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C# structures

This part of the C# tutorial covers structures.

C# structure definition

A structure is a value type. The type is defined with the struct keyword. Structures are very similar to the classes; they differ in some aspects. Structures are meant to represent lightweight objects like Point, Rectangle, Color and similar. In many cases, structures may be more efficient than classes. Structures are value types and are created on the stack. Note that primitive data types like int, bool, float are technically struct types.

All struct types inherit from System.ValueType and further from System.Object. Structures are never abstract and they are always implicitly sealed. So struct types do not support inheritance. Therefore, the struct data member cannot be declared protected. The abstract and sealed modifiers are not permitted for a struct definition. A struct is not permitted to declare a parameterless constructor.

Structures can also contain constructors, constants, fields, methods, properties, indexers, operators, events, and nested types. However, if we need to implement more of these features, we might consider using a class instead. Structures can implement an interface. A struct can be used as a nullable type and can be assigned a null value.

Simple structure example

The following example creates a simple structure.

Program.cs
using System;

namespace SimpleStructure
{
    public struct Point
    {
        private int x;
        private int y;

        public Point(int x, int y)
        {
            this.x = x;
            this.y = y;
        }

        public override string ToString()
        {
            return String.Format("Point x:{0}, y:{1}", x, y);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var p = new Point(2, 5);
            Console.WriteLine(p);
        }
    }
}

The example creates a Point structure. The point could be represented by a class too, but with struct we are more efficient; especially, if we dealt with lots of points.

public struct Point
{
    ...
}

The structure is declared with the struct keyword.

public override string ToString()
{
    return String.Format("Point x:{0}, y:{1}", x, y);
}

The inheritance is not supported for struct types. But we can use the override keyword for methods, from which the struct type implicitly inherits. The ToString() method is such a case.

var p = new Point(2, 5);
Console.WriteLine(p);

We create the Point structure and call the ToString() method upon it.

$ dotnet run
Point x:2, y:5

This is the output of the example.

No new keyword

It is possible to create an instance of the struct type without the new keyword.

Program.cs
using System;

namespace NoNewKeyword
{
    public struct Person
    {
        public string name;
        public int age;
    }

    class Program
    {
        static void Main(string[] args)
        {
            Person p;
            p.name = "Jane";
            p.age = 17;

            Console.WriteLine("{0} is {1} years old",
                p.name, p.age);
        }
    }
}

We have a Person structure with two public members.

Person p;

First we declare a Person structure.

p.name = "Jane";
p.age = 17;

Later we initialize the structure with some data.

$ dotnet run
Jane is 17 years old

This is the output of the program.

C# structures are value types

The structure types are value types. They are created on the stack. When a value type is created only a single space in memory is allocated to store the value. An assignment of a value type copies the value.

Program.cs
using System;

namespace ValueTypes
{
    public struct Person
    {
        public Person(string name, int age) : this()
        {
            this.Name = name;
            this.Age = age;
        }

        public string Name { get; set; }

        public int Age { get; set; }

        public override string ToString()
        {
            return String.Format("{0} is {1} years old", Name, Age);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var p1 = new Person("Beky", 18);
            var p2 = p1;

            Console.WriteLine(p2);
            p2.Name = "Jane";
            p2.Age = 17;

            Console.WriteLine(p2);
            Console.WriteLine(p1);
        }
    }
}

We have a Person structure with two data members. We have a two parameter constructor and we also use automatic properties.

public string Name { get; set; }
public int Age { get; set; }

Automatic properties can be used in struct types.

var p1 = new Person("Beky", 18);
var p2 = p1;

Here we create a struct. And then the created struct is assigned to another struct. We create a copy of the structure.

p2.Name = "Jane";
p2.Age = 17;

We change the data of the second structure. The first one is not affected, since we work on the copy of the original struct type.

$ dotnet run
Beky is 18 years old
Jane is 17 years old
Beky is 18 years old

This is the output of the program.

Primitive types are structures

The primitive data types like int, float, or bool are structures under the hood. This differs from languages like C++ or Java.

Program.cs
using System;

public class PrimitiveTypes
{
    static void Main()
    {
        float x = 12.3f;
        int y = 34;
        bool z = false;

        Console.WriteLine(x.GetType());
        Console.WriteLine(y.GetType());
        Console.WriteLine(z.GetType());
    }
}

We have three variables: a float, an int, and a bool. We call the GetType() method on each of them.

Console.WriteLine(x.GetType());

We call the GetType() method on the float value. Each structure implicitly inherits from the System.ValueType class which contains the GetType() method.

$ dotnet run
System.Single
System.Int32
System.Boolean

This is the output of the example. We can look up in the documentation that these types are structures.

In this part of the C# tutorial, we covered structures.