Productive Rage

Dan's techie ramblings

Implementing F#-inspired "with" updates for immutable classes in C#

I've been prototyping a data service for a product at work that communicates with immutable types and one of the feedback comments was a question as to whether the classes supported a flexible F#-esque "with" method that would allow multiple properties to be changed without the garbage collection churn of creating intermediate references for each individual property (since, of course, the property values aren't actually changed on an instance, a new instance is generated that reflects the requested changes).

To pull an example straight from the excellent F# for fun and profit site:

let p1 = {first="Alice"; last="Jones"}
let p2 = {p1 with last="Smith"}

This creates a new record p2 that takes p1 and changes one of the fields. Multiple fields may be altered in one use "with" statement

let p2 = {p1 with first="John";last="Smith"}

To start with a very simple example in C#, take the following class:

public class RoleDetails
{
  public RoleDetails(string title, DateTime startDate, DateTime? endDateIfAny)
  {
    Title = title;
    StartDate = startDate;
    EndDateIfAny = endDateIfAny;
  }

  public string Title { get; private set; }
  public DateTime StartDate { get; private set; }
  public DateTime? EndDateIfAny { get; private set; }
}

This is a very close parallel to the F# record type since it just assigns read-only properties (they're not strictly read-only since they don't use the "readonly" keyword but they're not externally alterable and are only set once within the class so it's close enough).

If I was writing something like this for real use, I would probably try to make more guarantees.. or at least, document behaviour. Something like:

public class RoleDetails
{
  public RoleDetails(string title, DateTime startDate, DateTime? endDateIfAny)
  {
    if (string.IsNullOrWhiteSpace(title))
      throw new ArgumentException("title");
    if ((endDateIfAny != null) && (endDateIfAny <= startDate))
      throw new ArgumentException("title");

    Title = title.Trim();
    StartDate = startDate;
    EndDateIfAny = endDateIfAny;
  }

  /// <summary>
  /// This will never be null or blank, it will not have any leading or trailing whitespace
  /// </summary>
  public string Title { get; private set; }

  public DateTime StartDate { get; private set; }

  /// <summary>
  /// If non-null, this will greater than the StartDate
  /// </summary>
  public DateTime? EndDateIfAny { get; private set; }
}

As I've said before, this validation and commenting is really a poor substitute for code contracts which would allow for compile time detection of invalid data rather than relying on runtime exceptions (speaking of which, I need to give the .net code contracts solution another go - last time I got stuck in I hit some problems which hopefully they've ironed out by now).

Another variation on the "aggressive validation" illustrated above would be a type that represents a non-blank string to prevent duplicating calls to IsNullOrWhiteSpace and trim. This concept could be taken even further to "strongly type" string values so that a "Title" can not be passed into a function that expects a "Notes" string value, for example. This is far from an original idea but it was something I was experimenting again with recently.

Incidentally, there is talk of a future version of C# getting a record type which would reduce boilerplate code when defining simple immutable types. For example (from the InfoQ article Easier Immutable Objects in C# 6 and VB 12) -

public record class Cartesian(double x: X, double y: Y);

This will define an immutable class with two read-only properties that are set through a constructor call. This future C# specification is also apparently going to allow read-only auto properties - so in my RoleDetails class above instead of "get; private set;" properties, which are externally unalterable but could actually be changed within the instance, the properties could be truly readonly. This is possible currently but it requires a private readonly field and a property with a getter that returns that field's value, which is even more boring boilerplate.

