Daniel Simionescu

Value and Reference types

Let's see the difference between value types and reference types:

The easiest way is to compare them in code 😄

I am going to create 2 numbers:

var number1 = 1;
var number2 = 2;

These are two simple integer varibles.

Let's print their value to the console:

Console.WriteLine(number1); // 1
Console.WriteLine(number2); // 2

Now, let's assign the value of the second one to the first one and print again:

number1 = number2;

Console.WriteLine(number1); // 2
Console.WriteLine(number2); // 2

Nothing is unusual here (a simple assigment).

Now, let's change the value of number2 to something else and print them again:

number2 = 3;

Console.WriteLine(number1); // 1
Console.WriteLine(number2); // 3

So, we can see our assigments work well. These variables hold their values separately.

Now, let's create 2 lists which hold a number:

var list1 = new List<int> { 1 };
var list2 = new List<int> { 2 };

If we print the first item of these lists we get:

Console.WriteLine(list1[0]); // 1
Console.WriteLine(list2[0]); // 2

Now, let's assign the second list to the first one as we did before with the numbers:

list1 = list2;

Console.WriteLine(list1[0]); // 2
Console.WriteLine(list2[0]); // 2

And we indeed see the same behavior: they have the same item inside.

Now, let's change the number from the second list and see what we get:

list2[0] = 3;

Console.WriteLine(list1[0]); // 3
Console.WriteLine(list2[0]); // 3

Now, here is something unusual: they both have the same value even though we didn't change the value of the first list explicitely. 😨

Explanation

This is the difference between value types and reference types.

Value types hold the values directly, whereas reference types hold references to values.

When we did this:

list1 = list2;

we pointed both variables to the same object value.

So, these two references get the same value.

When we change the contents of list2 afterwards, we change the value of the object. When we get the values of the lists, obviously we get the new value because they retrieve the same object.

Summary

Numbers are value types whilst Lists are objects. They store their data differently.

Let's see another example, this time with a struct and a custom class.

struct is a value type, wheres class (or objects) are reference types.

Example 2

I am going to create a Rectangle as a struct and as a class to show the same concept.

Let's start with the struct:

public struct Rectangle
{
    public int Length { get; set; }

    public int Width { get; set; }
}

Then, let's create two rectangles and, for brevity, we are going to look at the Width property:

var rectangle1 = new Rectangle
{
    Width = 1,
    Length = 5
};

var rectangle2 = new Rectangle
{
    Width = 5,
    Length = 10
};

Console.WriteLine(rectangle1.Width); // 1
Console.WriteLine(rectangle2.Width); // 5

Now, assign the second to the first one and change the Width of the second one:

rectangle1 = rectangle2;

rectangle2.Width = 8;

Console.WriteLine(rectangle1.Width); // 5
Console.WriteLine(rectangle2.Width); // 8

These two rectangles are independent from each other.

We are copying only the value from one to another one, not like references.

Now, you only need to change struct to class, and we get for the last ones;

Console.WriteLine(rectangle1.Width); // 8
Console.WriteLine(rectangle2.Width); // 8

You can see again that these two rectangles point to the same location in memory where the object is stored.

These two instances reference the same object basically which results that they get the same value.

So, we have an object and a reference to that object.

Null values

Now, one more thing that you have to consider here is the null value.

We can assign null to any reference type:

var list1 = new List<int> { 1 };
list1 = null;

This is impossible to do for value types:

var number1 = 1;
number1 = null; // massive error

Default values

All types have default values:

int number1;      // 0
char character1;  // '\0'
bool boolean;     // false
string string1;   // null
List<int> list1;  // null

It means that if we don't assign any value to a variable, then it gets the default type of that type.

If you need the default* value of a type, there's a special keyword called default:

var defaultIntValue = default(int);
Console.WriteLine(defaultIntValue); // 0

But, what happens if we want to assign null to a number?

You know it's a difference between null and 0. 0 means something, but null means nothing (it points to no object).

nullable types

This is where we can use nullable types which adds the ability to any type to get the null value:

int? number1 = null;

So, we can do this by adding the ? question mark after the type.

Now, even though it is a number (a value type), it can have null as a value. :grin`:

Summary

Value types are:

  • numbers (int, double, float)
  • booleans (bool)
  • characters (char)
  • enums (enum)
  • structs (struct)

All the others are reference types:

  • objects (string, custom objects)
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