Tweaking immutable objects with C# and Fody

Immutable data structures provide a lot of benefits to programmers, especially when representing domain value objects. Immutability is an essential part of functional programming paradigm. The concept is useful in object-oriented languages too, but you have to pay some price to get this advantage.

In C# immutable classes are usually implemented with read-only properties which are populated from constructor parameters. One of the disadvantages of this approach is the verbosity of creating a copy of an object with one property value modified.


Let’s have a look at an illustration of this problem. Let’s say we have a value type representing poker player statistics:

public class PlayerStats
    public PlayerStats(
        int hands, 
        int daysOnline,
        Money won, 
        Money expectedValue)
        this.Hands = hands;
        this.DaysOnline = daysOnline;
        this.Won = won;
        this.ExpectedValue = expectedValue;

    public int Hands { get; }
    public int DaysOnline { get; }
    public Money Won { get; }
    public Money ExpectedValue { get; }

We already see that it’s quite verbose: basically we repeat each property name five times. But the issue I’m discussing today is related to how we create a new object based on another object. Let’s say we need to make a copy of a given statistics, but with Hands property increased by 1:

var increasedHands = new PlayerStats(
    existing.Hands + 1,

Not as simple as we could hope. Also, there is some room for mistakes here. For instance, we could swap Won and ExpectedValue property calls and compiler won’t let us know because the types are the same. So we probably want to use explicit constructor parameter names:

var increasedHands = new PlayerStats(
    hands: existing.Hands + 1,
    daysOnline: existing.DaysOnline,
    won: existing.Won,
    expectedValue: existing.ExpectedValue);

But that leads to even more typing and repetition…


F# is a functional-first language with immutability as first-class concept. In F# value objects are usually modelled with Records, here is our example reimplemented:

type PlayerStats = {
    Hands: int
    DaysOnline: int
    Won: Money
    ExpectedValue: Money

Creation of new objects based on other objects is also solved properly in F#, thanks to the with keyword :

let increasedHands = { existing with Hands = existing.Hands + 1 }

All the properties are copied from the source record except for the ones explicitly mentioned in the expression.

Defining With in C#

There’s no with operator in C#, but we can try to come up with an alternative. We can define some fluent methods which would change property values one by one (they don’t change the original object, but return a copy with changed value):

public PlayerStats WithHands(int hands) 
    return new PlayerStats(
        hands: hands,
        daysOnline: existing.DaysOnline,
        won: existing.Won,
        expectedValue: existing.ExpectedValue);

public PlayerStats WithDaysOnline(int daysOnline) { ... }
public PlayerStats WithWon(Money won) { ... }
public PlayerStats WithExpectedValue (Money expectedValue) { ... }

The method implementation is very tedious but the usage gets much cleaner:

var increasedHands = existing.WithHands(existing.Hands + 1);

One way to avoid repetitive code is to generate it.

With.Fody Plugin

In one of my previous posts I described how C# value objects can be made less painful with Fody - a tool which changes your assembly at compilation time to provide some desired properties in automated and reliable fashion.

Please welcome the new Fody plugin With.Fody which auto-implements With method bodies for C# immutable classes.

Here is how to use this plugin for our imaginary example.

First, add a reference to NuGet pakages Fody and With.Fody.

Then, keep the PlayerStats class definition, but get rid of the bodies of WithXyz methods. Keep the signature but return something trivial like null or this:

public PlayerStats WithHands(int hands) => this;
public PlayerStats WithDaysOnline(int daysOnline) => this;
public PlayerStats WithWon(Money won) => this;
public PlayerStats WithExpectedValue (Money expectedValue) => this;

Compile the project and you will see the following line in Build Output:

>      Fody/With:   Added method 'With' to type 'PlayerStats'.

It means that the method bodies were re-implemented with calls to class constructor with proper parameter values.

The method stubs are needed to satisfy code completion tools like IntelliSense and Resharper, otherwise we could skip them altogether.

Single With() for Multiple Properties

In case you avoid Primitive Obsession antipattern, you will often end up with classes which have unique types of properties, e.g.

public class TripProfile
    public TripProfile(
        Distance totalDistance,
        Speed averageSpeed,
        Volume fuelConsumed)
        this.TotalDistance = totalDistance;
        this.AverageSpeed = averageSpeed;
        this.FuelConsumed = fuelConsumed;

    public Distance TotalDistance { get; }
    public Speed AverageSpeed { get; }
    public Volume FuelConsumed { get; }

In this case, the plugin can be smart enough to figure out which property you want to modify just by looking at the type of the argument. The single stub method can look like this:

public TripProfile With<T>(T value) => this;

This would get compiled into 3 strongly typed With methods with Distance, Speed and Volume arguments respectively. Resharper is still happy. And if you make a mistake and try to call the generic With method with an argument of wrong type (say int), the compiler will give you an error.

It is safe to call With methods in the same assembly where the class is defined: the calls get adapted to the real implementation automatically.

How to Get Started with Your Classes

Here are the requirements for the classes to be picked up by Fody.With:

  1. Have a single constructor.
  2. The constructor should have more than one argument.
  3. For each constructor agrument, there must be a gettable property with same name (case insensitive).
  4. At least one With stub must be defined as described above.

You can check out more examples, look at the source code or raise an issue in With.Fody github repository.

Give it a try and let me know what your think!

Cloud developer and researcher.
Software engineer at Pulumi. Microsoft Azure MVP.

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