The obvious, verbose and potentially more GC-churny way

To prevent callers from having to call the constructor every time a property needs to be altered, update methods for each "mutable" property may be added (they don't really mutate the values since a new instance is returned rather than the value changed on the current instance). This prevents the caller from having to repeat all of the constructor arguments that are not to be changed whenever one property needs altering. Forcing callers to call constructors in this way is particularly annoying if a constructor argument is added, removed or re-ordered at a later date; this can result in a lot of calling code that needs correcting.

public class RoleDetails
{
  public RoleDetails(string title, DateTime startDate, DateTime? endDateIfAny)
  {
    Title = title;
    StartDate = startDate;
    EndDateIfAny = endDateIfAny;
  }

  public string Title { get; private set; }
  public DateTime StartDate { get; private set; }
  public DateTime? EndDateIfAny { get; private set; }

  public RoleDetails Update(string title)
  {
    return (title == Title)
      ? this
      : new RoleDetails(title, StartDate, EndDateIfAny);
  }
  public RoleDetails UpdateStartDate(DateTime startDate)
  {
    return (startDate == StartDate)
      ? this
      : new RoleDetails(Title, startDate, EndDateIfAny);
  }
  public RoleDetails UpdateEndDateIfAny(DateTime? endDateIfAny)
  {
    return (endDateIfAny == EndDateIfAny)
      ? this
      : new RoleDetails(Title, StartDate, endDateIfAny);
  }
}

To update two properties on a given instance, you would need to call

var updatedRoleDetails = existingRoleDetails
  .UpdateStartDate(new DateTime(2014, 9, 21))
  .UpdateEndDateIfAny(new DateTime(2014, 11, 21));

If either of the new values is the same as the property value that it should be replacing, then no new instance is required for that property update - since the Update{Whatever} method will return back the same instance. But if both properties are changed then two new instances are required even though the first, intermediate value is immediately discarded and so is "wasted".

There could be an Update method that takes multiple parameters for the different properties but then you're basically just mirroing the constructor. Or there could be various Update methods that took combinations of properties to try to cover either the most common cases or all combinations of cases, but neither of these are particularly elegant and they would all result in quite a lot of code duplication.

A better way

It struck me that it should be possible to do something with named and optional method arguments (support for which was added to C# when .net 4 came out, if I remember correctly). Something like

public RoleDetails UpdateWith(
  string title = Title,
  DateTime startDate = StartDate,
  DateTime? endDateIfAny = EndDateIfAny)
{
  if ((title == Title) && (startDate == StartDate) && (endDateIfAny == EndDateIfAny))
    return this;
  return new RoleDetails(title, startDate, endDateIfAny);
}

would allow for only a subset of the arguments to be specified and for those that are left unspecified to default to the current property value of the instance. So the earlier update code becomes

var updatedRoleDetails = existingRoleDetails
  .UpdateWith(startDate: new DateTime(2014, 9, 21), endDateIfAny: new DateTime(2014, 11, 21));

However, this won't fly. The compiler gives the errors

Default parameter value for 'title' must be a compile-time constant

Default parameter value for 'startDate' must be a compile-time constant

Default parameter value for 'endDateIfAny' must be a compile-time constant

That's a bummer.

Another thought that briefly crossed my mind was for the default argument values to all be null. This would work if the arguments were all reference types and would result in the method body looking something like

if ((title == null) && (startDate == null) && (endDateIfAny == null))
  return this;
return new RoleDetails(title ?? Title, startDate ?? StartDate, endDateIfAny ?? EndDateIfAny);

But that is too restrictive a constraint since in this case we have a non-reference type argument (startDate) and we also have a reference type for which null is a valid value (endDateIfAny).

So what we really need is a wrapper type around the arguments that indicates when no value has been specified. Since we're being conscious of avoiding GC churn, this should be a struct since structs essentially avoid adding GC pressure since they are always copied when passed around - this means that no struct is referenced by multiple scopes and so they don't have to be traced in the same way as reference types; when the scope that has access to the struct is terminated, the struct can safely be forgotten as well. This is not a particularly precise description of what happens and more details can be found in the MSDN article Choosing Between Class and Struct. Particularly see the paragraph

The first difference between reference types and value types we will consider is that reference types are allocated on the heap and garbage-collected, whereas value types are allocated either on the stack or inline in containing types and deallocated when the stack unwinds or when their containing type gets deallocated. Therefore, allocations and deallocations of value types are in general cheaper than allocations and deallocations of reference types.

