Interfaces in C# - What they are, how to use them, and why they are so powerful.

Getting help online can be difficult. People get mad at you, they do not understand your question, or the answers you get are not correct. What do you do? Well, you watch this video and learn how to ask the right question the right way to get the right results.
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- Source Code: leadmagnets.app/?Resource=Int...
- Design Patterns as referenced in the video - • Design Patterns: Don't...
0:00 - Intro
1:06 - Demo application walk-through
4:50 - Explaining the problem: putting instances of different classes (product models) in one list (shopping cart)
7:54 - Creating an interface
12:53 - Implementing the interface
20:29 - List of type Interface
23:43 - Note about items in the List of type Interface: Accessing a Class member not in the contract
(Interface)
25:07 - recap: basics of the Interface
26:48 - Class Inheritance and Interface
implementation
29:02 - Extracting the Interface from a Class
31:19 - Interface implementing another Interface
34:54 - Accessing a Class member outside the initial contract (Interface) in the List of type Interface
40:51 - Power of Interfaces: Adding a new Class to your project, why to use Interfaces

[@mauricemakesmovies]

This channel is kind of like an interface. It even has the capital "I" at the start of the name.

[@IAmTimCorey]

An interface for what, though?

[@mauricemakesmovies]

@@IAmTimCorey Any project can inherit from your teachings. ;)

[@IAmTimCorey]

I like it.

[@HDEditzz]

@@mauricemakesmovies not to nitpick right now, but wouldn‘t you implement it, rather than inherit? ;)

[@MaamuBhanjiVines]

@@HDEditzz spot on

[@IMADETHISACCFORRAY]

God - I watched the entire session which Mosh did on interfaces on udemy and really couldn't grasp it at all. I then watch you, and you just make it so more intuitive and repeat things. It really clicked now. Thank you Tim, you are Godsent.

[@IAmTimCorey]

I'm glad you found it so helpful.

[@pitaszekfistaszek]

Agreed. Mosh did completely non useful example with unit testing.

[@vitaliysakun-duvalko8946]

@@farfaraway2212 Nice that's really cool. Where?

[@vitaliysakun-duvalko8946]

Lol that's the same thing that happened to me. I still feel like he is a great instructor, and he helped me learn a lot in other areas.

[@joeman123964]

finally, a video where the content creator actually DEMONSTRATES the power of interfaces, not just creates one and then say like and subscribe.

[@IAmTimCorey]

I’m glad you liked it.

[@estebanlombardi]

After years of trying, this is my first time that I understood Interfaces ever. Thanks!!!

[@IAmTimCorey]

Excellent!

[@lassel1644]

All good...caution pays👍

[@dimasjimenez3396]

Same goes for me. Thanks a ton Tim!

[@AudioJunkie79]

This is probably the cleanest and easiest explanation of interfaces I have watched or read. Thank you very much for the video. This was super helpful. I come from the industrial automation world where these concepts are just starting to creep in for machine automation code development. This is super helpful.

[@IAmTimCorey]

You are most welcome. Thanks for watching.

[@sekki2554]

Man, your videos might be long, but we don't have to go through it thousand times to understand it. Thanks for your time.

[@IAmTimCorey]

You are welcome

[@maiksonstrife]

That's the best explanation I've ever seen about Interfaces, it links different topics, and cover their different use while expanding its concepts with real-life use but yet simple to understand.

[@IAmTimCorey]

Awesome! I am glad it was so helpful.

[@dylanbaird7987]

I've been using interfaces in my code because that's what the code base was like, that's what the books told me to do, and I understood it as a "best practice". This video helped me understand why those three statements hold up. Thanks for that. Great video.

[@madhavmehendale7324]

What a brilliant brilliant brilliant video! Every hour, every day I learn little more..... or, in the case of your videos "a whole lot more"!

[@IAmTimCorey]

Great to hear!

[@greentaco3402]

Watched multiple tutorials to try and understand what the practical point of interfaces was and none of them really explain why they're so useful. All of them explained what they are and will say they're super useful, but they never really explained why I would actually need to use them. This tutorial explained the why perfectly and now I get it, they are very useful. Thank you.

[@IAmTimCorey]

I am glad it was helpful.

[@kramesh3821]

Hi Tim. I have been in engineering programming for two decades. I was brushing up some advanced C# topics with your videos. These are absolutely fantastic, very clearly explained taking into account all aspects. Your teaching method is by far the best. Thanks.

[@ShayanFiroozi1]

Every time i search something about C# , your channel would be on top !! you are a hero Tim.👍👍

[@IAmTimCorey]

Glad you enjoy it!

[@Sunstriderko]

Tim this video is GOLDEN. I cant find words which describe that amount of help this video provided to me.

[@IAmTimCorey]

Awesome! I am glad my content has been so helpful.

