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?
CodePudding user response:
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.
CodePudding user response:
Basically, the with
operator will create a new object instance (records only, for now), by "coping values" from the "source" object and override some named properties in the destination object.
For example, instead of doing this:
var person = new Person("John", "Doe")
{
MiddleName = "Patrick"
};
var modifiedPerson = new Person(person.FirstName, person.LastName)
{
MiddleName = "William"
};
you can do this:
var modifiedPerson = person with
{
MiddleName = "Patrick"
};
Basically, you will write less code.
Use this source to get more details on the example above and official documentation for more examples.