I am new in c# development. I just trying to study the interface feature. Based on the articles and notes I read about interfaces, I tried to write a sample code to implement interface based on what I understood from those notes and articles.
But when I debugging the project I got a build error
"Inconsistent accessibility: parameter type 'StartMachine' is less accessible than method 'SwitchBoard.switchPress(StartMachine)'".
What is the problem here ?. or Did I implemented the interface in correct way ? or Is my concept about interface is wrong ?..
Please help.
Thanks in advance.
I posted my code below.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
StartMachine s1 = new Machine1();
SwitchBoard switch1 = new SwitchBoard();
switch1.switchPress(s1);
}
}
interface StartMachine
{
void startMachine();
}
public class Machine1 : StartMachine
{
public void startMachine()
{
HttpContext.Current.Response.Write("Machine 1 Started");
}
}
public class Machine2 : StartMachine
{
public void startMachine()
{
HttpContext.Current.Response.Write("Machine 2 Started");
}
}
public class SwitchBoard
{
public void switchPress(StartMachine switchNum)
{
switchNum.startMachine();
}
}
You need to put a public access modifier in front of your interface. The Machine1/2 classes are public, but your interface is defaulting to internal. This is less accessible than public.
Set interface to public access modifer:
public interface StartMachine {...}
Alternatively, you could change your classes to be:
internal class Machine1: StartMachine {...}
internal class Machine2: StartMachine {...}
For more information on access modifiers, take a look on MSDN.
Correction::
The Switchboard class and switchPress method are public. However, the switchPress method is attempting to access the StartMachine.startMachine (ugh, ambigious naming) method, which is internal (by default). You need to either change the StartMachine interface to be public, or change Switchboard/switchPress tointernal.
Consider the error message, you have an "inconsistent accessibility" between your method and your interface. The fix is simple, mark your interface public.
public interface StartMachine
You have a public method that accepts a StartMachine, yet the interface itself was not marked public. External code would be able to see the method yet not be able to provide the appropriate argument to it.
Make your interface public.
public interface StartMachine
{
...
}
As others have said, mark the interface public. I'll add that I think the confusion here is that while all interface members are automatically public, the interface itself does not have to be. You can have private interfaces for use only internally to a library.
Related
Been reading all day on interfaces and abstract classes trying to get a grasp on them to better understand the amazon library I'm working with. I have this code:
using MWSClientCsRuntime;
namespace MarketplaceWebServiceOrders.Model
{
public interface IMWSResponse : IMwsObject
{
ResponseHeaderMetadata ResponseHeaderMetadata { get; set; }
}
and
namespace MWSClientCsRuntime
{
public interface IMwsObject
{
void ReadFragmentFrom(IMwsReader r);
string ToXML();
string ToXMLFragment();
void WriteFragmentTo(IMwsWriter w);
void WriteTo(IMwsWriter w);
}
}
My first questions is I thought Interfaces cannot contain fields, however they can contain properties usch as ResponseHeaderMetadata?
Second, in my main program I have this line of code:
IMWSResponse response = null;
with response being later used to store the information that amazon sends back after a method call is invoked. But what is the meaning behind setting a variable of an interface type to null?
Also, a interface can implement another interface? It isn't only classes that can implement interfaces, but interfaces themselves as well?
Pproperties can be present in interfaces since properties are actually methods - the use of T GetSomeValue() alongside void SetSomeValue(T value) became so common in other languages, that C# implements these as properties.
The meaning behind setting an interface member to null is the same as setting anyother property to null - since a property's set accessor is a method, it's like calling any other method on the interface. What null means where is up to the implementation.
Interfaces do not implement each other, since and interface cannot contain code and therefore is not implementing; Interface inheritance allows one to require one interface in another. A big example is IEnumerable<T>, which is so closely tied to IEnumerable that it inherits, thus meaning any class implementing IEnumerable<T> must also implement IEnumerable.
