In this post, we are going to learn about the different types of polymorphism in C#, how they work and how we can use them in our code.<\/p>\n
Let’s start.<\/p>\n
There are two types of polymorphism:<\/p>\n
Also, generic programming is sometimes referred to as another type of polymorphism (parametric polymorphism). That’s because a generic class can operate in a different way, depending on the classes we use as type parameters.<\/p>\n
You can find more about generic programming in our article about C# Generics<\/a>.<\/p>\n Compile-time polymorphism in C# is the existence of multiple methods with the same name, but with different arguments in type and\/or number.<\/strong> We also call it method overloading. We can use it in situations where we need to implement multiple methods with similar functionality and we opt to give them the same name:<\/p>\n In this example, class The second method provides a default value (Info) for the log level. The third method accepts an integer as input and converts it to the corresponding log level. Both methods eventually use the first one, in order to write the message to the log file.<\/p>\n A special case of method overloading is operator overloading. Here we can define multiple implementations for each operator method. Also, constructors are frequently overloaded; we can have constructors with multiple parameters as well as the default constructor.<\/p>\n Another reason to use method overloading is when we need to change the definition of a method by adding one or more parameters to it. Since this change will probably break existing code, sometimes it is preferable to introduce a new method that will overload the initial one.<\/p>\n In the case of run-time polymorphism in C#, an object behaves in a different way depending on the context in which it is used.<\/p>\n To understand the concepts better, let’s define a good example that can help us.<\/p>\n First, let’s create a The We’re also going to add another class called This class has a slightly different delivery cost and delivery date calculation:<\/p>\n And we’ll add another class called With polymorphism, we can have a base class reference point to an object of a subclass:<\/p>\n Note that this operation is only possible due to the base class \u2013 subclass relationship between Furthermore, it’s not possible for unrelated classes to point at each other:<\/p>\n Now, let’s try to call the We can see that we actually call the method defined in the Now, let\u2019s insert the <\/a>Compile-Time Polymorphism<\/h2>\n
public class Logger\r\n{\r\n private StreamWriter LogFile;\r\n public Logger(StreamWriter logFile)\r\n {\r\n LogFile = logFile;\r\n }\r\n\r\n public void Log(string message, LogLevels level)\r\n {\r\n LogFile.Write(level + \" ---\");\r\n LogFile.Write(DateTime.Now.ToString() + \" ---\");\r\n LogFile.WriteLine(message);\r\n LogFile.Flush();\r\n }\r\n\r\n public void Log(string message)\r\n {\r\n Log(message, LogLevels.Info);\r\n }\r\n\r\n public void Log(string message, int level)\r\n {\r\n if (Enum.IsDefined(typeof(LogLevels), level))\r\n {\r\n Log(message, (LogLevels)Enum.Parse(typeof(LogLevels), level.ToString()));\r\n }\r\n else\r\n {\r\n throw new Exception(\"Log level value does not exist\");\r\n }\r\n }\r\n}\r\n<\/pre>\nLogger<\/code> defines 3 methods and makes use of a LogLevels<\/code> enum:<\/p>\npublic enum LogLevels\r\n{\r\n Info = 1,\r\n Warning = 2,\r\n Error = 3\r\n}<\/pre>\n<\/a>Run-Time Polymorphism<\/h2>\n
How it works<\/h3>\n
