An abstract class is a special type of class that cannot be instantiated. An abstract class is designed to be inherited by subclasses that either implement or override its methods. In other words, abstract classes are either partially implemented or not implemented at all.
Data abstraction is the process of hiding specific details and showing only essential information to the user. To achieve abstraction, the keyword abstract is used.
- Abstract class: is a restricted class that cannot be used to create objects (to access it, it must be inherited from another class).
- Abstract method: It can only be used in an abstract class and does not have a body. The body is provided by the derived class (inherited from).
Below is an example of DataAccess an abstract class with three methods:
LoadConnectionString - A virtual method provides an implementation option, and the derived class can override.
LoadData and SaveData - Are abstract methods and do not provide an implementation option meaning the class that inherits it will have to override it.
public abstract class DataAccess
{
public virtual string LoadConnectionString(string name)
{
Console.WriteLine("Load Connection String");
return "testConnectionString";
}
public abstract void LoadData(string sql);
public abstract void SaveData(string sql);
}Below is a snippet of code where SqliteDataAccess class inherits from DataAccess, therefore it overrides the methods from the abstract class.
public class SqliteDataAccess : DataAccess
{
public override string LoadConnectionString(string name)
{
string output = base.LoadConnectionString(name);
return output += " (from SQLite)";
}
public override void LoadData(string sql)
{
Console.WriteLine("Loading SQLite Data");
}
public override void SaveData(string sql)
{
Console.WriteLine("Saving data to SQLite");
}
}- Abstract Method resides in abstract class and it has no body.
- Abstract Method must be overridden in non-abstract child class.
- Virtual Method can reside in abstract and non-abstract class.
- It is not necessary to override virtual method in derived but it can be.
- Virtual method must have body can be overridden by "override keyword".
Abstract classes usually provide default or template functionality, and private members. Interfaces are much more minimal, only giving public methods. This means that interfaces are great for the 'I' in SOLID coding, if a class A implements interface I, then other functions that take objects of type I as a param can work while knowing almost nothing about that param. Functions that expect an Abstract Class as a parameter (or attribute) have to know a lot about it, which creates tighter coupling.