The other guidelines in that article around cases where structs may be appropriate (if the type "logically represents a single value", "has an instance size under 16 bytes", "is immutable" and "will not have to be boxed frequently") are followed by this type:

public struct Optional<T>
{
  private T _valueIfSet;
  private bool _valueHasBeenSet;

  public T GetValue(T valueIfNoneSet)
  {
    return _valueHasBeenSet ? _valueIfSet : valueIfNoneSet;
  }

  public bool IndicatesChangeFromValue(T value)
  {
    if (!_valueHasBeenSet)
      return false;

    if ((value == null) && (_valueIfSet == null))
      return false;
    else if ((value == null) || (_valueIfSet == null))
      return true;

    return !value.Equals(_valueIfSet);
  }

  public static implicit operator Optional<T>(T value)
  {
    return new Optional<T>
    {
      _valueIfSet = value,
      _valueHasBeenSet = true
    };
  }
}

This type allows us to write an UpdateWith method

public RoleDetails UpdateWith(
  Optional<string> title = new Optional<string>(),
  Optional<DateTime> startDate = new Optional<DateTime>(),
  Optional<DateTime?> endDateIfAny = new Optional<DateTime?>())
{
  if (!title.IndicatesChangeFromValue(Title)
  && !startDate.IndicatesChangeFromValue(StartDate)
  && !endDateIfAny.IndicatesChangeFromValue(EndDateIfAny))
    return this;

  return new RoleDetails(
    title.GetValue(Title),
    startDate.GetValue(StartDate),
    endDateIfAny.GetValue(EndDateIfAny)
  );
}

The Optional type could have exposed properties for has-a-value-been-set and get-value-if-any but since each property comparison (to detemine whether a new instance is actually required) would have to follow the pattern if-value-has-been-set-and-if-value-that-has-been-set-does-not-equal-current-value, it made sense to me to hide the properties and to instead expose only the access methods "IndicatesChangeFromValue" and "GetValue". The "IndicatesChangeFromValue" method returns true if the Optional describes a value that is different to that passed in and "GetValue" returns the wrapped value if there is one, and returns the input argument if not. This enables the relatively succinct "UpdateWith" method format shown above.

The other method on the struct is an implicit operator for the wrapped type which makes the "UpdateWith" calling code simpler. Instead of having to do something like

var updatedRoleDetails = existingRoleDetails
  .UpdateWith(startDate = Optional<DateTime>(new DateTime(2014, 9, 21)));

the implicit conversion allows you to write

var updatedRoleDetails = existingRoleDetails
  .UpdateWith(startDate = new DateTime(2014, 9, 21));

because the DateTime will be implicitly converted into an Optional<DateTime>. In fact, I went one step further and made it such that this is the only way to create an Optional that wraps a value. There is no constructor that may be used to initialise an Optional with a value, you must rely upon the implicit conversion. This means that it's very clear that there's only one way to use this type. It also happens to be very similar to the most common way that the Nullable type is used in C# - although that does have a public constructor that accepts the value to wrap, in practice I've only ever seen values cast to Nullable (as opposed to the Nullable constructor being passed the value).

Turning it up to eleven

Now this is all well and good and I think it would be a solid leap forward simply to leave things as they are shown above. Unnecessary GC pressue is avoided since there are no "intermediary" instances when changing properties, while the use of structs means that we're not generating a load of property-update-value references that need to be collected either.

