In software development, SOLID is an acronym that represents a set of principles for designing well-structured and maintainable object-oriented code. The SOLID principles help in achieving code that is easier to understand, test, and maintain. These principles were introduced by Robert C. Martin (also known as Uncle Bob). Let's briefly go through each principle:
Single Responsibility Principle (SRP): A class should have only one reason to change. It states that a class should have a single responsibility and should focus on doing one thing well. This promotes modular and cohesive code.
The Single Responsibility Principle gives us a good way of identifying classes at the design phase of an application, and it makes you think of all the ways a class can change. However, a good separation of responsibilities is done only when we have the full picture of how the application should work. Let us check this with an example.
public class UserService
{
public void Register(string email, string password)
{
if (!ValidateEmail(email))
throw new ValidationException("Email is not an email");
var user = new User(email, password);
SendEmail(new MailMessage("mysite@nowhere.com", email) { Subject="HEllo foo" });
}
public virtual bool ValidateEmail(string email)
{
return email.Contains("@");
}
public bool SendEmail(MailMessage message)
{
_smtpClient.Send(message);
}
}It looks fine, but it is not following SRP. The SendEmail and ValidateEmail methods have nothing to do with the UserService class. Let's refract it.
public class UserService
{
EmailService _emailService;
DbContext _dbContext;
public UserService(EmailService aEmailService, DbContext aDbContext)
{
_emailService = aEmailService;
_dbContext = aDbContext;
}
public void Register(string email, string password)
{
if (!_emailService.ValidateEmail(email))
throw new ValidationException("Email is not an email");
var user = new User(email, password);
_dbContext.Save(user);
emailService.SendEmail(new MailMessage("myname@mydomain.com", email) {Subject="Hi. How are you!"});
}
}
public class EmailService
{
SmtpClient _smtpClient;
public EmailService(SmtpClient aSmtpClient)
{
_smtpClient = aSmtpClient;
}
public bool virtual ValidateEmail(string email)
{
return email.Contains("@");
}
public bool SendEmail(MailMessage message)
{
_smtpClient.Send(message);
}
}Open/Closed Principle (OCP): Software entities (classes, modules, functions, etc.) should be open for extension but closed for modification. This principle suggests that you should design your code in a way that allows adding new functionality by extending existing code, rather than modifying the existing code.
Here "Open for extension" means we must design our module/class so that the new functionality can be added only when new requirements are generated. "Closed for modification" means we have already developed a class, and it has gone through unit testing. We should then not alter it until we find bugs. As it says, a class should be open for extensions; we can use inheritance. OK, let's dive into an example.
Suppose we have a Rectangle class with the properties Height and Width.
public class Rectangle{
public double Height {get;set;}
public double Wight {get;set; }
}Our app needs to calculate the total area of a collection of Rectangles. Since we already learned the Single Responsibility Principle (SRP), we don't need to put the total area calculation code inside the rectangle. So here, I created another class for area calculation.
public class AreaCalculator {
public double TotalArea(Rectangle[] arrRectangles)
{
double area;
foreach(var objRectangle in arrRectangles)
{
area += objRectangle.Height * objRectangle.Width;
}
return area;
}
}Hey, we did it. We made our app without violating SRP. No issues for now. But can we extend our app so that it can calculate the area of not only Rectangles but also the area of Circles? Now we have an issue with the area calculation issue because the way to calculate the circle area is different. Hmm. Not a big deal. We can change the TotalArea method to accept an array of objects as an argument. We check the object type in the loop and do area calculations based on the object type.
