2.1. Data Types¶
2.1.1. Static vs. Dynamic Typing¶
C# is a statically typed language. When a variable or parameter is declared in a statically typed language, the data type for the value must be specified. Once the declaration is made, the variable or parameter cannot refer to a value of any other type.
1 2 3 4
let dynamicVariable = "dog"; console.log(typeof(dynamicVariable)); dynamicVariable = 42; console.log(typeof(dynamicVariable));
After line 1 executes,
dynamicVariable holds a
string data type. After
line 3 runs,
dynamicVariable becomes a
is allowed to hold values of different types, which can be reassigned as
needed when the program runs.
However, the corresponding code in C# will result in a build error:
string staticVariable = "dog"; staticVariable = 42;
Error CS0029: Cannot implicitly convert type 'int' to 'string' (CS0029)
The compiler error occurs when we try to assign
42 to a variable of type
Take-home lesson: We must declare the type of every variable and parameter in
a statically typed language. This is done by declaring the data type for the
variable or parameter BEFORE its name, as we did in the example above:
string staticVariable = "dog".
We only need to specify the type of a variable or parameter when declaring it. Further use of the variable or parameter does not require us to identify its type. Doing so will result in an error.
It is allowed in some situations in C# to declare a variable without
specifying a type by using the keyword
var, as in
var x = "dog";. In this case, C# still assigns a type to
through inference. It looks and sees that we are assigning
"dog", which is a
x has type
and attempting to assign
x = 42 will still result in a build error.
We recommend avoiding use of
var while you are learning C#. Even
after you become more experienced with the language you will still only
want to use it sparingly and in specific circumstances. Explicitly
declaring the type of your variables makes for more readable code, in
Dynamic and static typing are examples of different type systems. The type system of a programming language is one of the most important high-level characteristics that programmers use when discussing the differences between languages. Here are a few examples of popular languages falling into these two categories:
Static: C#, C, C++, Java, TypeScript
Because we need to give plenty of attention to types when writing C# code, let’s begin by exploring the most common data types in this language.
2.1.2. Built-In Types¶
In C#, all of the basic data types are objects — we’ll get into this idea shortly. Though the so-called built-in data types also have short names that differ from typical class name conventions.
We provide here a list of some of the most common types, along with the official .NET class name. Recall that .NET gives us a class library with object types. We’ll generally prefer to use the short names for each of these.
Doubles are twice as precise (i.e. can hold much longer decimal numbers than floats)
A single Unicode character. Must be enclosed in single quotes
Note that booleans in C# are not capitalized as they are in Python
As we will see in a later section, the
float data type sacrifices some
accuracy for speed of calculation. Thus, evaluating 1.11111 + 3 results in an
answer of 4.1111097 instead of 4.11111.
Anytime you need to perform calculations with decimal values, consider using
double type instead of
Not all built-in data types in C# are listed here, only the most commonly used types that beginners are likely to encounter. If you’re curious, read more about built-in types in C#.
126.96.36.199. Primitive Types¶
The types in the table above are known as primitive types. A primitive data type is a basic building block of a programming language. Using primitive data types, we can build more complex data structures.
188.8.131.52. Non-primitive Types¶
Primitive data types are immutable and can be combined to build larger data
structures. One example is forming the
string “LaunchCode” from multiple
char characters (
string is another built-in type in C# and it is also a non-primitive data type. We’ll delve into
how strings work in C# on the next page, as well as other complex data types.
Operators, such as
*, are type-dependent.
That is, we can only use them on allowed types, and their effects are
different depending on which types we use them on. The
+ operator is
a good example of this. We can use
+ to add numeric types together,
2 + 2 which results in
4. But we can also use it to
"2" + "2", for example, which results in
"22". What the operators do depends on the type they are operating
on, and we may not mix types in arbitrary ways (
"2" + 2 results in a
Numeric types such as
double may be freely mixed when
using numeric operators. Generally, the result of such mixing is that
the output has the type of the more precise input. For example, the
following snippet would print out
float a = 2; double b = 3; Console.WriteLine((a + b).GetType());
2.1.3. Reference and Value Types¶
We can group types in C# into two categories: value types and
reference types. Variables holding value types directly contain
their data, and include numeric types (
bool, and a handful of others that we won’t encounter in this
course. The primitive, built-in types we list above are all value types.
184.108.40.206. Class Types¶
A class is a template for creating objects. In addition to the built-in types provided by .NET, any class in C# defines its own type. A class is a template, or blueprint, for creating objects. We’ll have much more to say about classes and objects — this is an object-oriented course, after all. For now, you need to be comfortable seeing the basic syntax of class types and class creation.
If we have a class
Cat with a constructor that takes no arguments, we
declare and create a new instance of
Cat using its constructor.
Cat myCat = new Cat();
Cat myCatdeclares the variable
myCatand sets it to be of type
= new Cat()initializes the variable with a new
Any arguments that are required to build the new
Catobject must be included within the parentheses. In this case, there are no required arguments.
This statement creates a new variable that is initialized to
hold a new
Cat object. Note that in C#, we must declare the
variable’s type. Also note that we precede the constructor with the
new keyword. And, as we’ll see with all C# statements, the
declaration ends with a semi-colon.
Variables and parameters that are of the type of a class are said to be
of reference type (in contrast to primitive type). In plain
English, we would say of the C# example: “
myCat is a reference
variable of type
As mentioned above, classes define reference types. A variable of a
reference type (such as
myCat above) does not actually store the
object in question. Instead, it stores a reference to the object. A
reference is literally a memory address. We visualize references as an
arrow pointing to the object in memory.
Consider this code:
int catAge = 11; Cat myCat = new Cat(); Cat sameCat = myCat;
Visually, we can represent these three variables as shown here.
int is a value type, the variable
catAge functions as a
box holding the integer value 11. On the other hand,
myCat is a
reference variable, since it refers to an object of type
variable actually stores the memory address of the object, which we visualize
as an arrow from the variable box to the object. Instead of holding the actual
myCat stores directions for finding the data in memory.
When we assign
myCat to another variable, as in
Cat sameCat = myCat,
we do NOT create a second copy of the object or its data. Instead, we make a
second pointer to the same memory location.
The distinction between reference types and value types is important, but can be difficult to wrap your brain around at first. We will see that reference types are handled differently in essential and important ways in a lot of different situations.
As we mention above, all types in C# are treated as objects. Even value types. This can be accomplished through processes called boxing and unboxing. Converting from a value type to a reference type is called boxing, and the reverse process (reference to value) is called unboxing. C# is known as a unified type system because it implicitly boxes values types to be treated as objects.
int i = 123; // This is a value type. object o = i; // Boxing the value type into a reference type. int j = (int)o; // Unboxing the reference type back into a value type.
2.1.4. Check Your Understanding¶
Which of the following is NOT a number data type in C#:
Which of the following terms refers to C#’s behavior of treating all types as objects:
static type system
dynamic type system
reference type system
unified type system