But I just couldn't resist trying to push it a bit further since there's still quite a lot of boring code that needs to be written for every immutable type - the UpdateWith method needs to check all of the properties to ensure that they haven't changed and then it needs to pass values into a constructor. If a class has quite a lot of properties (which is not especially unusual if the types are representing complex data) then this UpdateWith method could grow quite large. Wouldn't it be nice if we could just write something like:

public RoleDetails UpdateWith(
  Optional<string> title = new Optional<string>(),
  Optional<DateTime> startDate = new Optional<DateTime>(),
  Optional<DateTime?> endDateIfAny = new Optional<DateTime?>())
{
  return magicUpdater(title, startDate, endDateIfAny);
}

Wouldn't it?? Yes it would.

And we can.. if we dip into some of the .net framework's crazier parts - reflection and stack tracing. With some LINQ expressions thrown in to make it work efficiently when called more than once or twice.

What this "magicUpdater" needs to do is take the names and values of the arguments passed to it and then analyse the target type (RoleDetails in this example) to find the constructor to call that will allow all of these named values to be passed into a new instance, using existing property values on the source instance for any constructor arguments that are not provided by the update arguments. It also needs to do the same work to determine whether the update arguments actually require a new instance to be generated - if only the StartDate is being provided to change but the new value is the same as the current value then no new instance is required, the source instance can be returned directly by the "magicUpdater".

This is handled by two steps. The first based around this line:

var callingMethod = new StackFrame(1).GetMethod();

It returns a MethodBase with metadata about the method that called the "magicUpdater" (the "1" in the call above is how many steps back to go in the call stack). From this the names of the arguments can be extracted and a delegate returned which will take the argument values themselves. So the call would actually look more like (if this "magicUpdater" method return a delegate which then must itself be called):

return magicUpdater()(title, startDate, endDateIfAny);

Before we move on to the second step, there are some important considerations in relation to the use of StackFrame. Firstly, there is some expense to performing analysis like this, as with using reflection - but we'll not worry about that here, some optimisations will be covered later which hopefully mean we can ignore it. What's more important is that analysing the call stack can seem somewhat.. unreliable, in a sense. In the real world, the code that gets executed is not always the code as it appears in the C# source. A release build will apply optimisations that won't be applied to debug builds and when code is manipulated by the JIT compiler more optimisations again may occur - one of the more well-known of which is "method inlining". Method inlining is when the compiler sees a chain of Method1 -> Method2 -> Method3 -> Method4 and observes that Method2 is so small that instead of being a distinct method call (which has a cost, as every method call does - the arguments have to be passed into a new scope and this must be considered by the garbage collector; as a very basic example of one of these costs) the code inside Method2 can be copied inside Method1. This would mean that if Method3 tried to access Method2's metadata through the StackFrame class, it would be unable to - it would be told it was called by Method1!

There's a short but informative article about this by Eric Gunnerson: More on inlining. In a nutshell it says that -

  • Methods that are greater than 32 bytes of IL will not be inlined.
  • Virtual functions are not inlined.
  • Methods that have complex flow control will not be in-lined. Complex flow control is any flow control other than if/then/else; in this case, switch or while.
  • Methods that contain exception-handling blocks are not inlined, though methods that throw exceptions are still candidates for inlining.
  • If any of the method's formal arguments are structs, the method will not be inlined.

This means that we shouldn't have to worry about the UpdateWith method being inlined (since its arguments are all Optional which are structs), but the "magicUpdater" method may be a concern. The way that my library gets around that is that the method "GetGenerator" on the UpdateWithHelper class (it's not really called "magicUpdater" :) has the attribute

[MethodImpl(MethodImplOptions.NoInlining)]
public UpdateWithSignature<T> GetGenerator<T>(int numberOfFramesFromCallSite = 1)

which tells the JIT compiler not to inline it and so, since the caller isn't inlined (because of the structs), we don't have to worry about stack "compressing".