public class Rectangle{
public double Height {get;set;}
public double Wight {get;set; }
}
public class Circle{
public double Radius {get;set;}
}
public class AreaCalculator
{
public double TotalArea(object[] arrObjects)
{
double area = 0;
Rectangle objRectangle;
Circle objCircle;
foreach(var obj in arrObjects)
{
if(obj is Rectangle)
{
area += obj.Height * obj.Width;
}
else
{
objCircle = (Circle)obj;
area += objCircle.Radius * objCircle.Radius * Math.PI;
}
}
return area;
}
}Wow. We are done with the change. Here we successfully introduced Circle into our app. We can add a Triangle and calculate its area by adding one more "if" block in the TotalArea method of AreaCalculator. But every time we introduce a new shape, we must alter the TotalArea method. So the AreaCalculator class is not closed for modification. How can we make our design to avoid this situation? Generally, we can do this by referring to abstractions for dependencies, such as interfaces or abstract classes, rather than using concrete classes. Such interfaces can be fixed once developed so the classes that depend upon them can rely upon unchanging abstractions. Functionality can be added by creating new classes that implement the interfaces. So let's refract our code using an interface.
public abstract class Shape
{
public abstract double Area();
}Inheriting from Shape, the Rectangle and Circle classes now look like this:
public class Rectangle: Shape
{
public double Height {get;set;}
public double Width {get;set;}
public override double Area()
{
return Height * Width;
}
}
public class Circle: Shape
{
public double Radius {get;set;}
public override double Area()
{
return Radius * Radus * Math.PI;
}
}Every shape contains its area with its way of calculation functionality, and our AreaCalculator class will become simpler than before.
public class AreaCalculator
{
public double TotalArea(Shape[] arrShapes)
{
double area=0;
foreach(var objShape in arrShapes)
{
area += objShape.Area();
}
return area;
}
}Now our code is following SRP and OCP both. Whenever you introduce a new shape by deriving from the "Shape" abstract class, you need not change the "AreaCalculator" class. Awesome. Isn't it?
Liskov Substitution Principle (LSP): Objects of a superclass should be replaceable with objects of its subclasses without affecting the correctness of the program. It defines that subtypes must be substitutable for their base types without causing any errors or unexpected behavior.
The Liskov Substitution Principle (LSP) states, "you should be able to use any derived class instead of a parent class and have it behave in the same manner without modification.". It ensures that a derived class does not affect the behavior of the parent class; in other words, a derived class must be substitutable for its base class.
This principle is just an extension of the Open Closed Principle, and we must ensure that newly derived classes extend the base classes without changing their behavior. I will explain this with a real-world example that violates LSP.
A father is a doctor, whereas his son wants to become a cricketer. So here, the son can't replace his father even though they belong to the same family hierarchy.
Now jump into an example to learn how a design can violate LSP. Suppose we need to build an app to manage data using a group of SQL files text. Here we need to write functionality to load and save the text of a group of SQL files in the application directory. So we need a class that manages the load and keeps the text of a group of SQL files along with the SqlFile Class.
public class SqlFile
{
public string FilePath {get;set;}
public string FileText {get;set;}
public string LoadText()
{
/* Code to read text from sql file */
}
public string SaveText()
{
/* Code to save text into sql file */
}
}
public class SqlFileManager
{
public List<SqlFile> lstSqlFiles {get;set}
public string GetTextFromFiles()
{
StringBuilder objStrBuilder = new StringBuilder();
foreach(var objFile in lstSqlFiles)
{
objStrBuilder.Append(objFile.LoadText());
}
return objStrBuilder.ToString();
}
public void SaveTextIntoFiles()
{
foreach(var objFile in lstSqlFiles)
{
objFile.SaveText();
}
}
}OK. We are done with our part. The functionality looks good for now. However, after some time, our leaders might tell us that we may have a few read-only files in the application folder, so we must restrict the flow whenever it tries to save them.
OK. We need to modify "SqlFileManager" by adding one condition to the loop to avoid an exception. We can do that by creating a "ReadOnlySqlFile" class that inherits the "SqlFile" class, and we need to alter the SaveTextIntoFiles() method by introducing a condition to prevent calling the SaveText() method on ReadOnlySqlFile instances.
public class SqlFileManager
{
public List<SqlFile? lstSqlFiles {get;set}
public string GetTextFromFiles()
{
StringBuilder objStrBuilder = new StringBuilder();
foreach(var objFile in lstSqlFiles)
{
objStrBuilder.Append(objFile.LoadText());
}
return objStrBuilder.ToString();
}
public void SaveTextIntoFiles()
{
foreach(var objFile in lstSqlFiles)
{
//Check whether the current file object is read-only or not.If yes, skip calling it's
// SaveText() method to skip the exception.
