Ravindra Devrani

When we create an application with .NET, we tend to use the Microsoft tech stack like Visual Studio IDE, Microsoft SQL Server, Windows Operating System, and Azure. However, things have changed since the introduction of .NET Core. We are no longer bound to a specific operating system and database.

In this blog post, we will learn how to create a Web API CRUD (Create, Read, Update, Delete) application using .NET Core and a Postgres database.

Higher-order functions (HOF) are functions that can take a function as an argument or return a function or both. In C#, higher-order functions are achieved using delegates, lambda expressions and expression trees.

Example 1: Using a Delegate

// Declare a delegate that takes an int and returns an int
public delegate int IntOperation(int x);

public class Program
{
    // Higher-order function that takes a delegate as an argument
    public static int PerformOperation(int value, IntOperation operation)
    {
    return operation(value);
    }

    public static void Main()
    {
        // Create a delegate instance that points to the Square method
        IntOperation square = x => x \* x;

        // Pass the delegate to the higher-order function
        int result = PerformOperation(5, square);
        Console.WriteLine(result); // Output: 25
    }

}

Example 2 : Using lambda expression

// Higher-order function that takes a lambda expression as an argument
public static int PerformOperation(int value, Func<int, int\> operation)
{
 return operation(value);
}

// main method

// Define a lambda expression that squares a number
Func<int, int> square = x => x * x;

// Pass the lambda expression to the higher-order function
int result = PerformOperation(5, square);

Example 3: Function as a Return Value

public class Program
{
    // Higher-order function that returns a function
    public static Func<int, int\> GetOperation(bool isSquare)
    {
        if (isSquare)
        {
            return x => x \* x;
        }
        else
        {
            return x => x + x;
        }
    }

    public static void Main()
    {
        // Get a function that squares a number
        Func<int, int> operation = GetOperation(true);

        // Use the returned function
        int result = operation(5);
        Console.WriteLine(result); // Output: 25
    }
}

In c#, higher order functions are everywhere. If one have used LINQ, must have used HO functions. Collection’s Where() is the good example.

I assume you have some knowledge of C# properties. If not, here’s a quick definition: A property is a class member that provides a flexible way to read, write, or compute the value of a private field. Don’t worry if you’re new to properties; we’ll be using them in this blog, and I’ll explain them as we go along.

1. Properties with getter and setter

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

Generally, we define properties like this. The Person class has two properties Name and Age, each with getters and setters. This enables you to set and read their values.

Pure functions

Pure function is that:

1. Always returns the same output given the same inputs.
2. Does not modify the state of object or any external state.
3. Does not have any side effects such as I/O operations, exceptions or modifying external variables.

• In other words, a pure function takes input, process it and returns the output without modifying anything else.
• Pure functions are easy to test and debug.

Examples

1. Mathematical calculations

public int Add(int a, int b)
{
 return a + b;
}

2. String manipulation

public string Concatenate(string a, string b)
{
 return a + b;
}

3. Array operations

public int[] SortArray(int[] array)
{
 return array.OrderBy(x => x).ToArray();
}

4. Data transformations

public string ConvertToString(int value)
{
 return value.ToString();
}

5. Validation

public bool IsValidEmail(string email)
{
 return email.Contains("@");
}

Impure Functions

Impure function is that

The Command Pattern is a behavioral design pattern that turns a request into a stand-alone object containing all the information about the request. This conversion allows you to parameterize methods with different requests, queue requests, and support undoable operations.

In simpler terms, the Command Pattern encapsulates a request as an object, thereby allowing users to decouple the sender (invoker) from the object that performs the action (receiver).

Key Concepts

1. Command: An object that encapsulates a request or action.

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The Adapter pattern is a structural design pattern that allows two incompatible interfaces to work together by converting the interface of one class into an interface expected by the clients. In other words, it acts as a bridge between two incompatible interfaces, enabling them to communicate with each other.

Adapters in real world?

For a phone that does not support a 3.5 mm audio jack, you can use a USB-C to 3.5 mm audio adapter. The phone has only a USB-C port, so the adapter converts the USB-C connection into a 3.5 mm audio jack, allowing you to use traditional wired headphones with your phone. The adapter bridges the gap between the old headphone interface and the new phone design.

The Facade pattern is a structural design pattern that provides a simplified interface to a complex system of classes or interfaces.

Let’s understand it with example. Imagine a home automation system.

Let’s implement it programmatically.

public class FacadePatternTestDrive
{
    public static void Main(string[] args)
    {
        ILights lights = new Lights();
        IAirConditioner airConditioner = new AirConditioner();
        ISecuritySystem securitySystem = new SecuritySystem();

        // When leaving home
        lights.TurnOff();
        airConditioner.TurnOff();
        securitySystem.Arm();
        Console.WriteLine("You have left home.");

        // When entering home
        lights.TurnOn();
        airConditioner.TurnOn();
        securitySystem.Disarm();
        Console.WriteLine("Welcome home!");
    }

}

Note: AirConditioner, SecuritySystem and Lights class is not present in the above example for the sake of simplicity. We will write those classes when we implement the facade pattern.

The Decorator pattern is a structural pattern that enables you to wrap an object with additional behaviors or responsibilities without changing its external interface. It’s like adding layers of decorations to a gift — you can add or remove decorations without altering the gift itself.

Example:

Let’s say, we have a book whose base price is 500 INR. Some buyers need a hard cover book which costs some extra money, some need a signed copy which also add some additional cost, some needs the both hard cover and a signed copy which adds more cost. May be in future we want to remove the option of hard cover book or may be we want to add some extra features to it. How do we make our system more flexible, that is the big question.

The Observer pattern is a behavioral design pattern that defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

In a simple words, it is like a publisher and subscriber pattern. When publisher publishes something, all it’s subscriber gets notified.

One Example could be, when we subscribe someone’s mailing list we get notified whenever the publisher publishes something.

The Abstract Factory pattern is a creational design pattern that provides an interface for creating families of related or dependent objects without specifying their concrete classes. It allows a client to create objects from a variety of related classes without needing to know the specifics of their implementations.

GUI Factory Example

Let’s say, we need to create GUI components for different themes in our application. Each theme includes a consistent set of components such as buttons and checkboxes. We want to ensure that the creation of these components is consistent and interchangeable.