What is non-destructive mutation in C# 9.0?

Non-destructive mutation provides a way to change immutable properties and fields of a record type. This is achieved by using the with expression, which creates a copy of the record type and replaces the existing values with the specified values in the copy.

Explanation

The compiler employs the copy constructor and the clone method to implement the non-destructive mutation of the record types. To customize the copy behavior, the users can write their own copy constructors, in which case the compile will not create a copy constructor.

The positional properties or properties created using standard property syntax work with the with expression. The non-positional properties with the init and set accessors also work with the with expression.

The copy of the record is a shallow copy. This means that for reference-type properties, only the reference is copied, and both the record type and its copy point to the same entity.

Example

The following example demonstrates how to use the with expression to create copies of the record and mutate properties.

  • The program creates an object student1 and creates its copy student2 using the with expression.
  • The program first changes FirstName to Mark and then the Marks to 50.
  • The second and third outputs display the changed values for the FirstName and Marks fields.
  • The == operator returns false in line 26 because the field values are different for student1 and student2.
  • The == operator returns true in the output of line 30 because the field values in the new copy student2 are identical to the field values of student1.

The == operator measures value equality in the record by default.

using System;
					
public class Program
{
	
	public record Student(string FirstName, string LastName)
	{
    public int Marks { get; init; }
	}
	public static void Main()
	{
		// create an object
		Student student1 = new("Sam", "Johnson") { Marks = 100 };
		// display object
		Console.WriteLine(student1);
		
		// create a copy of the object
		Student student2 = student1 with { FirstName = "Mark" };
		Console.WriteLine(student2);
		// change a non positional property
		student2 = student1 with { Marks = 50 };
		Console.WriteLine(student2);
		// display the two objects are different
		Console.WriteLine(student1 == student2); 
		
		// create a copy with identical fields
		student2 = student1 with { };
		Console.WriteLine(student1 == student2); 
	}
}

Output

Student { FirstName = Sam, LastName = Johnson, Marks = 100 }
Student { FirstName = Mark, LastName = Johnson, Marks = 100 }
Student { FirstName = Sam, LastName = Johnson, Marks = 50 }
False
True

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