if(! objFile is ReadOnlySqlFile)
objFile.SaveText();
}
}
}Here we altered the SaveTextIntoFiles() method in the SqlFileManager class to determine whether or not the instance is of ReadOnlySqlFile to avoid the exception. We can't use this ReadOnlySqlFile class as a substitute for its parent without altering the SqlFileManager code. So we can say that this design is not following LSP. Let's make this design follow the LSP. Here we will introduce interfaces to make the SqlFileManager class independent from the rest of the blocks.
public interface IReadableSqlFile
{
string LoadText();
}
public interface IWritableSqlFile
{
void SaveText();
}Now we implement IReadableSqlFile through the ReadOnlySqlFile class that reads only the text from read-only files. We implement both IWritableSqlFile and IReadableSqlFile in a SqlFile class by which we can read and write files.
public class SqlFile: IWritableSqlFile,IReadableSqlFile
{
public string FilePath {get;set;}
public string FileText {get;set;}
public string LoadText()
{
/* Code to read text from sql file */
}
public void SaveText()
{
/* Code to save text into sql file */
}
}Now the design of the SqlFileManager class becomes like this.
public class SqlFileManager
{
public string GetTextFromFiles(List<IReadableSqlFile> aLstReadableFiles)
{
StringBuilder objStrBuilder = new StringBuilder();
foreach(var objFile in aLstReadableFiles)
{
objStrBuilder.Append(objFile.LoadText());
}
return objStrBuilder.ToString();
}
public void SaveTextIntoFiles(List<IWritableSqlFile> aLstWritableFiles)
{
foreach(var objFile in aLstWritableFiles)
{
objFile.SaveText();
}
}
}Here the GetTextFromFiles() method gets only the list of instances of classes that implement the IReadOnlySqlFile interface. That means the SqlFile and ReadOnlySqlFile class instances. And the SaveTextIntoFiles() method gets only the list instances of the class that implements the IWritableSqlFiles interface, in this case, SqlFile instances. So now we can say our design is following the LSP. And we fixed the problem using the Interface segregation principle (ISP), identifying the abstraction and the responsibility separation method.
Interface Segregation Principle (ISP): Clients should not be forced to depend on interfaces they do not use. It encourages the creation of specific interfaces for clients, rather than having a single large interface that encompasses all possible behaviors. This prevents clients from being burdened with unnecessary dependencies.
We can define it in another way. An interface should be more closely related to the code that uses it than the code that implements it. So the methods on the interface are defined by which methods the client code needs rather than which methods the class implements. So clients should not be forced to depend upon interfaces they don't use.
Like classes, each interface should have a specific purpose/responsibility (refer to SRP). You shouldn't be forced to implement an interface when your object doesn't share that purpose. The larger the interface, the more likely it includes methods not all implementers can use. That's the essence of the Interface Segregation Principle. Let's start with an example that breaks the ISP. Suppose we need to build a system for an IT firm that contains roles like TeamLead and Programmer where TeamLead divides a huge task into smaller tasks and assigns them to his/her programmers or can directly work on them.
Based on specifications, we need to create an interface and a TeamLead class to implement it.
public Interface ILead
{
void CreateSubTask();
void AssginTask();
void WorkOnTask();
}
public class TeamLead : ILead
{
public void AssignTask()
{
//Code to assign a task.
}
public void CreateSubTask()
{
//Code to create a sub task
}
public void WorkOnTask()
{
//Code to implement perform assigned task.
}
}OK. The design looks fine for now. However, later another role, like Manager, who assigns tasks to TeamLead and will not work on the tasks, is introduced into the system. Can we directly implement an ILead interface in the Manager class, like the following?
public class Manager: ILead
{
public void AssignTask()
{
//Code to assign a task.
}
public void CreateSubTask()
{
//Code to create a sub task.