This "GetGenerator" method, then, has access to the argument names and argument types of the method that called it. The generic type param T is the immutable type that is being targeted by the "UpdateWith" method. UpdateWithSignature<T> is a delegate with the signature

public delegate T UpdateWithSignature<T>(T source, params object[] updateValues);

This delegate is what takes the property update values and creates a new instance (or returns the source instance if no changes are required). It does this by considering every public constructor that T has and determining what constructor arguments it can satisfy with update arguments. It does this by matching the update argument names to the constructor argument names and ensuring that the types are compatible. If a constructor is encountered with arguments that don't match any update arguments but T has a property whose name and type matches the constructor argument, then that will be used. If a constructor argument is encountered that can't be matched to an update argument or a property on T but the constructor argument has a default value, then the default value may be used if the constructor.

If a constructor does not have at least one argument that can be matched to each update argument name, then that constructor is ignored (otherwise an update argument would be ignored, which would make the UpdateWith somewhat impotent!). If there are multiple constructors that meet all of these conditions, they are sorted by the number of arguments they have that are fulfilled by update arguments and then sorted by the number of arguments that are satisfed by other properties on T - the best match from this sorted set it used.

The return UpdateWithSignature<T> delegate itself is a compiled LINQ expression so, once the cost of generating it has been paid the first time that it's required, the calls to this delegate are very fast. The "GetGenerator" method caches these compiled expressions, so the method

public RoleDetails UpdateWith(
  Optional<string> title = new Optional<string>(),
  Optional<DateTime> startDate = new Optional<DateTime>(),
  Optional<DateTime?> endDateIfAny = new Optional<DateTime?>())
{
  return DefaultUpdateWithHelper.GetGenerator<RoleDetails>()(this, title, startDate);
}

can be called repeatedly and cheaply.

Note that in the above example, the DefaultUpdateWithHelper is used. This is a static wrapper around the UpdateWithHelper which specifies a default configuration. The UpdateWithHelper takes arguments that describe how to match update argument names to constructor argument names, for example (amongst other configuration options). The implementation in the DefaultUpdateWithHelper matches by name in a case-insensitive manner, which should cover the most common cases. But the relevant UpdateWithHelper constructor argument is of type

public delegate bool UpdateArgumentToConstructorArgumentComparison(
  ParameterInfo updateArgument,
  ConstructorInfo constructor,
  ParameterInfo constructorArgument);

so a custom implementation could implement any complex scheme based upon target type or constructor or update argument type.

The UpdateWithHelper also requires a cache implementation for maintaining the compiled expressions, as well as matchers for other comparisons (such as property name to constructor argument name, for constructor arguments that can't be matched by an update argument). If a custom UpdateWithHelper is desired that only needs to override some behaviour, the DefaultUpdateWithHelper class has a static nested class DefaultValues with properties that are the references that it uses for the UpdateWithHelper constructor arguments - some of these may be reused by the custom configuration, if appropriate.

I considered going into some detail about how the LINQ expressions are generated since I think it's hard to find a good "how-to" walkthrough on these. It's either information that seems too simple or fine-grained that it's hard to put it together into something useful or it's the other extreme; dense code that's hard to get to grips with if you don't know much about them. But I feel that it would balloon this post too much - so maybe another day!

Incidentally, the DefaultUpdateWithHelper's static "GetGenerator" method inserts another layer into the call stack, which is why the UpdateWithHelper's method requires an (optional) "numberOfFramesFromCallSite" argument - so that it can be set to 2 in this case, rather than the default 1 (since it will need to step back through the DefaultUpdateWithHelper method before getting to the real "UpdateWith" method). This also means that DefaultUpdateWithHelper has the "MethodImplOptions.NoInlining" attribute on its "GetGenerator" method.

It's also worthy of note that the "GetGenerator" methods support extension methods for "UpdateWith" implementations, as opposed to requiring that they be instance methods. So the following is also acceptable

public static RoleDetails UpdateWith(
  this RoleDetails source,
  Optional<string> title = new Optional<string>(),
  Optional<DateTime> startDate = new Optional<DateTime>(),
  Optional<DateTime?> endDateIfAny = new Optional<DateTime?>())
{
  return DefaultUpdateWithHelper.GetGenerator<RoleDetails>()(source, title, startDate);
}

The analysis detects that the first argument is not an OptionalType<T> and asserts that its type is assignable to the type param T and then ignores it when generating the translation expression. The extension method will pass through the "source" reference where "this" was used in the instance method implementation shown earlier.

