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This article will cover almost every OOP concept that a novice/beginner developer hunt for, and not only beginners, the article’s purpose is to be helpful to experience professionals also who need to sometimes brush-up their concepts or who prepare for interviews. Pre-requisites Since this is the first part of the series, my readers should have basic knowledge of C# and should be aware of OOP concepts and terminology. OOP 1. What is OOP and what is advantage of OOP? OOP stands for "Object-Oriented Programming." Remember, it's OOP not OOPS,’S’ may stand for system, synopsis, structure etc. It is a programming approach entirely based on objects, instead of just functions and procedures like in procedural languages. It is like a programming language model organized around objects rather than "actions" and data rather than logic. An "object" in an OOP language refers to a specific type, or "instance," of a class. Each object has a structure exactly similar to other objects in a class, but can have individual properties/values. An object can also invoke methods, specific to that object OOP makes it easier for developers to structure and organize software programs. Individual objects can be modified without affecting other aspects of the program therefore it is also easier to update and change programs written in object-oriented languages. Since the nature of software programs have grown larger over the years, OOP has made developing these large programs more manageable and readable. 2. What are OOP Concepts? Following are OOP concepts explained in brief, we’ll take the topics in detail. Data Abstraction: Data Abstraction is a concept in which the internal and superfluous details of the implementation of a logic is hidden from an end user(who is using the program) .A user can use any of the data and method from the class without knowing about how this is created or what is the complexity behind it. In terms of a real world example, when we drive a bike and change the gears we don’t have to care about how internally its working, like how lever is pulled or how chain is set. Inheritance: Inheritance is most popular Concept in OOP’s .This provides a developer an advantage called reusability of code. Suppose a class is written having functions with specific logic, then we can derive that class into our newly created class and we don’t have to write the logic again for derived class functions, we can use them as it is. Data Encapsulation: Wrapping up of member data and member functions of a class in a single unit is called encapsulation. The visibility of the member functions,data members is set via access modifiers used in class. Polymorphism: Poly means many and morphism means changing or alterable. The Concepts Introduces in the form of Many behaviours of an object. Message Communication: Message Communication means when an object passes the call to method of class for execution. OK, we covered lots of theory, now it’s time for action. I hope that will be interesting. We’ll cover the topics in a series as follows: 3. Polymorphism: In this article we will cover almost all the scenarios of compile type polymorphism, the use of params keyword in detail, and case study or hands on to different possible combinations of the thoughts coming to our mind while coding. Method Overloading or Early Binding or Compile Time Polymorphism Note: Each and every code snippet written in this article is tried and tested. 1. Let's create a simple console application named InheritanceAndPolymorphism, and add a class named Overload.cs and add three methods named DisplayOverload having varied parameters as follows: Overload.cs public class Overload { public void DisplayOverload(int a){ System.Console.WriteLine("DisplayOverload " + a); } public void DisplayOverload(string a){ System.Console.WriteLine("DisplayOverload " + a); } public void DisplayOverload(string a, int { System.Console.WriteLine("DisplayOverload " + a + ; } } In the main method in Program.cs file, add the following code: Program.cs class Program { static void Main(string[] args) { Overload overload = new Overload(); overload.DisplayOverload(100); overload.DisplayOverload("method overloading"); overload.DisplayOverload("method overloading", 100); Console.ReadKey(); } } Now when you run the application, the output is: The class Overload contains three methods named DisplayOverload, they only differ in the datatype of the parameters they consist of. In C# we can have methods with the same name, but the datatypes of their parameters should differ. This feature of C# is called method overloading. Therefore, we need not to remember lots of method names if a method differs in behavior, only providing different parameters to the methods can call a method individually. Point to remember: C# recognizes the method by its parameters and not only by its name. A signature signifies the full name of the method. So the name of a method or its signature is the original method name + the number and data types of its individual parameters. If we run project using following code: public void DisplayOverload() { } public int DisplayOverload(){ } We certainly get a compile time error as: Here we had two functions who differ only in the data type of the value that they return, but we got a compile time error, therefore, another point to remember comes, Point to remember: The return value/parameter type of a method is never the part of method signature if the names of the methods are same. So this is not polymorphism. If we run the project using following code: static void DisplayOverload(int a) { } public void DisplayOverload(int a) { } public void DisplayOverload(string a){ } We again get a compile time error: Can you differentiate with the modification done in the above code, we now have two DisplayOverload methods, that accept an int (integer). The only difference is that one method is marked static. Here the signature of the methods will be considered same as modifiers such as static are also not considered to be a part of method signature. Point to remember: Modifiers such as static are not considered as part of method signature. If we run the program as per following code, considering the method signature is different now: private void DisplayOverload(int a) { } private void DisplayOverload(out int a) { a = 100; } private void DisplayOverload(ref int a) { } We again get a compile time error: The signature of a method not only consists of the data type of the parameter but also the type/kind of parameter such as ref or out etc. Method DisplayOverload takes an int with different access modifiers i.e. out/ref etc, the signature on each is different. Point to remember: The signature of a method consists of its name, number and types of its formal parameters. The return type of a function is not part of the signature. Two methods can not have the same signature and also non-members cannot have the same name as members. 