{"id":440,"date":"2022-01-09T10:08:22","date_gmt":"2022-01-09T03:08:22","guid":{"rendered":"https:\/\/csharptutorial.net\/csharp-tutorial\/c-interface-default-implementation\/"},"modified":"2022-06-03T15:05:35","modified_gmt":"2022-06-03T08:05:35","slug":"csharp-interface-default-implementation","status":"publish","type":"page","link":"https:\/\/www.csharptutorial.net\/csharp-tutorial\/csharp-interface-default-implementation\/","title":{"rendered":"C# Interface with Default Implementation"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to define a C# interface with a default implementation in C# 8 or later.<\/p>\n\n\n\n

Introduction to the C# interface default implementation<\/h2>\n\n\n\n

Suppose we have an interface<\/a> ILogger<\/code> that has one method Log()<\/code> and two classes ConsoleLogger<\/code> and FileLogger<\/code> that implements the interface:<\/p>\n\n\n

interface<\/span> ILogger<\/span>\n{\n    void<\/span> Log<\/span>(string<\/span> message<\/span>)<\/span>;\n}\n\nclass<\/span> ConsoleLogger<\/span> : ILogger<\/span>\n{\n    public<\/span> void<\/span> Log<\/span>(string<\/span> message<\/span>)<\/span>\n    {\n        Console.WriteLine(message);\n    }\n}\n\nclass<\/span> FileLogger<\/span> : ILogger<\/span>\n{\n    public<\/span> string<\/span> Filename { get<\/span>; set<\/span>; }\n\n    public<\/span> FileLogger<\/span>(string<\/span> filename<\/span>)<\/span>\n    {\n        Filename = filename;\n    }\n    public<\/span> void<\/span> Log<\/span>(string<\/span> message<\/span>)<\/span>\n    {\n        File.AppendAllText(Filename, message + Environment.NewLine);\n    }\n\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
If you want to add a new method to the ILogger<\/code> interface, your program won’t work unless you implement this method in all the existing classes that implement the interface. <\/p>\n\n\n\n
For example:<\/p>\n\n\n
interface<\/span> ILogger<\/span>\n{\n    void<\/span> Log<\/span>(string<\/span> message<\/span>)<\/span>;\n    void<\/span> Log<\/span>(Exception exception<\/span>)<\/span>;\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, we add a new Log()<\/code> method that has a parameter as an instance of the Exception<\/code> class. Howeer, this will break the program.<\/p>\n\n\n\n
To resolve this, C# 8 or later allows the interface members to have default implementations.<\/p>\n\n\n\n
For example, a method of the interface can have a default implementation. If the implementing classes do not provide an implementation for that member, no error will occur. Instead, the default implementation is used.<\/p>\n\n\n\n
The following example adds the Log()<\/code> method that accepts an exception instance. This Log()<\/code> method has an implementation that shows the exception message to the console:<\/p>\n\n\n
interface<\/span> ILogger<\/span>\n{\n    void<\/span> Log<\/span>(string<\/span> message<\/span>)<\/span>;\n    void<\/span> Log<\/span>(Exception exception<\/span>)<\/span> => Console.WriteLine(exception.Message);\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
In this case, the existing classes don’t need to implement the new Log()<\/code> method. Hence, the program will compile successfully and work properly.<\/p>\n\n\n\n
The following creates a new instance of the FileLogger<\/code> class and calls the Log()<\/code> method with an exception argument:<\/p>\n\n\n
\/\/ Program.cs<\/span>\nILogger logger = new<\/span> FileLogger(@\"C:\\temp\\logs.txt\"<\/span>);\nlogger.Log(new<\/span> Exception(\"An exception occurred.\"<\/span>));<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Because the FileLogger<\/code> doesn’t implement the new Log()<\/code> method, the new Log()<\/code> method uses the default implementation in the interface to show the exception message to the console.<\/p>\n\n\n\n
The following FileLogger<\/code> class provides an implementation for the new Log()<\/code> method:<\/p>\n\n\n
class<\/span> FileLogger<\/span> : ILogger<\/span>\n{\n    public<\/span> string<\/span> Filename { get<\/span>; set<\/span>; }\n\n    public<\/span> FileLogger<\/span>(string<\/span> filename<\/span>)<\/span>\n    {\n        Filename = filename;\n    }\n    public<\/span> void<\/span> Log<\/span>(string<\/span> message<\/span>)<\/span>\n    {\n        File.AppendAllText(Filename, message + Environment.NewLine);\n    }\n\n    public<\/span> void<\/span> Log<\/span>(Exception exception<\/span>)<\/span>\n    {\n        Log(exception.Message);\n    }\n\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
If you run the program, you’ll see that it logs the exception message to a log file:<\/p>\n\n\n
\/\/ Program.cs<\/span>\nILogger logger = new<\/span> FileLogger(@\"C:\\temp\\logs.txt\"<\/span>);\nlogger.Log(new<\/span> Exception(\"An exception occurred.\"<\/span>));<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Put it all together.<\/p>\n\n\n
interface<\/span> ILogger<\/span>\n{\n    void<\/span> Log<\/span>(string<\/span> message<\/span>)<\/span>;\n    void<\/span> Log<\/span>(Exception exception<\/span>)<\/span> => Console.WriteLine(exception.Message);\n}\n\nclass<\/span> ConsoleLogger<\/span> : ILogger<\/span>\n{\n    public<\/span> void<\/span> Log<\/span>(string<\/span> message<\/span>)<\/span>\n    {\n        Console.WriteLine(message);\n    }\n}\n\nclass<\/span> FileLogger<\/span> : ILogger<\/span>\n{\n    public<\/span> string<\/span> Filename { get<\/span>; set<\/span>; }\n\n    public<\/span> FileLogger<\/span>(string<\/span> filename<\/span>)<\/span>\n    {\n        Filename = filename;\n    }\n    public<\/span> void<\/span> Log<\/span>(string<\/span> message<\/span>)<\/span>\n    {\n        File.AppendAllText(Filename, message + Environment.NewLine);\n    }\n\n    public<\/span> void<\/span> Log<\/span>(Exception exception<\/span>)<\/span>\n    {\n        Log(exception.Message);\n    }\n\n}<\/code><\/span>Code language:<\/span> C#<\/span> (<\/span>cs<\/span>)<\/span><\/small><\/pre>\n\n\n
Summary<\/h2>\n\n\n\n
  • Provide default implementation for an interface member to keep the program backward compatibility.<\/li><\/ul>\n
    \n\t
    \n\t\t
    Was this tutorial helpful ?<\/div>\n\t\t
    \n\t\t\t\n\t\t\t\t\n\t\t\t\t\t<\/path>\n\t\t\t\t<\/svg>\n\t\t\t\t Yes <\/span>\n\t\t\t<\/button>\n\n\t\t\t\n\t\t\t\t\n\t\t\t\t\t<\/path>\n\t\t\t\t<\/svg>\n\t\t\t\t No <\/span>\n\t\t\t<\/button>\n\t\t<\/div>\n\t<\/header>\n\n\t
    \n\t\t
    \n\t\t\t
    <\/div>\n\t\t\t\n\t\t\t