Further performance optimisations

Although the compiled "generator" expressions are cached, the cache key is based upon the "UpdateWith" method's metadata. This means that the cost of accessing the StackFrame is paid for every "UpdateWith" call, along with the reflection access to get the UpdateWith argument's metadata. If you feel that this might be an unbearable toll, a simple alternative is something like

private static UpdateWithSignature<RoleDetails> updater
  = DefaultUpdateWithHelper.GetGenerator<RoleDetails>(typeof(RoleDetails).GetMethod("UpdateWith"));
public RoleDetails UpdateWith(
  Optional<string> title = new Optional<string>(),
  Optional<DateTime> startDate = new Optional<DateTime>(),
  Optional<DateTime?> endDateIfAny = new Optional<DateTime?>())
{
  return updater(this, title, startDate);
}

The "GetGenerator" methods have alternate signatures that accept a MethodBase reference relating to the "UpdateWith" method, rather than relying upon StackFrame to retrieve it. And using a static "updater" reference means that "GetGenerator" is only ever called once, so subsequent calls that would require reflection in order to check for a cached expression are avoided entirely. The trade-off is that the method must be named in a string, which would break if the method was renamed. Not quite as convenient as relying upon stack-tracing magic.

If you really want to get crazy, you can go one step further. If part of the reason for this experiment was to reduce GC pressure, then surely the params array required by the UpdateWithSignature<T> is a step backwards from the less-automated method, where the number of update arguments is known at compile time? (Since that didn't require a params array for a variable number of arguments, there were no method calls where the precise number of update arguments was unknown). Well that params array can be avoided if we make some more trade-offs. Firstly, we may only use an approach like above, which doesn't rely on expression caching (ie. use a static property that requests a generator only once). Secondly, there may only be up to nine update arguments. The first reason is because the cache that the UpdateWithHelper uses records UpdateWithSignature<T> references, which are no good since they use the params array that we're trying to avoid. The second reason is because a distinct delegate is required for each number of arguments, as is a distinct method to construct the generator - so there had to be a limit somewhere and I chose nine. The methods are

public UpdateWithSignature1<T> GetUncachedGenerator1<T>(MethodBase updateMethod)
public UpdateWithSignature2<T> GetUncachedGenerator2<T>(MethodBase updateMethod)
public UpdateWithSignature3<T> GetUncachedGenerator3<T>(MethodBase updateMethod)
// .. etc, up to 9

and the delegates are of the form

public delegate T UpdateWithSignature1<T>(T source, object arg0);
public delegate T UpdateWithSignature2<T>(T source, object arg0, object arg1);
public delegate T UpdateWithSignature3<T>(T source, object arg0, object arg1, object arg2);
// .. etc, up to 9

They may be used in a similar manner to that already shown, but you must be careful to match the number of arguments required by the "UpdateWith" method. In a way, there is actually a compile-time advantage here - if you choose the wrong one, then the compiler will warn you that you have specified three update arguments when the delegate requires four (for example). With the generic form (the non-numbered "GetGenerator" method), the params array means that you can specify any number of update arguments and you won't find out until runtime that you specified the wrong amount.

