What is the "with" operator for in C#?

Question

I've came across this code:

var rectangle = new Rectangle(420, 69);
var newOne = rectangle with { Width = 420 }

I was wondering about with keyword in C# code. What is it for? And how can it be used? And what benefits does it bring to the language?

Answer 1

It's an operator used in expressions for easier duplication of an object, overriding some of it's public properties/fields (optional) with expression - MSDN

Currently it can only be used with records. But maybe there will be no such restriction in the future (assumption).

Here's an example how it can be used:

// Declaring a record with a public property and a private field
record WithOperatorTest
{
    private int _myPrivateField;

    public int MyProperty { get; set; }

    public void SetMyPrivateField(int a = 5)
    {
        _myPrivateField = a;
    }
}

Now let's see how with operator can be used:

var firstInstance = new WithOperatorTest
{
    MyProperty = 10
};
firstInstance.SetMyPrivateField(11);
var copiedInstance = firstInstance with { };
// now "copiedInstance" also has "MyProperty" set to 10 and "_myPrivateField" set to 11.

var thirdCopiedInstance = copiedInstance with { MyProperty = 100 };
// now "thirdCopiedInstance " also has "MyProperty" set to 100 and "_myPrivateField" set to 11.

thirdCopiedInstance.SetMyPrivateField(-1);
// now "thirdCopiedInstance " also has "MyProperty" set to 100 and "_myPrivateField" set to -1.

NOTE for reference types from MSDN:

In the case of a reference-type member, only the reference to a member instance is copied when an operand is copied. Both the copy and original operand have access to the same reference-type instance.

That logic can be modified by modifying the copy constructor of a record type. Quote from MSDN:

By default, the copy constructor is implicit, that is, compiler-generated. If you need to customize the record copy semantics, explicitly declare a copy constructor with the desired behavior.

protected WithOperatorTest(WithOperatorTest original)
{
   // Logic to copy reference types with new reference
}

And in terms of what benefits it gives, I think it should be quite obvious now, that it makes copying of instances much easier and convenient.