{"id":374,"date":"2022-01-05T23:12:45","date_gmt":"2022-01-05T16:12:45","guid":{"rendered":"https:\/\/csharptutorial.net\/?page_id=374"},"modified":"2022-06-03T14:58:41","modified_gmt":"2022-06-03T07:58:41","slug":"csharp-inheritance-constructor","status":"publish","type":"page","link":"https:\/\/www.csharptutorial.net\/csharp-tutorial\/csharp-inheritance-constructor\/","title":{"rendered":"C# Inheritance & Constructor"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how C# calls the constructors of the base class and subclass.<\/p>\n\n\n\n

The constructor calling order<\/h2>\n\n\n\n

When you create a new instance of a subclass, C# calls the constructor<\/a> of the base class first and then the constructor of the subclass. For example:<\/p>\n\n\n\n

First, define a Person<\/code> class with a parameterless constructor:<\/p>\n\n\n

class<\/span> Person<\/span>\n{ \n    public<\/span> Person<\/span>(<\/span>)<\/span>\n    {\n        Console.WriteLine(\"Called the Person's constructor\"<\/span>);\n    }\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Note that a parameterless constructor of a class is the constructor that has no parameter. If you don’t explicitly specify a parameterless constructor, the C# compiler will automatically generate one for the class.<\/p>\n\n\n\n
class<\/span> Employee<\/span> : Person<\/span>\n{\n    public<\/span> Employee<\/span>(<\/span>)<\/span>\n    {\n        Console.WriteLine(\"Called the Employee's constructor\"<\/span>);\n    }\n\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, create a new instance of the Employee<\/code> class:<\/p>\n\n\n
var<\/span> employee = new<\/span> Employee();<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
Called the Person's constructor\nCalled the Employee'<\/span>s constructor<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
As you can see from the output, C# executes the constructor of the Person<\/code> class first and then the constructor of the Employee<\/code> class.<\/p>\n\n\n\n
In fact, C# always calls the parameterless constructor of the parent class. Consider the following example.<\/p>\n\n\n\n
First, add a constructor with the name<\/code> parameter to both Person<\/code> and Employee<\/code> classes:<\/p>\n\n\n
class<\/span> Person<\/span>\n{ \n    public<\/span> Person<\/span>(<\/span>)<\/span>\n    {\n        Console.WriteLine(\"Called the Person's constructor\"<\/span>);\n    }\n\n    public<\/span> Person<\/span>(string<\/span> name<\/span>)<\/span>\n    {\n        Console.WriteLine(\"Called the Person's constructor with a parameter\"<\/span>);\n    }\n}\n\nclass<\/span> Employee<\/span> : Person<\/span>\n{\n    public<\/span> Employee<\/span>(<\/span>)<\/span>\n    {\n        Console.WriteLine(\"Called the Employee's constructor\"<\/span>);\n    }\n\n    public<\/span> Employee<\/span>(string<\/span> name<\/span>)<\/span>\n    {\n        Console.WriteLine(\"Called the Employee's constructor with a parameter\"<\/span>);\n    }\n\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, create a new instance of the Employee<\/code> class:<\/p>\n\n\n
var<\/span> employee = new<\/span> Employee(\"John Doe\"<\/span>);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
Called the Person's constructor\nCalled the Employee'<\/span>s constructor with a parameter<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
In this case, C# executes the parameterless constructor of the Person<\/code> class (the base class) first and then the constructor with a parameter of the Employee<\/code> class (the subclass).<\/p>\n\n\n\n
If you remove the parameterless constructor of the Person<\/code> class and\/or the Employee<\/code> class, you’ll get an error. For example:<\/p>\n\n\n
class<\/span> Person<\/span>\n{\n    public<\/span> Person<\/span>(string<\/span> name<\/span>)<\/span>\n    {\n        Console.WriteLine(\"Called the Person's constructor with a parameter\"<\/span>);\n    }\n}\n\nclass<\/span> Employee<\/span> : Person<\/span>\n{\n    public<\/span> Employee<\/span>(string<\/span> name<\/span>)<\/span>\n    {\n        Console.WriteLine(\"Called the Employee's constructor with a parameter\"<\/span>);\n    }\n\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Error:<\/p>\n\n\n