Package<\/code> class defines the common information necessary to deliver a package to its destination (recipient name and address). It also defines the characteristics of the package (weight and receipt date).<\/p>\nPackage<\/code> class also defines two methods: GetDeliveryCost()<\/code> and GetDeliveryDate()<\/code>. Those methods calculate the expected cost and delivery date of the package. We base the calculation on a package weight, and receipt date:<\/p>\npublic class Package\r\n{\r\n public string Recipient { get; set; }\r\n public string Address { get; set; }\r\n public decimal Weight { get; set; }\r\n public DateTime SendDate { get; set; }\r\n\r\n \/\/ ...\r\n\r\n public decimal GetDeliveryCost()\r\n {\r\n return 3 * Weight;\r\n }\r\n\r\n public DateTime GetDeliveryDate()\r\n {\r\n if (SendDate.DayOfWeek == DayOfWeek.Friday)\r\n return SendDate.AddDays(4);\r\n if (SendDate.DayOfWeek == DayOfWeek.Thursday)\r\n return SendDate.AddDays(4);\r\n else\r\n return SendDate.AddDays(2);\r\n }\r\n}\r\n<\/pre>\nExpeditedPackage<\/code> which inherits from the Package<\/code> class.<\/p>\npublic class ExpeditedPackage : Package\r\n{\r\n \/\/ ...\r\n\r\n public new decimal GetDeliveryCost()\r\n {\r\n return 4 * Weight + 2;\r\n }\r\n\r\n public new DateTime GetDeliveryDate()\r\n {\r\n return SendDate.AddDays(1);\r\n }\r\n}<\/pre>\nInternationalPackage<\/code> which takes the destination country into account:<\/p>\npublic class InternationalPackage : Package\r\n{\r\n public string CountryCode { get; set; }\r\n\r\n \/\/ ...\r\n public new decimal GetDeliveryCost()\r\n {\r\n if (CountryCode == \"US\")\r\n return 6 * Weight + 3;\r\n else if (CountryCode == \"UK\")\r\n return 5 * Weight + 4;\r\n else if (CountryCode == \"DE\")\r\n return 6 * Weight;\r\n else\r\n return 6 * Weight + 2;\r\n }\r\n\r\n public new DateTime GetDeliveryDate()\r\n {\r\n if (CountryCode == \"US\")\r\n return SendDate.AddDays(3);\r\n else if (CountryCode == \"UK\")\r\n return SendDate.AddDays(2);\r\n else if (CountryCode == \"DE\")\r\n return SendDate.AddDays(1);\r\n else\r\n return SendDate.AddDays(2);\r\n }\r\n}<\/pre>\nExpeditedPackage ep = new ExpeditedPackage(\"Sender B\", \"Address B\", 10, DateTime.Now);\r\nPackage p = ep;\r\n<\/pre>\n
Package<\/code> and ExpeditedPackage<\/code>. It does not work the other way round:\u00a0 for instance, we cannot have an ExpeditedPackage<\/code> reference pointing to a Package<\/code> object:<\/p>\nPackage p = new Package(\"Sender A\", \"Address A\", 10, DateTime.Now);\r\nExpeditedPackage ep = p; \/\/compiler error<\/pre>\n
Package p = new Package(\"Sender A\", \"Address A\", 10, DateTime.Now);\r\nPerson pr = p; \/\/compiler error<\/pre>\n
GetDeliveryCost()<\/code> method using the reference to the ExpeditedPackage<\/code>:<\/p>\nExpeditedPackage ep = new ExpeditedPackage(\"Sender B\", \"Address B\", 10, DateTime.Now);\r\nPackage p = ep;\r\nConsole.WriteLine(\"Cost: \" + p.GetDeliveryCost());<\/pre>\n
Package<\/code> class. This happens because the reference of type Package<\/code> knows only about the inner workings of the Package<\/code> class. This behavior will change as soon as we introduce virtual methods into our classes.<\/p>\nPolymorphism with Virtual Methods<\/h3>\n
virtual<\/code> keyword in the declaration of the two methods in the Package<\/code> class:<\/p>\npublic class Package\r\n{\r\n \/\/ ...\r\n\r\n public virtual decimal GetDeliveryCost()\r\n {\r\n return 3 * Weight;\r\n }\r\n\r\n public virtual DateTime GetDeliveryDate()\r\n {\r\n if (SendDate.DayOfWeek == DayOfWeek.Friday)\r\n return SendDate.AddDays(4);\r\n if (SendDate.DayOfWeek == DayOfWeek.Thursday)\r\n return SendDate.AddDays(4);\r\n else\r\n return SendDate.AddDays(2);\r\n }\r\n}\r\n<\/pre>\n