Why does C# 9.0 have a "with" keyword instead of using existing syntax?

Question

The C# 9.0 with keyword facilitates the creation of new records from existing records.

My question is: what technical reasons did or could motivate introducing this specific feature with a new keyword and syntax instead of just generating a new function with named parameters.

For example consider the example from the docs:

    Person person1 = new("Nancy", "Davolio") { PhoneNumbers = new string[1] };
    Person person2 = person1 with { FirstName = "John" };

Instead we could have had something like:

    var person1 = new Person("Nancy", "Davolio", new string[1]);
    var person2 = person1.With(FirstName:"John");

I assume that adding new syntax to a widely used language like C# based on careful consideration and that possibly it is documented somewhere.

Answer 1

How would you implement this With method? Let's make a try:

public Person With(string firstName = null, string lastName = null,
                   string[] phoneNumbers = null)
{
    return new (
        firstName ?? this.FirstName,
        lastName ?? this.LastName,
        phoneNumbers ?? this.PhoneNumbers
    );
}

The problem with this approach is that we are not able to set a field to null as in

Person person2 = person1 with { phoneNumbers = null };

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Therefore another approach is required. We could add overloads of every possible parameter combination

public Person With(string firstName);
public Person With(string lastName);
public Person With(string[] phoneNumbers);
public Person With(string firstName, string lastName);
public Person With(string firstName, string[] phoneNumbers);
public Person With(string lastName, string[] phoneNumbers);
public Person With(string firstName, string lastName, string[] phoneNumbers);

The problem is that the number of required overloads grows exponentially with the number of parameters:

2ⁿ - 1.

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Yet another approach would be to let the compiler handle a call like person1.With(FirstName:"John"). It would be syntactic sugar for compiler generated code. This introduces other problems:

• It does not behave as we would expect from other methods.
• How would Intellisense display the parameters of this pseudo method? As in our first attempt? This would be misleading because of this nulling problem.
• It could conflict with a user defined method of the same name.

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Some interesting links relating to the implementation of Non-destructive mutation (with keyword):

• Records as a collection of features #3137 ( MadsTorgersen, 1 Feb 2020).
• Nominal records #3226 (MadsTorgersen , 22 Feb 2020).
• C# 9.0 has introduced Records and Nondestructive mutation through with-expressions.

Answer 2

It was originally proposed with a With method.

• https://github.com/dotnet/csharplang/blob/00de1a1c7469c01c87e2e4d855ee24ff2a0eb203/proposals/records.md

In order to default parameter values from another object instance, there was a complicated new language feature proposed: caller-receiver parameters

The new design preserves the convenience of the old design but is much simpler for the compiler to implement.