So, to illustrate -

private static UpdateWithSignature3<RoleDetails> updater
  = DefaultUpdateWithHelper.GetUncachedGenerator3<RoleDetails>(
    typeof(RoleDetails).GetMethod("UpdateWith"));

public RoleDetails UpdateWith(
  Optional<string> title = new Optional<string>(),
  Optional<DateTime> startDate = new Optional<DateTime>(),
  Optional<DateTime?> endDateIfAny = new Optional<DateTime?>())
{
  return updater(this, title, startDate, endDateIfAny);
}

If I'm being honest, however, if you really think that this optimisation is beneficial (by which, I mean you've done performance analysis and found it to be a bottleneck worth addressing), you're probably better replacing this automated approach with the hand-written code that I showed earlier. It's not all that long and it removes all of this "magic" and also gives the compiler more opportunity to pick up on mistakes. But most importantly (in terms of performance) may be the fact that all update arguments are passed as "object" in these delegates. This means that any value types (ints, structs, etc..) will be boxed when they are passed around and then unboxed when used as constructor arguments. This is explained very clearly in the article 5 Basic Ways to Improve Performance in C# and more information about the use of the heap and stack can be found at Six important .NET concepts: Stack, heap, value types, reference types, boxing, and unboxing - I'd not seen this article before today but I thought it explained things really clearly.

Chances are that you won't have to worry about such low level details as whether values are being boxed-unboxed 99% of the time and I think there's a lot of mileage to be had from how convenient this automated approach is. But it's worth bearing in mind the "what ifs" of performance for the times when they do make a difference.

Any other downsides to the automagical way?

I can't claim to have this code in production anywhere yet. But I'm comfortable enough with it at this stage that I intend to start introducing it into prototype projects that it will be applicable to - and then look to using it in real-world, scary, production projects before too long! My only concern, really, is about making silly mistakes with typos in update argument names. If I mistyped "tittle" in the RoleDetails "UpdateWith" example I've been using, I wouldn't find out until runtime that I'd made the mistaken - at which point, the "GetGenerator" call would throw an exception as it wouldn't be able to match "tittle" to any argument on any accessible constructor. I think the trade-off here would be that every "UpdateWith" method that used this library would need a unit test so that discovering the problem at "runtime" doesn't mean "when I hit code in manual testing that triggers the exception" but rather equates to "whenever the test suite is run - whether locally or when pushed to the build server". I doubt that Update methods of this type would normally get a unit test since they're so basic (maybe you disagree!) but in this case the convenience of using the automated "GetGenerator" method still wins even with the (simple) unit test recommended for each one.

Now that I think about it, this is not a dissimilar situation to using a Dependency Injection framework or using AutoMapper in your code - there is a lot of convenience to be had, but at the risk that configuration errors are not exposed until the code is executed.

In summary, until I find a good reason not to use this library going forward, I intend to do so! To revisit my (F#) inspiration, how can it not be enticing to be able to write

// F#
let p2 = {p1 with first="Jim";last="Smith"}

// C#
var p2 = p1.UpdateWith(first:"Jim",last:"Smith");

with so little code having to be written to enable it?!

Go get the code at bitbucket.org/DanRoberts/updatewith!

Or alternatively, pull the NuGet package straight down from nuget.org/packages/CSharpImmutableUpdateWith.

Update (19th September 2014): There's been quite a lot of interest in this post and some good comments made here and at the discussion on Reddit/implementing-f-sharp-inspired-with-updates-for-immutable-classes-in-c-sharp. I intend to write a follow-up post that talks about some of the observations and includes some performance stats. In summary, though, I may have to admit to considering a slight about-turn in the crazy magical approach and strike that up as a convenience for rattling out code quickly but probably something that won't make it into production code that I write. The idea of using an "UpdateWith" method with named, optional arguments (using the Optional struct) will make it into my "real world" code, though! It's also strikingly similar to some of the code in Roslyn, it was pointed out (I'll touch on this in the follow-up too). I still had a lot of fun with the "Turning it up to eleven" investigation and I think there's useful information in here and in the library code I wrote - even more so when I get round to documenting how I approach writing the LINQ expression-generating code. But maybe it didn't result in something that should always be everyone's immediate go-to method for writing this sort of code. Such is life! :)

Update (2nd October 2014): See A follow-up to "Implementing F#-inspired 'with' updates in C#".

Posted at 23:12