Within structs, you have access to many built-in types, which could be used either as keywords or types from the System namespace.

One of them is the Boolean type (the bool keyword), which makes it possible to store a logical value, that is, one of two values, namely true or false.

As for storing integer values, you can use one of the following types: Byte (the byte keyword), SByte (sbyte), Int16 (short), UInt16 (ushort), Int32 (int), UInt32 (uint), Int64 (long), and UInt64 (ulong). They differ by the number of bytes for storing values and therefore by the range of available values. As an example, the short data type supports values in the range from -32,768 to 32,767 while uint supports values in the range from 0 to 4,294,967,295. Another type within the integral types is Char (char), which represents a single Unicode character such as 'a' or 'M'.

In the case of floating-point values, you can use two types, namely Single (float) and Double (double). The first uses 32 bits, while the second uses 64 bits. Thus, their precision differs significantly.

What's more, the Decimal type (the decimal keyword) is available. It uses 128 bits and is a good choice for monetary calculations.

An example declaration of a variable in the C# programming language is as follows:

int number; 

You can assign a value to a variable using the equals sign (=), shown as follows:

number = 500; 

Of course, declaration and assignment could be performed in the same line:

int number = 500; 

If you want to declare and initialize an immutable value, that is, a constant, you can use the const keyword, as shown in the following line of code:

const int DAYS_IN_WEEK = 7; 

Apart from structs, the value types contain enumerations. Each has a set of named constants to specify the available set of values. For instance, you can create the enumeration for available languages or supported currencies. An example definition is as follows:

enum Language { PL, EN, DE }; 

Then, you can use the defined enumeration as a data type, as shown as follows:

Language language = Language.PL; 
switch (language) 
{ 
    case Language.PL: /* Polish version */ break; 
    case Language.DE: /* German version */ break; 
    default: /* English version */ break; 
} 

It is worth mentioning that enumerations allow you to replace some magical strings (such as "PL" or "DE") with constant values and this has a positive impact on code quality.

There is often the necessity to store some text values. You can achieve this goal using the String built-in reference type from the System namespace, which is also available using the string keyword. The string type is a sequence of Unicode characters. It can have zero chars, one or more chars, or the string variable can be set to null.

You can perform various operations on string objects, such as concatenation or accessing a particular char using the [] operator, as shown as follows:

string firstName = "Marcin", lastName = "Jamro"; 
int year = 1988; 
string note = firstName + " " + lastName.ToUpper()  
   + " was born in " + year; 
string initials = firstName[0] + "." + lastName[0] + "."; 

At the beginning, the firstName variable is declared, and the "Marcin" value is assigned to it. Similarly, "Jamro" is set as a value of the lastName variable. In the third line, you concatenate five strings (using the + operator), namely, the current value of firstName, the space, the current value of lastName converted to the upper-case string (by calling the ToUpper method), the string " was born in ", and the current value of the year variable. In the last line, the first chars from firstName and lastName variables are obtained, using the [] operator, as well as concatenated with two dots to form the initials, that is, M.J., which are stored as a value of the initials variable.

The Format static method could also be used for constructing the string, as follows:

string note = string.Format("{0} {1} was born in {2}",  
   firstName, lastName.ToUpper(), year); 

In this example, you specify the composite format string with three format items, namely the firstName (represented by {0}), upper-case lastName ({1}), and the year ({2}). The objects to format are specified as the following parameters.

It is also worth mentioning the interpolated string, which uses interpolated expressions to construct a string. To create a string using this approach, the $ character should be placed before ", as shown in the following example:

string note = $"{firstName} {lastName.ToUpper()}  
   was born in {year}"; 

The Object class, declared in the System namespace, performs a very important role while developing applications in the C# language because it is the base class for all classes. It means that built-in value types and built-in reference types, as well as user-defined types, are derived from the Object class, which is also available by using the object alias.