}
public void WorkOnTask()
{
throw new Exception("Manager can't work on Task");
}
}Since the Manager can't work on a task and, at the same time, no one can assign tasks to the Manager, this WorkOnTask() should not be in the Manager class. But we are implementing this class from the ILead interface; we must provide a concrete Method. Here we are forcing the Manager class to implement a WorkOnTask() method without a purpose. This is wrong. The design violates ISP. Let's correct the design.
Since we have three roles, 1, managers can only divide and assign tasks, 2. TeamLead can divide and assign the jobs and work on them, 3. We need to divide the responsibilities by segregating the ILead interface for the programmer that can only work on tasks—an interface that provides a contract for WorkOnTask().
public interface IProgrammer
{
void WorkOnTask();
}An interface that provides contracts to manage the tasks:
public interface ILead
{
void AssignTask();
void CreateSubTask();
}Then the implementation becomes.
public class Programmer: IProgrammer
{
public void WorkOnTask()
{
//code to implement to work on the Task.
}
}
public class Manager: ILead
{
public void AssignTask()
{
//Code to assign a Task
}
public void CreateSubTask()
{
//Code to create a sub taks from a task.
}
}TeamLead can manage tasks and can work on them if needed. Then the TeamLead class should implement both the IProgrammer and ILead interfaces.
public class TeamLead: IProgrammer, ILead
{
public void AssignTask()
{
//Code to assign a Task
}
public void CreateSubTask()
{
//Code to create a sub task from a task.
}
public void WorkOnTask()
{
//code to implement to work on the Task.
}
}Wow. Here we separated responsibilities/purposes, distributed them on multiple interfaces, and provided good abstraction.
Dependency Inversion Principle (DIP): High-level modules/classes should not depend on low-level modules/classes. Both should depend on abstractions. Abstractions should not depend on details; details should depend on abstractions. This principle promotes loose coupling between modules and emphasizes the use of interfaces or abstract classes to define dependencies.High-level modules/classes implement business rules or logic in a system (application). Low-level modules/classes deal with more detailed operations; in other words, they may write information to databases or pass messages to the operating system or services.
A high-level module/class that depends on low-level modules/classes or some other class and knows a lot about the other classes it interacts with is said to be tightly coupled. When a class knows explicitly about the design and implementation of another class, it raises the risk that changes to one class will break the other. So we must keep these high-level and low-level modules/classes loosely coupled as much as possible. To do that, we need to make both of them dependent on abstractions instead of knowing each other. Let's start with an example.
Suppose we need to work on an error-logging module that logs exception stack traces into a file. Simple, isn't it? The following classes provide the functionality to log a stack trace into a file.
public class FileLogger
{
public void LogMessage(string aStackTrace)
{
//code to log stack trace into a file.
}
}
public class ExceptionLogger
{
public void LogIntoFile(Exception aException)
{
FileLogger objFileLogger = new FileLogger();
objFileLogger.LogMessage(GetUserReadableMessage(aException));
}
private GetUserReadableMessage(Exception ex)
{
string strMessage = string. Empty;
//code to convert Exception's stack trace and message to user readable format.
....
....
return strMessage;
}
}A client class exports data from many files to a database.
public class DataExporter
{
public void ExportDataFromFile()
{
try {
//code to export data from files to the database.
}
catch(Exception ex)
{
new ExceptionLogger().LogIntoFile(ex);
}
}
}Looks good. We sent our application to the client. But our client wants to store this stack trace in a database if an IO exception occurs. Hmm... OK, no problem. We can implement that too. Here we need to add one more class that provides the functionality to log the stack trace into the database and an extra method in ExceptionLogger to interact with our new class to log the stack trace.
public class DbLogger
{
public void LogMessage(string aMessage)
{
//Code to write message in the database.
}
}
public class FileLogger
{
public void LogMessage(string aStackTrace)
{
//code to log stack trace into a file.
}
}
public class ExceptionLogger
{
public void LogIntoFile(Exception aException)
{
FileLogger objFileLogger = new FileLogger();
objFileLogger.LogMessage(GetUserReadableMessage(aException));
}
public void LogIntoDataBase(Exception aException)
{
DbLogger objDbLogger = new DbLogger();
objDbLogger.LogMessage(GetUserReadableMessage(aException));
}
private string GetUserReadableMessage(Exception ex)
{
string strMessage = string.Empty;
//code to convert Exception's stack trace and message to user readable format.
