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POExplanation of Interface usage (C# perspective)
 

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  1. POExplanation of Interface usage (C# perspective)
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    <p>I'm working with C# and .NET for some time now and even though I've been working with OOP languages before (Java to be more precise), while working on .NET applications I see interfaces used a lot more than I've seen in Java. I don't have a lot of experience at all so I'm not saying that interfaces are more used in .NET but now I felt the need to get deeper understanding of interfaces and what are the benefits which make people to use them so often.</p> <p>I've some stuff on the internet, some stuff here, and the explanation that made most sense for me was this - <a href="https://stackoverflow.com/questions/444245/how-will-i-know-when-to-create-an-interface">How will I know when to create an interface?</a>. </p> <p>And here comes my question. I don't expect some magical explanation of interfaces at all, cause I've read enough to see that there is not such thing. I hope that with experience and coding the deeper understanding will come by itself so I decided to try the example pointed as benefit of using the interfaces from the accepted answer in the link. I'll copy-paste it here too :</p> <p>you have a, b, c, d of 4 different types. all over your code you have something like:</p> <pre><code>a.Process(); b.Process(); c.Process(); d.Process(); </code></pre> <p>Why not have them implement <code>IProcessable</code>, and then do</p> <pre><code>List&lt;IProcessable&gt; list; foreach(IProcessable p in list) p.Process(); </code></pre> <p>This will scale much better when you add, say, 50 types of classes that all do the same thing.</p> <p>The problem is that it seems I haven't got this. After I read it I open my Visual Studio 2010 and tried to recreate the example like this - I made a project and created four classes - <code>a</code>, <code>b</code>, <code>c</code>, <code>d</code>. All those classes are the same :</p> <pre><code>using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace ConsoleApplication1 { class a : ITest { public void Test() { System.Console.WriteLine("Hi from a"); } } } </code></pre> <p>Just 4 different names and the method <code>Test()</code> printing the name. Here is the interface:</p> <pre><code>using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace ConsoleApplication1 { interface ITest { public void Test(); } } </code></pre> <p>And by what I understand in my <code>Program.cs</code> I have this:</p> <pre><code>using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace ConsoleApplication1 { class Program { List&lt;ITest&gt; list; static void Main(string[] args) { foreach (ITest p in list) p.Test(); } } } </code></pre> <p>Well. It seems that I really got the things wrong. I thought that by implementing the same interface I can automatically get a list of all classes and use them, for example like in my <code>Main()</code> method above. However I get the following error:</p> <blockquote> <p><em>An object reference is required for the non-static field, method, or property</em></p> </blockquote> <p>Well, can someone lower the level to me and explain me in terms of this example how I exactly benefit like this by using an interface.</p>
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    1. COThe interfaces aren't the problem here -- it's the `list` field of `Program`. That needs to be a static field to use it from the static method `Main`. (You'll probably also want to put some things in that list)
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    2. COThis would be a good read - http://stackoverflow.com/q/14482483/218882
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