4. Role of Params Parameter in Polymorphism A method can be called by four different types of parameters: Pass by value, Pass by reference, As an output parameter, Using parameter arrays. As explained earlier the parameter modifier is never the part of method signature. Now let’s focus on Parameter Arrays. A method declaration means creating a separate declaration space in memory. So anything created will be lost at the end of the method. Running following code: public void DisplayOverload(int a, string a) { } public void Display(int a) { string a; } Results in compile time error: Point to remember: Parameter names should be unique. And also we can not have a parameter name and a declared variable name in the same function as same. In the case of pass by value, the value of the variable is passed and in the case of ref and out, the address of the reference is passed. When we run the following code: Overload.cs public class Overload { private string name = "Akhil"; public void Display() { Display2(ref name, ref name); System.Console.WriteLine(name); } private void Display2(ref string x, ref string y) { System.Console.WriteLine(name); x = "Akhil 1"; System.Console.WriteLine(name); y = "Akhil 2"; System.Console.WriteLine(name); name = "Akhil 3"; } } Program.cs class Program { static void Main(string[] args) { Overload overload = new Overload(); overload.Display(); Console.ReadKey(); } } We get out put as: We are allowed to pass the same ref parameter as many times as we want. In the method Display the string name has a value of Akhil. Then by changing the string x to Akhil1, we are actually changing the string name to Akhil1 as name is passed by reference. Variables x and name refer to the same string in memory. Changing one changes the other. Again changing y also changes name variable as they refer to the same string anyways. Thus variables x, y and name refer to the same string in memory. When we run the following code: Overload.cs public class Overload { public void Display() { DisplayOverload(100, "Akhil", "Mittal", "OOP"); DisplayOverload(200, "Akhil"); DisplayOverload(300); } private void DisplayOverload(int a, params string[] parameterArray) { foreach (string str in parameterArray) Console.WriteLine(str + " " + a); } } Program.cs class Program { static void Main(string[] args) { Overload overload = new Overload(); overload.Display(); Console.ReadKey(); } } We get output: We will often get into a scenario where we would like to pass n number of parameters to a method. Since C# is very particular in parameter passing to methods, if we pass an int where a string is expected, it immediately breaks down. But C# provides a mechanism for passing n number of arguments to a method, we can achieve it with the help of params keyword. Point to remember: This params keyword can only be applied to the last argument of the method. So the n number of parameters can only be at the end. In the case of method DisplayOverload, the first argument has to be an integer, the rest can be from zero to an infinite number of strings. If we add a method like: private void DisplayOverload(int a, params string[] parameterArray, int { } We get a compile time error as: Thus is is proved that params keyword will be the last parameter in a method, this is already stated in the latest point to remember. Overload.cs public class Overload { public void Display() { DisplayOverload(100, 200, 300); DisplayOverload(200, 100); DisplayOverload(200); } private void DisplayOverload(int a, params int[] parameterArray) { foreach (var i in parameterArray) Console.WriteLine(i + " " + a); } } Program.cs class Program { static void Main(string[] args) { Overload overload = new Overload(); overload.Display(); Console.ReadKey(); } } When we run the code we get: Therefore: Point to Remember: C# is very smart to recognize if the penultimate argument and the params have the same data type. The first integer is stored in the variable a, the rest are made part of the array parameterArray. private void DisplayOverload(int a, params string[][] parameterArray) { } private void DisplayOverload(int a, params string[,] parameterArray) { } For the above written code, we again get a compile time error and a new point to remember as well: Point to remember: same as error above. The data type of the params argument must be a single dimensional array. Therefore [ ][ ] is allowed but not [,]. We also not allowed to combine the params keyword with ref or out. Overload.cs public class Overload { public void Display() { string[] names = {"Akhil", "Ekta", "Arsh"}; DisplayOverload(3, names); } private void DisplayOverload(int a, params string[] parameterArray) { foreach (var s in parameterArray) Console.WriteLine(s + " " + a); } } Program.cs class Program { static void Main(string[] args) { Overload overload = new Overload(); overload.Display(); Console.ReadKey(); } } Output We are, therefore, allowed to pass a string array instead of individual strings as arguments. Here, names is a string array which has been initialized using the short form. Internally when we call the function DisplayOverload, C# converts the string array into individual strings. Overload.cs public class Overload { public void Display() { string [] names = {"Akhil","Arsh"}; DisplayOverload(2, names, "Ekta"); } private void DisplayOverload(int a, params string[] parameterArray) { foreach (var str in parameterArray) Console.WriteLine(str + " " + a); } } Program.cs class Program { static void Main(string[] args) { Overload overload = new Overload(); overload.Display(); Console.ReadKey(); } } Output So, we got two errors. For the above mentioned code, C# does