An interface is like a contractual agreement. By inheriting an interface from a class, you are saying, "I agree to implement all of the methods defined in this interface". So if you have an interface like this:
public interface IWorker {
void DoWork();
}
and you use that interface like this:
public class Employee : IWorker
{
// you are forced to implement this method
void DoWork {}
}
public class Contractor: IWorker
{
// you are forced to implement this method
void DoWork {}
}
By "inheriting" interfaces by other interfaces, you are simply agreeing to implement any methods in the other interfaces, like so (from MSDN):
interface IBase
{
void F();
}
interface IDerived: IBase
{
void G();
}
class C: IDerived
{
void IBase.F() {...}
void IDerived.G() {...}
}
class D: C, IDerived
{
public void F() {...}
public void G() {...}
}
You do not have to set a variable of an interface type to null, though you have the power to do so. The great thing about interfaces is that you are able to set a variable of the type of interface, to anything that "inherits" that interface.
Why will my interface not import into my class? I just want to test some interfaces and need to have the one method extended into my class that implements my interface. This I thought would be simple but it is throwing me an error which I will list at the bottom. My method is public and it is throwing me and error. In fact I took the public keyword away and it is still throwing me an error.
Is there a short cut key to import all of the methods into a class.
Code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace FlowControl
{
public abstract class Grades : iBase, iFoot
{
public abstract void AddGrade(float grade);
public abstract void ComputeStatistics();
public void WriteGrades(string t)
{
}
public int AddTwo(int a, int b)
{
return 0;
}
void findYourScore() { }
}
public interface iBase
{
public void findYourScore();
}
public interface iFoot
{
}
}
But I get this error:
The modifier 'public' is not valid for this item
You can't specify access modifiers on interface methods, properties, events, or indexers. The interface itself is marked public, therefore all of its members are also implicitly public. However, you will have to specify an access modifier where you're implementing the method (unless you're writing an explicit interface member implementation).
For example:
public abstract class Grades : iBase, iFoot
{
...
public void findYourScore() { }
}
public interface iBase
{
void findYourScore();
}
Further Reading
interface (C# Reference)
Interfaces (C# Programming Guide)
You can't specify the access modifiers inside of interface.You just need to specify signature of the method
public interface iBase
{
void findYourScore();
}
From documentation:
Interfaces can contain methods, properties, events, indexers, or any combination of those four member types. An interface can't contain constants, fields, operators, instance constructors, destructors, or types. Interface members are automatically public, and they can't include any access modifiers. Members also can't be static.
It is as simple as
public interface iBase
{
void findYourScore();
}
Interface members don't need access qualifiers. public is then not valid in this context, as the compiler says.
Interfaces do not specify the protection levels or methods and do not implement them either. Removes the public/private piece and your code will work fine. Classes which implement an interface determine what the access level is. If you need to specify access level use an abstract class instead of an interface.
I would like to have an interface for a problem called IProblem. With two methods: Solve() and CheckArguments(). The Problem class will implement the CheckArguments() function because it will be the same for all the problems. But then I have different types of problems like EasyProblem and HardProblem that have different implementations of Solve() method but the CheckArguments() method always be the same and I always want to use the base class Problem()'s implementation.
I would like to have correct modifiers and I'm a bit confused on which method being defined in which class/interface. Not to mention I also have a test project for both these functions.
I'm not sure if your question is "what to use", but I'd suggest an interface and an abstract class:
public interface IProblem {
void Solve();
void CheckArguments();
}
public abstract class Problem : IProblem {
public abstract void Solve();
public void CheckArguments() {
...
}
}
public class EasyProblem : Problem
{
public override void Solve()
{
....
}
}
This way, check arguments is implemented in the base class, all derived classes implement IProblem and every derived class must implement Solve.
If you leave out the interface and only support classes which derive from Problem, you'll make sure that a given class can't give it's own implementation of CheckArguments().
public abstract class Problem {
public abstract void Solve();
public void CheckArguments() {
...
}
}
public class EasyProblem : Problem
{
public override void Solve()
{
....
}
}
...
static Main(string[] args)
{
List<Problem> problemsToSolve = ...
foreach(var problem in problemsToSolve)
{
problem.CheckArguments();
problem.Solve();
}
}
You can try something like:
public interface ISupportArguments
{
bool CheckArguments();
}
public abstract class AbstractProblem : ISupportArguments
{
public bool CheckArguments() {
return true;
}
public abstract void SolveProblem();
}
so every your class derives from AbstractProblem and override it's own version of
SolveProblem(..)