As the object type is the base entity for all value types, it means that it is possible to convert a variable of any value type (for example, int or float) to the object type, as well as to convert back a variable of the object type to a specific value type. Such operations are named boxing (the first one) and unboxing (the other). They are shown as follows:

int age = 28; 
object ageBoxing = age; 
int ageUnboxing = (int)ageBoxing; 

Apart from the types already described, the dynamic one is available for developers. It allows the bypassing of type checking during compilation so that you can perform it during the run time. Such a mechanism is useful while accessing some application programming interfaces (APIs), but it will not be used in this book.

As already mentioned, C# is an object-oriented language and supports declaration of classes together with various members, including constructors, finalizers, constants, fields, properties, indexers, events, methods, and operators, as well as delegates. Moreover, classes support inheritance and implementing interfaces. Static, abstract, and virtual members are available, as well.

An example class is shown as follows:

public class Person 
{ 
    private string _location = string.Empty; 
    public string Name { get; set; } 
    public int Age { get; set; } 
 
    public Person() => Name = "---"; 
 
    public Person(string name, int age) 
    { 
        Name = name; 
        Age = age; 
    } 
 
    public void Relocate(string location) 
    { 
        if (!string.IsNullOrEmpty(location)) 
        { 
            _location = location; 
        } 
    } 
 
    public float GetDistance(string location) 
    { 
        return DistanceHelpers.GetDistance(_location, location); 
    } 
} 

The Person class contains the _location private field with the default value set as the empty string (string.Empty), two public properties (Name and Age), a default constructor that sets a value of the Name property to --- using the expression body definition, an additional constructor that takes two parameters and sets values of properties, the Relocate method that updates the value of the private field, as well as the GetDistance method that calls the GetDistance static method from the DistanceHelpers class and returns the distance between two cities in kilometers.

You can create an instance of the class using the new operator. Then, you can perform various operations on the object created, such as calling a method, as shown as follows:

Person person = new Person("Mary", 20); 
person.Relocate("Rzeszow"); 
float distance = person.GetDistance("Warsaw");  

In the previous section, a class was mentioned that could implement one or more interfaces. It means that such a class must implement all methods, properties, events, and indexers, that are specified in all implemented interfaces. You can easily define interfaces in the C# language using the interface keyword.

As an example, let's take a look at the following code:

public interface IDevice 
{ 
    string Model { get; set; } 
    string Number { get; set; } 
    int Year { get; set; } 
 
    void Configure(DeviceConfiguration configuration); 
    bool Start(); 
    bool Stop(); 
} 

The IDevice interface contains three properties, namely those representing a device model (Model), serial number (Number), and production year (Year). What's more, it has signatures of three methods, which are Configure, Start, and Stop. When a class implements the IDevice interface, it should contain the mentioned properties and methods.

The delegate reference type allows specification of the required signature of a method. The delegate could then be instantiated, as well as invoked, as shown in the following code:

delegate double Mean(double a, double b, double c); 
 
static double Harmonic(double a, double b, double c) 
{ 
    return 3 / ((1 / a) + (1 / b) + (1 / c)); 
} 
 
static void Main(string[] args) 
{ 
    Mean arithmetic = (a, b, c) => (a + b + c) / 3; 
    Mean geometric = delegate (double a, double b, double c) 
    { 
        return Math.Pow(a * b * c, 1 / 3.0); 
    }; 
    Mean harmonic = Harmonic; 
    double arithmeticResult = arithmetic.Invoke(5, 6.5, 7); 
    double geometricResult = geometric.Invoke(5, 6.5, 7); 
    double harmonicResult = harmonic.Invoke(5, 6.5, 7); 
} 

In the example, the Mean delegate specifies the required signature of the method for calculating the mean value of three floating-point numbers. It is instantiated with the lambda expression (arithmetic), anonymous method (geometric), and named method (harmonic). Each delegate is invoked by calling the Invoke method.