not permit mix and match. We assumed that the last string “Ekta” would be added to the array of strings names or convert names to individual strings and then add the string “Ekta” to it. Quite logical. Internally before calling the function DisplayOverload, C# accumulates all the individual parameters and converts them into one big array for the params statement. Overload.cs public class Overload { public void Display() { int[] numbers = {10, 20, 30}; DisplayOverload(40, numbers); Console.WriteLine(numbers[1]); } private void DisplayOverload(int a, params int[] parameterArray) { parameterArray[1] = 1000; } } Program.cs class Program { static void Main(string[] args) { Overload overload = new Overload(); overload.Display(); Console.ReadKey(); } } Output We see that the output produced is the proof of concept. The member parameterArray[1] of array has an initial value of 20 and in the method DisplayOverload, we changed it to 1000. So the original value changes, this shows that the array is given to the method DisplayOverload, Hence proved. Overload.cs public class Overload { public void Display() { int number = 102; DisplayOverload(200, 1000, number, 200); Console.WriteLine(number); } private void DisplayOverload(int a, params int[] parameterArray) { parameterArray[1] = 3000; } } Program.cs class Program { static void Main(string[] args) { Overload overload = new Overload(); overload.Display(); Console.ReadKey(); } } Output In the above mentioned scenario C# creates an array containing 1000 102 and 200. We now change the second member of array to 3000 which has nothing to do with the variable number. As DisplayOverload has no knowledge of number, so how can DisplayOverload change the value of the int number? Therefore it remains the same. Overload.cs public class Overload { public void Display() { DisplayOverload(200); DisplayOverload(200, 300); DisplayOverload(200, 300, 500, 600); } private void DisplayOverload(int x, int y) { Console.WriteLine("The two integers " + x + " " + y); } private void DisplayOverload(params int[] parameterArray) { Console.WriteLine("parameterArray"); } } Program.cs class Program { static void Main(string[] args) { Overload overload = new Overload(); overload.Display(); Console.ReadKey(); } } Output Now we'll talk about method overloading. C# is extremely talented though partial. It does not appreciate the params statement and treats it as a stepchild. When we invoke DisplayOverload only with one integer, C# can only call the DisplayOverload that takes a params as a parameter as it matches only one int. An array can contain one member too. The fun is with the DisplayOverload that is called with two ints now. So here we have a dilemma. C# can call the params DisplayOverload or DisplayOverload with the two ints. As discussed earlier, C# treats the params as a second class member and therefore chooses the DisplayOverload with two ints. When there are more than two ints like in the third method call, C# is void of choice but to grudgingly choose the DisplayOverload with the params. C# opts for the params as a last resort before flagging an error. Now a bit tricky example, yet important: Overload.cs public class Overload { public static void Display(params object[] objectParamArray) { foreach (object obj in objectParamArray) { Console.Write(obj.GetType().FullName + " "); } Console.WriteLine(); } } Program.cs class Program { static void Main(string[] args) { object[] objArray = { 100, "Akhil", 200.300 }; object obj = objArray; Overload.Display(objArray); Overload.Display((object)objArray); Overload.Display(obj); Overload.Display((object[])obj); Console.ReadKey(); } } Output In the first instance we are passing the method Display an array of object that looks like object. Since all the classes are derived from a common base class object, we can do that. The method Display gets an array of objects objectParamArray. In the foreach object class has a method named GetType that returns an object that looks like Type, which too has a method named FullName that returns the name of the type. Since three different types displayed. In the second method call of Display we are casting objArray to an object. Since there is no conversion available from converting an object to an object array i.e. object [ ], so only a one element object [ ] is created. It's the same case in the third invocation and the last explicitly casts to an object array. For proof of concept: Overload.cs public class Overload { public static void Display(params object[] objectParamArray) { Console.WriteLine(objectParamArray.GetType().FullName); Console.WriteLine(objectParamArray.Length); Console.WriteLine(objectParamArray[0]); } } Program.cs class Program { static void Main(string[] args) { object[] objArray = { 100, "Akhil", 200.300 }; Overload.Display((object)objArray); Console.ReadKey(); } } Output 5. Conclusion In this article of our Diving in OOP series we learnt about compile time polymorphism, it is also called early binding or method overloading. We catered most of the scenarios specific to polymorphism.We also learned about the use of powerful params keyword and its use in polymorphism. To sum up lets list down all the point to remembers once more: C# recognizes the method by its parameters and not only by its name. The return value/parameter type of a method is never the part of method signature if the names of the methods are same. So this is not polymorphism. Modifiers such as static are not considered as part of method signature. The signature of a method consists of its name, number and types of its formal parameters. The return type of a function is not part of the signature. Two methods can not have the same signature and also non-members cannot have the same name as members. Parameter names should be unique. And also we can not have a parameter name and a declared variable name in the same function as same. In case of pass by value, the value of the variable is passed and in the case of ref and out, the address of the reference is passed. This params keyword can only be applied to the last argument of the method.So the n number of parameters can only be at the end. C# is very smart to recognize if the penultimate argument and the params have the same data type. Parameter array must be a single dimensional array. Sursa