There is<\/span> no argument given that corresponds to the required formal parameter 'name'<\/span> of 'Person.Person(string)'<\/span><\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, the Person<\/code> class doesn’t have a parameterless constructor. When you create a new instance of the Employee<\/code> class, C# calls a parameterless constructor of the Person<\/code> class. <\/p>\n\n\n\n
However, it cannot find the parameterless constructor in the Person class. Therefore, it issues an error. <\/p>\n\n\n\n
Change the calling order using base()<\/h2>\n\n\n\n
To fix the issue, you can explicitly call a specific constructor of a parent class instead of letting C# call the parameterless constructor. <\/p>\n\n\n\n
To do that, you use the base()<\/code> after the declaration of the subclass’ constructor as follows:<\/p>\n\n\n
class<\/span> Person<\/span>\n{\n    public<\/span> Person<\/span>(string<\/span> name<\/span>)<\/span>\n    {\n        Console.WriteLine(\"Called the Person's constructor with a parameter\"<\/span>);\n    }\n}\n\nclass<\/span> Employee<\/span> : Person<\/span>\n{\n    public<\/span> Employee<\/span>(string<\/span> name<\/span>): base<\/span>(name<\/span>)<\/span>\n    {\n        Console.WriteLine(\"Called the Employee's constructor with a parameter\"<\/span>);\n    }\n\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Now, if you create a new instance of the Employee<\/code> class:<\/p>\n\n\n
\/\/ Program.cs<\/span>\n\nvar<\/span> employee = new<\/span> Employee(\"John Doe\"<\/span>);<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
C# will call the constructor of the Person<\/code> class first and then the constructor of the Employee<\/code> class:<\/p>\n\n\n
Called the Person's constructor with a parameter\nCalled the Employee'<\/span>s constructor with a parameter<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
C# inheritance and constructor example<\/h2>\n\n\n\n
First, define the Person class with a constructor that has three parameters:<\/p>\n\n\n
\/\/ Person.cs<\/span>\nclass<\/span> Person<\/span>\n{\n    public<\/span> string<\/span> FirstName { get<\/span>; set<\/span>; }\n\n    public<\/span> string<\/span> LastName { get<\/span>; set<\/span>; }\n\n    public<\/span> byte<\/span> Age { get<\/span>; set<\/span>; }\n\n    public<\/span> string<\/span> FullName => $\"{FirstName}<\/span> {LastName}<\/span>\"<\/span>;\n\n    public<\/span> Person<\/span>(string<\/span> firstName, string<\/span> lastName, byte<\/span> age<\/span>)<\/span>\n    {\n        FirstName = firstName;\n        LastName = lastName;\n        Age = age;\n    }\n\n    public<\/span> string<\/span> Introduce<\/span>(<\/span>)<\/span> => $\"Hi, I'm {FullName}<\/span>.\"<\/span>;\n\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, define the Employee with a constructor that has five parameters:<\/p>\n\n\n
\/\/ Employee.cs<\/span>\nclass<\/span> Employee<\/span> : Person<\/span>\n{\n    public<\/span> string<\/span> JobTitle { get<\/span>; set<\/span>; }\n    public<\/span> decimal<\/span> Salary { get<\/span>; set<\/span>; }\n\n    public<\/span> Employee<\/span>(string<\/span> firstName, string<\/span> lastName, byte<\/span> age, string<\/span> jobTitle, decimal<\/span> salary<\/span>)\n        : base<\/span>(firstName, lastName, age<\/span>)<\/span>\n    {\n        JobTitle = jobTitle;\n        Salary = salary;\n    }\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
In this class, the Employee’s constructor calls the Person’s constructor using the base()<\/code> syntax.<\/p>\n\n\n\n
Summary<\/h2>\n\n\n\n
  • In inheritance, C# always calls the parameterless constructor of the parent class unless you use the base()<\/code> to call the specific constructor of the parent class.<\/li><\/ul>\n
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Second, define an Employee<\/code> class that inherits<\/a> from the Person<\/code> class:<\/p>\n\n\n