Why do I get an error instantiating an interface

Question

I have a class and an interface  and when I try to instantiate the interface  I get an error      Cannot create an instance of the abstract class or interface   My code is below   namespace MyNamespace       public interface IUser               int Property1   get  set            string Property2   get  set            string Property3   get  set            void GetUser               public class User   IUser               public int Property1   get  set            public string Property2   get  set            public string Property3   get  set             public void GetUser                          some logic here                             When I try to instantiate IUser user   new IUser    I get an error      Cannot create an instance of the abstract class or interface   What am I doing wrong here

User · Answer

You can t instantiate interfaces or abstract classes   That s because it wouldn t have any logic to it   Interfaces provide a contract of the methods that should be in a class  without implementation   So there s no actual logic in the interface    Abstract classes provide basic logic of a class  but are not fully functional  not everything is implemented   So again  you won t be able to do anything with it

User · Answer

IUser is the interface  you can t instantiate the interface   You need to instantiate the concrete class that implements the interface   IUser user   new User      or   User user   new User

User · Answer

You cannot instantiate an abstract class or interface   You must inherit it  if its an abstract class  or implement it if it s an interface  e g       private class User   IUser            User u   new User

User · Answer

It is what it says  you just cannot instantiate an abstract class  You need to implement it first  then instantiate that class   IUser user   new User

User · Answer

The error message seems self-explanatory  You can t instantiate an instance of an interface  and you ve declared IUser as an interface   The same rule applies to abstract classes   The whole point of an interface is that it doesn t do anything mdash there is no implementation provided for its methods   However  you can instantiate an instance of a class that implements that interface  provides an implementation for its methods   which in your case is the User class   Thus  your code needs to look like this   IUser user   new User      This instantiates an instance of the User class  which provides the implementation   and assigns it to an object variable for the interface type  IUser  which provides the interface  the way in which you as the programmer can interact with the object    Of course  you could also write   User user   new User      which creates an instance of the User class and assigns it to an object variable of the same type  but that sort of defeats the purpose of a defining a separate interface in the first place

User · Answer

Imagine if one went into a store and asked for a device with a power switch   You didn t say whether you wanted a copier  television  vacuum cleaner  desk lamp  waffle maker  or anything   You asked for a device with a power switch   Would you expect the clerk to offer you something that could only be described as  a device with a power switch    A typical interface would be analogous to the description  a device with a power switch    Knowing that a piece of equipment is   a device with a power switch  would allow one to do some operations with it  i e  turn it on and off   and one might plausibly want a list of e g   devices with power switches that will need to be turned off at the end of the day   without the devices having to share any characteristic beyond having a power switch  but such situations generally only apply when applying some common operation to devices that were created for some more specific purpose   When creating something from scratch  one would more likely wand a  copier    television    vacuum cleaner   or other particular type of device  than some random  device with a power switch    There are some circumstances where one may want a vaguely-defined object  and really not care about what exactly it is    Give me your cheapest device that can boil water    It would be nice if one could specify that when someone asks for an arbitrary object with  water boiling  ability  they should be offered an Acme 359 Electric Teakettle  and indeed when using classes it s possible to do that   Note  however  that someone who asks for a  device to boil water  would not be given a  device to boil water   but an  Acme 359 Electric Teakettle

[c#] Why do I get an error instantiating an interface?

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