2.1. Data Types

2.1.1. Static vs. Dynamic Typing

In a dynamically typed programming language (like JavaScript or Python), a variable or parameter can refer to a value of any data type (string, number, object, etc.) at any time. When the variable is used, the interpreter figures out what type it is and behaves accordingly.

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.

For example, this is legal in JavaScript, a dynamically typed language:


let dynamicVariable = "dog";
dynamicVariable = 42;



After line 1 executes, dynamicVariable holds a string data type. After line 3 runs, dynamicVariable becomes a number type. dynamicVariable 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 string.

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 x through inference. It looks and sees that we are assigning x the value "dog", which is a string. Thus, x has type string 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 general.

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:

  1. Dynamic: Python, Ruby, JavaScript, PHP
  2. 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.

Some Built-In Types in C#
Short name .NET Class Examples Notes
int Int32 -5, 1024  
float Single 1.212, 3.14  
double Double 3.14159, 2.0 Doubles are twice as precise (i.e. can hold much longer decimal numbers than floats)
char Char ‘a’, ‘!’ A single Unicode character. Must be enclosed in single quotes '' to be a character; double quotes "" indicate a string
bool Boolean true, false 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 the double type instead of float.

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#. 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. 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 ('L', 'a', 'u', etc.).

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. Operations

Operators, such as + and *, 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, such as 2 + 2 which results in 4. But we can also use it to concatenate strings: "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 compiler error).


Numeric types such as int and 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 System.double.

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 (int, double, etc.), 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. 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();
  1. Cat myCat declares the variable myCat and sets it to be of type Cat.
  2. = new Cat() initializes the variable with a new Cat object.
  3. Any arguments that are required to build the new Cat object 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 Cat.”

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.

Reference Types

Reference Types

Since 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 Cat. The 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 Cat data, 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. Boxing

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#:

  1. number
  2. int
  3. float
  4. double


Which of the following terms refers to C#’s behavior of treating all types as objects:

  1. static type system
  2. dynamic type system
  3. reference type system
  4. unified type system