If we follow the functional paradigm, we should refrain from state mutation altogether: once created, an object never changes. This is the subject of an entire chapter of the C# FP Orange Book

Here I give an overview of Value Types, Reference types and immutability.

Benefits of avoiding mutation

The options discussed in the book are:

  • Immutability by convention (but mutation can creep in!)
  • Define immutable objects in C# (required some extra work in defining constructors)
  • Write data objects in F#

1. Value Types

Variables of value types directly contain the data:

  • simple types: bool, byte, char, decimal, int etc…
  • structs (eg Point), enum
// Value types
// the value of 42 is copied to to the variable i
int i = 42;
void Update(int j)
    WriteLine(j); // 42
    j = 43;
    WriteLine(j); // 43
WriteLine(i); // 42

2. Reference Types

Variables of reference type store references to their data eg:

  • class
  • object
  • string etc..
public class Thing
    public string Name { get; set; }

var q = new Thing { Name = "test1" };
// A function with side effects 
void UpdateThing(Thing r)
    // q and r reference the same object 
    r.Name = "test2";
WriteLine(q.Name); // test2

3. String - Reference type but behaves like a Value Type

var aa = "test1";
void UpdateString(string bb) => bb = "test2";
WriteLine(aa); // test1

Strings are not value types since they can be huge.

Value types are stored on the stack which is 1MB for 32-bit and 4MB for 64-bit, so you’d incur many penalties for doing this.

Reference types are stored on the heap.

It matters whether an object is a value or a reference type but the rest is an implementation detail

String vs string - string is an alias for System.String and can be used interchangeably. R# suggests to use the alias string.

4. Immutable Data Object - Reference type

To enforce immutability on our Data Object / Custom Type / Anemic Object, which is a reference type, we can make the public property read only ie no set.

public class Person
    public string Name { get; } // C#6 getter-only auto-property
    public int Age { get; }

    public Person(string name, int age)
        Name = name;
        Age = age;
    public override string ToString() => $"{Name} : {Age}";

// Immutable data object (so can only set public property on construction)
var s = new Person("test1", 45);
var u = UpdatePersonName(s);
// Make a copy of the Person object and return with an updated value
Person UpdatePersonName(Person t) => new Person("test2", t.Age);
WriteLine(s); // test1, 45
WriteLine(u); // test2, 45

This has been a brief look at Value types (eg int), Reference types (eg custom classes) and making Immutable Objects.

Have a look at my series on leaning FP in C#