your changes on the right side.

public class Program
{
    public static void Main(string[] args)
    {
        Customer customer = new Customer
        {
            Name = "Mark",
            Age = 34,
            Address = "23 ashdown",
            Country = "UK"
        };

        var newCustomer = customer with { Name = "John" };

        Console.WriteLine(customer.Name);
        Console.WriteLine(newCustomer.Name);
    }
}

record Customer
{
    public string Name { get; set; }
    public int Age { get; set; }
    public string Address { get; set; }
    public string Country { get; set; }
}

As you can see in the above code, we created a customer record, and using the with keyword another customer record is created only changing its name and all other properties remain as is.

• It will take all the values of customer, and creates a new record newCustomer with the Name changed.
• With-expressions use object initializer syntax to state what’s different in the new object from the old object. You can specify multiple properties.
• The with expression causes the copy constructor to get called and then applies the object initializer on top to change the properties accordingly.