Problem with interface

Question

Hi

there are two kinds of bank accounts: normal account and saving account.For each, a diiferent method Debit is applied.

I'm trying to implement a method for both accounts. I have already done it with absctract class, but now i want to do it with using interface.

Class Bank is ok.

The first error i meet is public class SavingAccount : Bank 'base class Bank is less accessible than class SavingAccount.

The next error is at public override void Debit(double amount): cannot override inherited member because it's not marked as abstract ... But if I set abstract at line public abstract void Debit(double amount) in class Bank, i get the error: "cannot declare a body because it's abstract.

Thanks

using System;
interface IBank
{
void Debit(double amount);
}

class Bank : IBank
{
private string owner;
private double balance;

public string Owner
{
get { return owner; }
}

public double Balance
{
get { return balance; }
set { balance = value; }
}

public string See()
{
string descr = balance.ToString();
return descr;
}

public Bank(string theOwner, double theBalance)
{
owner = theOwner;
balance = theBalance;
}

public void Debit(double amount)
{

}
}

public class SavingAccount : Bank
{
public SavingAccount(string theOwner, double theBalance) : base(theOwner, theBalance)
{

}
public override void Debit(double amount)
{
Balance -= amount - 5;
}
}

public class NormalAccount : Bank
{
public NormalAccount(string theOwner, double theBalance) : base(theOwner,theBalance)
{

}
public override void Debit(double amount)
{
Balance -= amount - 10;
}
}

public class Program
{
static void Main(string[] args)
{
SavingAccount saveBob = new SavingAccount("Bob", 1000);
saveBob.Debit(400);
Console.WriteLine(saveBob.See());

NormalAccount normalBob = new NormalAccount("Bob", 200);
normalBob.Debit(200);
Console.WriteLine(saveBob.See());
}
}

Riccardo Rigutini · Answer

Hi, the first error is caused by the fact that your derived classes ( SavingAccount , NormalAccount ) are marked as public , while the base class ( Bank ) is internal (a class by default is internal if you don't specify an access modifier). See for instance the Microsoft documentation https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/access-modifiers: Derived classes and derived records can't have greater accessibility than their base types. You can't declare a public class B that derives from an internal class A. If allowed, it would have the effect of making A public, because all protected or internal members of A are accessible from the derived class. To fix it, you can either mark the Bank class as public: public class Bank : IBank { ... } or remove the public access modifier from the derived classes: class SavingAccount : Bank { ... } class NormalAccount : Bank { ... } For the second error, your Debit method in the Bank class is declaring a (empty) body, and a method that has a body cannot be declared as abstract. The abstract modifier indicates that a method has a missing implementation, and the error is caused by the fact that you are actually providing an implementation. Moreover, only abstract classes can contain abstract methods. From your example I'm assuming you are using Bank only as a base class, and you'll never need to instantiate it directly (e.g. var bank = new Bank() ), so you can safely mark it as abstract. You can the fix the error by marking both Bank and its Debit method as abstract, and removing the body from Debit: abstract class Bank : IBank { private string owner; private double balance; public string Owner { get { return owner; } } public double Balance { get { return balance; } set { balance = value; } } public string See() { string descr = balance.ToString(); return descr; } public Bank( string theOwner, double theBalance) { owner = theOwner; balance = theBalance; } public abstract void Debit( double amount); } As an alternative, if you don't want to use an abstract class, you can mark the method as virtual instead of abstract: class Bank : IBank { private string owner; private double balance; public string Owner { get { return owner; } } public double Balance { get { return balance; } set { balance = value; } } public string See() { string descr = balance.ToString(); return descr; } public Bank( string theOwner, double theBalance) { owner = theOwner; balance = theBalance; } public virtual void Debit( double amount) { } }

Sachin Singh · Answer

Point you missed 1. Methods inside the interface are public by default but not the interface itself. This means you need to make Interface public and other classes also public to make it accessible. 2. All methods within the interface are by default abstract so need to use abstract/override explicitly. modify the code as 3. To override a method i n C#, the method in the base class needs to be explicitly marked as virtual . It doesn't matter if the method implements an interface method or not. using System; public Interface IBank { void Debit( double amount); } public class Bank : IBank { private string owner; private double balance; public string Owner { get { return owner; } } public double Balance { get { return balance; } set { balance = value; } } public string See() { string descr = balance.ToString(); return descr; } public Bank(string theOwner, double theBalance) { owner = theOwner; balance = theBalance; } public virtual void Debit( double amount) // notice here { } } public class SavingAccount : Bank { public SavingAccount(string theOwner, double theBalance) : base(theOwner, theBalance) { } public override void Debit( double amount) { Balance -= amount - 5; } } public class NormalAccount : Bank { public NormalAccount(string theOwner, double theBalance) : base(theOwner,theBalance) { } public override void Debit( double amount) { Balance -= amount - 10; } } public class Program { static void Main(string[] args) { SavingAccount saveBob = new SavingAccount( "Bob" , 1000); saveBob.Debit(400); Console.WriteLine(saveBob.See()); NormalAccount normalBob = new NormalAccount( "Bob" , 200); normalBob.Debit(200); Console.WriteLine(saveBob.See()); } } also, when you are using an interface then you don't need to inherit classes from another class until you really need them. try to keep the code clean by inheriting from the interface and implementing the interface methods in child classes so that your whole project will have a uniform vocabulary, any new developer will find the purpose of the methods easily by just analyzing your interface.

Valerie Meunier · Answer

Thanks both for replying. What do you mean with " also, when you are using an interface then you don't need to inherit classes from another class until you really need them". Is the class Bank superfluous in this case, accoding to you? But then, i have to define in each child class (SavingAccount and NormalAccount) the same fields owner and balance ? One more question: in this case, is it better to use an abstract class or an interface? Thanks

Problem with interface

Answers (3)