C#: how to define an extension method as “with” in F#?

Question

F# has a convenient feature "with", example:

type Product = { Name:string; Price:int };;
let p = { Name="Test"; Price=42; };;
let p2 = { p with Name="Test2" };;

F# created keyword "with" as the record types are by default immutable.

Now, is it possible to define a similar extension in C#? seems it's a bit tricky, as in C# i'm not sure how to convert a string

Name="Test2"

to a delegate or expression?

Answer 1

public static T With<T, U>(this T obj, Expression<Func<T, U>> property, U value)
    where T : ICloneable {
    if (obj == null)
        throw new ArgumentNullException("obj");
    if (property == null)
        throw new ArgumentNullException("property");
    var memExpr = property.Body as MemberExpression;
    if (memExpr == null || !(memExpr.Member is PropertyInfo))
        throw new ArgumentException("Must refer to a property", "property");
    var copy = (T)obj.Clone();
    var propInfo = (PropertyInfo)memExpr.Member;
    propInfo.SetValue(copy, value, null);
    return copy;
}

public class Foo : ICloneable {
    public int Id { get; set; } 
    public string Bar { get; set; }
    object ICloneable.Clone() {
        return new Foo { Id = this.Id, Bar = this.Bar };
    }
}

public static void Test() {
    var foo = new Foo { Id = 1, Bar = "blah" };
    var newFoo = foo.With(x => x.Bar, "boo-ya");
    Console.WriteLine(newFoo.Bar); //boo-ya
}

Or, using a copy constructor:

public class Foo {
    public Foo(Foo other) {
        this.Id = other.Id;
        this.Bar = other.Bar;
    }
    public Foo() { }
    public int Id { get; set; } 
    public string Bar { get; set; }
}

public static void Test() {
    var foo = new Foo { Id = 1, Bar = "blah" };
    var newFoo = new Foo(foo) { Bar = "boo-ya" };
    Console.WriteLine(newFoo.Bar);
}

And a slight variation on George's excellent suggestion, that allows for multiple assignments:

public static T With<T>(this T obj, params Action<T>[] assignments)
    where T : ICloneable {
    if (obj == null)
        throw new ArgumentNullException("obj");
    if (assignments == null)
        throw new ArgumentNullException("assignments");
    var copy = (T)obj.Clone();
    foreach (var a in assignments) {
        a(copy);
    }
    return copy;
}

public static void Test() {
    var foo = new Foo { Id = 1, Bar = "blah" };
    var newFoo = foo.With(x => x.Id = 2, x => x.Bar = "boo-ya");
    Console.WriteLine(newFoo.Bar);
}

I would probably use the second one since (1) any general purpose solution is going to be unnecessarily slow and convoluted; (2) it has the closest syntax to what you want (and the syntax does what you expect); (3) F# copy-and-update expressions are implemented similarly.

Answer 2

Maybe something like this:

void Main()
{
    var NewProduct = ExistingProduct.With(P => P.Name = "Test2");
}

// Define other methods and classes here

public static class Extensions
{
    public T With<T>(this T Instance, Action<T> Act) where T : ICloneable
    {
        var Result = Instance.Clone();
        Act(Result);

        return Result;
    }
}

Answer 3

As an alternative to lambda function, you can use parameters with default values. The only minor issue is that you have to pick some default value that means do not change this parameter (for reference types), but null should be a safe choice:

class Product {
   public string Name { get; private set; }
   public int Price { get; private set; }
   public Product(string name, int price) {
     Name = name; Price = price;
   }

   // Creates a new product using the current values and changing
   // the values of the specified arguments to a new value
   public Product With(string name = null, int? price = null) {
     return new Product(name ?? Name, price ?? Price);
   }
 }

 // Then you can write:
 var prod2 = prod1.With(name = "New product");

You have to define the method yourself, but that's always the case (unless you're going to use reflection, which less efficient). I think the syntax is reasonably nice too. If you want to make it as nice as in F#, then you'll have to use F# :-)

Answer 4

There is no native ability to do this in C# short of an extension method, but at what cost? a and b are reference types and any suggestion that b is based ("with") on a causes immediate confusion as to how many objects we are working with. Is there only one? Is b a copy of a ? Does b point to a ?

C# is not F#.

Please see a previous SO question of mine as answered by Eric Lippert:

"Amongst my rules of thumb for writing clear code is: put all side effects in statements; non-statement expressions should have no side effects."

More fluent C# / .NET