....
....
return strMessage;
}
}
public class DataExporter
{
public void ExportDataFromFile()
{
try {
//code to export data from files to database.
}
catch(IOException ex)
{
new ExceptionLogger().LogIntoDataBase(ex);
}
catch(Exception ex)
{
new ExceptionLogger().LogIntoFile(ex);
}
}
}Looks fine for now. But whenever the client wants to introduce a new logger, we must alter ExceptionLogger by adding a new method. Suppose we continue doing this after some time. In that case, we will see a fat ExceptionLogger class with a large set of practices that provide the functionality to log a message into various targets. Why does this issue occur? Because ExceptionLogger directly contacts the low-level classes FileLogger and DbLogger to log the exception. We need to alter the design so that this ExceptionLogger class can be loosely coupled with those classes. To do that, we need to introduce an abstraction between them so that ExcetpionLogger can contact the abstraction to log the exception instead of directly depending on the low-level classes.
public interface ILogger
{
void LogMessage(string aString);
}Now our low-level classes need to implement this interface.
public class DbLogger: ILogger
{
public void LogMessage(string aMessage)
{
//Code to write message in database.
}
}
public class FileLogger: ILogger
{
public void LogMessage(string aStackTrace)
{
//code to log stack trace into a file.
}
}Now, we move to the low-level class's initiation from the ExcetpionLogger class to the DataExporter class to make ExceptionLogger loosely coupled with the low-level classes FileLogger and EventLogger. And by doing that, we are giving provision to DataExporter class to decide what kind of Logger should be called based on the exception that occurs.
public class ExceptionLogger
{
private ILogger _logger;
public ExceptionLogger(ILogger aLogger)
{
this._logger = aLogger;
}
public void LogException(Exception aException)
{
string strMessage = GetUserReadableMessage(aException);
this._logger.LogMessage(strMessage);
}
private string GetUserReadableMessage(Exception aException)
{
string strMessage = string.Empty;
//code to convert Exception's stack trace and message to user readable format.
....
....
return strMessage;
}
}
public class DataExporter
{
public void ExportDataFromFile()
{
ExceptionLogger _exceptionLogger;
try {
//code to export data from files to database.
}
catch(IOException ex)
{
_exceptionLogger = new ExceptionLogger(new DbLogger());
_exceptionLogger.LogException(ex);
}
catch(Exception ex)
{
_exceptionLogger = new ExceptionLogger(new FileLogger());
_exceptionLogger.LogException(ex);
}
}
}We successfully removed the dependency on low-level classes. This ExceptionLogger doesn't depend on the FileLogger and EventLogger classes to log the stack trace. We no longer need to change the ExceptionLogger's code for the new logging functionality. We must create a new logging class that implements the ILogger interface and adds another catch block to the DataExporter class's ExportDataFromFile method.
public class EventLogger: ILogger
{
public void LogMessage(string aMessage)
{
//Code to write a message in system's event viewer.
}
}And we need to add a condition in the DataExporter class as in the following:
public class DataExporter
{
public void ExportDataFromFile()
{
ExceptionLogger _exceptionLogger;
try {
//code to export data from files to database.
}
catch(IOException ex)
{
_exceptionLogger = new ExceptionLogger(new DbLogger());
_exceptionLogger.LogException(ex);
}
catch(SqlException ex)
{
_exceptionLogger = new ExceptionLogger(new EventLogger());
_exceptionLogger.LogException(ex);
}
catch(Exception ex)
{
_exceptionLogger = new ExceptionLogger(new FileLogger());
_exceptionLogger.LogException(ex);
}
}
}Looks good. But we introduced the dependency here in the DataExporter class's catch blocks. So, someone must be responsible for providing the necessary objects to the ExceptionLogger to get the work done.
By following these SOLID principles, you can achieve code that is more flexible, maintainable, and easier to test. These principles encourage modular design, separation of concerns, and abstraction, leading to code that is less prone to bugs and easier to extend or modify in the future.
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