The class structure has been shown by Matten very well.
As regards access modifiers: I'd propose a defensive approach, so that you use the most restrictive access modifier that solves the problem. It is easier to be less restrictive afterwards than to be more restrictive as you might have to explain to some users of your code why they cannot use it anymore.
So for the types (interface and classes): if you don't need them in other assemblies, rather define them as internal. If you want to access the types from your test project, you can use the InternalsVisibleTo attribute to be able to access them from specific assemblies. You add the attribute to the assembly containing the types and provide the name (and for strong named assemblies some additional data) of the test assembly as a parameter.
The same applies to the members. You can also think about implementing the interface explicitly, so you can access the methods only if you access the class via the interface.
Im kind of new to the whole C#, but basically im writing a plugin based architecture for an app im working on. Every plugin will need to have some basic things as such I have an interface as follows:
interface IPlugin
{
string Username {get;}
string Password {get;}
}
The problem is that the username and password will only be used within the class implementing the interface, as such there is no need to make it public.
So that means I cant use an interface since it is only allowed to be public. I was thinking i could use an abstract class but what is the correct access modifier I would need to put on a class member so that I can implement I can see it when I inherit from the class.
I tried the following but it never worked, and i know why it doesn't, i just don't know what the correct modifier is.
abstract class Plugin
{
private string Username;
}
class Imp : Plugin
{
this.Username = "Taylor";
}
Try to use protected modifier, so that fields can be accessible from subclases
abstract class Plugin
{
protected string Username;
protected string Password;
}
class Imp : Plugin
{
public Imp()
{
base.Username = "Taylor";
base.Password = "Pass";
}
}
You can omit base accesor or use this instead, but I've used to explicitly state what I am changing. It make code a little bit more readable and less ambiguous.
You are looking for the protected modifier.
I think you're looking for the protected keyword, like this:
abstract class Plugin
{
protected string Username;
}
The correct modifier is protected. You are right about using an abstract class and not interface in this case - interface is a contract so that the outside world knows some capabilities of the implementors, while abstract class may (and often does) contain some logic and protected members used by that logic.
You are correct in that Interfaces only expose Public methods and properties. You cannot set access modifiers in interfaces.
Given your case, creating an abstract is probably a correct approach. To make a field or property visible only to classes which inherit from your abstract class, you should use the protected access modifier.
For more information: protected access modifier
In your example:
abstract class Plugin
{
protected string Username;
}
class Imp : Plugin
{
public Imp()
{
this.Username = "Taylor"; // No error here...
}
}
As others said the right approach is to have an abstract class as a base class. This means only your Imp class will be able to access Username. But you can achieve close to that with interfaces, though not exactly.
interface IPlugin
{
string Username { get; }
}
class Imp : IPlugin
{
string IPlugin.Username
{
get { return "Taylor"; }
}
}
The key is explicit implementation of interfaces. Now you wont be able to do:
new Imp().Username; //error
But you will able to do:
((IPlugin)new Imp()).Username; //works
In explicit implementation, Username is public only to the interface instance, not the derived type instance.
As to why private is not allowed, see Non Public Members for C# Interfaces
This is something curious that I saw in my coding today.
Here is the sample code:
public class SomeClass
{
public IUtils UtilitiesProperty { get; set; }
}
public interface IUtils
{
void DoSomething();
}
public class Utils : IUtils
{
void DoSomething();
}
This compiles fine.
So what is UtilitiesProperty? Is it a Util? What if more than one class implemented IUTil? Would it fail the compile then?
It doesn't have any value until you give it one (or rather, it has the value null). If you assign it a Utils reference, then yes: it is a Utils, exposed via the IUtils interface. You can only give it null or things that implement IUtils.
It's a property that can hold an object that implements your IUtils interface. More classes can implement this interface and using the interface allows you a level of abstraction (the consumer doesn't care as long as the class adheres to the interface contract).
I'd suggest you read up on the use of interfaces, abstract classes and the like.
For example the MSDN docs.