[FIXED] What is the "with" operator for in C#?

Issue

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?

Solution

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.

Answered By - David Oganov

Answer Checked By - Marilyn (PHPFixing Volunteer)