SOLID Principles In C# - Liskov Substitution Principle

This class does not follow the “Liskov Substitution Principle” as whenever we create an instance of the RegularDataFileUser class and try to replace the AccessDataFile base class, we will get an error when we try to call the “WriteFile” function. However, this works fine from the base class directly. Hence, the Liskov Substitution Principle is not followed in this design.

//Calling class not following Liskov Substitution Principle  
  
AccessDataFile accessDataFile = new AdminDataFileUser();  
accessDataFile.FilePath = @"c:\temp\a.txt";  
accessDataFile.ReadFile();  
accessDataFile.WriteFile();  
  
AccessDataFile accessDataFileR = new RegularDataFileUser();  
accessDataFileR.FilePath = @"c:\temp\a.txt";  
accessDataFileR.ReadFile();  
//accessDataFileR.WriteFile();  // Throws exception  

We can fix this as below,

// Following the Liskov Substitution Principle    
    
    public interface IFileReader     
    {    
        void ReadFile(string filePath);    
    }    
    
    public interface IFileWriter     
    {    
        void WriteFile(string filePath);    
    }    
    
    public class AdminDataFileUserFixed : IFileReader, IFileWriter    
    {    
        public void ReadFile(string filePath)    
        {    
            // Read File logic    
            Console.WriteLine($"File {filePath} has been read");    
        }    
    
        public void WriteFile(string filePath)    
        {    
            //Write File Logic    
            Console.WriteLine($"File {filePath} has been written");    
        }    
    }    
    
    public class RegularDataFileUserFixed : IFileReader    
    {    
        public void ReadFile(string filePath)    
        {    
            // Read File logic    
            Console.WriteLine($"File {filePath} has been read");    
        }    
    }    

In the above code, we see that we have created two separate interfaces for the file reader and file writer. Now our classes would implement only the interfaces needed and then the interfaces would be replaceable without any issues as below,

//Calling class following Liskov Substitution Principle  
  
IFileReader fileReader = new AdminDataFileUserFixed();  
fileReader.ReadFile(@"c:\temp\a.txt");  
  
IFileWriter fileWriter = new AdminDataFileUserFixed();  
fileWriter.WriteFile(@"c:\temp\a.txt");  
  
IFileReader fileReaderR = new RegularDataFileUserFixed();  
fileReaderR.ReadFile(@"c:\temp\a.txt");  

Summary

In this article, we have looked at implementing the Liskov Substitution Principle (SRP) in a practical example. I would recommend you look through your existing classes and identify places where you have violated this principle and then think of ways to fix it. This will help to get you thinking in terms of applying this principle and help you to apply it to your code in the future as well. In my next article, we will look at the Interface Segregation Principle.