[@jakezepeda1267]

Thanks for this. So many videos explain what an interface is and how to use it, but not what problems it solves over basic inheritance and why you would want to use it other than because someone told you to. This was a great practical example :)

[@IAmTimCorey]

Excellent! I'm glad you found it valuable.

[@Ahmedibrahim0100]

I think you know perfectly what a student needs. Great job, Tim.

[@IAmTimCorey]

Thank you!

[@michaelkelly3241]

I normally do not comment on videos, but... First, I must say the video explained all of the pressure points I have previously had with interfaces, even though if I am being honest I was only looking them up today because I was confused about one specific piece of implementation for a Space Engineers Script I am working on.... Second, this video is over 2 years old... It is still relevant and helping people... and most remarkably IAmTimCorey still seems to reply to basically every comment, no matter how trivial. This is amazing... You sir have earned my Thumbs Up, Subscription, and recommendation to others and I look forward to watching other videos you have to offer. Thank you again!

[@IAmTimCorey]

I am glad it was helpful. Thank you for the kind words.

[@owiz9212]

Tim, your explanation for Interfaces is the best I've seen. You really make a concept that seems rather complex quite simple. I appreciate the time and effort that you put into these videos.

[@IAmTimCorey]

I am glad it was so helpful.

[@maurices.3194]

Have to agree! I never understood the concept of interfaces and abstract classes because it was too complex for me to understand and most explanations i found didn't bring over the message, only examples with animals or cars. Now I got it, thanks to your Videos. I also got a bit more into Async developoing. I appreciate your videos, they do really open a big new field for me.

[@ajaykumark107]

Probably the best video on interfaces, I have seen on internet! Great job!!!

[@IAmTimCorey]

Thank you!

[@joshr3739]

Just wanted to say that I'm pretty new to c# having come from a Javascript functional programming perspective. This is by far the best video on interfaces I've seen, most videos just ended up confusing me even more but you're super clear and easy to understand. Thank you so much! Really appreciate the time you spent to help people understand :)

[@IAmTimCorey]

Glad it helped!

[@KevinBFG]

I never understood the use of interfaces until now, what an impeccable explanation on the subject. It's truly appreciated Tim, thank you!

[@IAmTimCorey]

Awesome!

[@ookennedy]

This is the clearest explanation of interface I have seen. Thank you

[@IAmTimCorey]

You are welcome.

[@VitaliyParitskiy]

Slow, steady and well explained. You got me!

[@IAmTimCorey]

Glad to hear it

[@KevinNguyen1]

Definitely cleared things up. Before I watched this video, I thought interfaces were like header files in C++, but now after watching the video, they do something else than what I thought.

[@IAmTimCorey]

I’m glad it was illuminating.

[@rayl1244]

Went through this lesson a number of videos ago. What I appreciate most is the time Tim takes to explain pitfalls and benefits among other facets of using different parts of a structure, class or what have you. About 12 years ago I build an Interface app in C++ from the ground up. So now I know more of what I did was all about. But I don't think I miss all the background code that VS puts in, you don't even have to worry about the entry point. Although I program mostly as a hobby, I pretty much am self taught. Beginning with writing FORTRAN code on punch cards, a few years ago, and now learning C# and working with ADO during CV-19 quarantine these instructional videos have been, well, fun.

[@IAmTimCorey]

I am glad you enjoyed them.

[@samadhistate9637]

Some people are just naturally good at teaching and explaining. It's a rare skill that not many have. TC is one such person. This is the second video I've watched in full and while I haven't needed to use Interfaces in C# much, I now have a much better appreciation of their use and why they're so powerful and useful. It's rare also to find a publisher who takes the time to answer questions in the comments often with a verbose answer, as opposed to less helpful short cryptic responses.

[@tomthelestaff-iamtimcorey7597]

Thanks for growing your skills and sharing your insights with us.

[@manamahalo1]

This is my 3rd tutorials by Tim I have watched. I have to say he is the most amazing teacher I have ever had. I felt there was a big gap between C# fundamental to the actual .NET development. I went through 40+ tutorials and many were either too easy, scratched only the surface or too difficult; none ever explained so well. The more I watch the more I am convinced that I got to the right person. Cannot thank him enough!!

[@IAmTimCorey]

I am glad you have found my content so valuable.

[@andresndergaard1712]

This is just straight up fantastic! Explanations, illustrations and examples are clear and concise. 10/10

[@IAmTimCorey]

Thank you!

[@mkd1964]

This is THE BEST explanation of Interfaces I've come across. Thank you for the "real-world" example. Before seeing this, everyone's else's explanation just seemed like "another layer of stuff", as you put it. I just couldn't see the point. But this really made clear, how useful Interfarcs can be.

[@IAmTimCorey]

Glad it was helpful!

[@Dual12d]

You deserve much more views than that. This was the best explanation I have ever seen so far.

[@IAmTimCorey]

I appreciate the kind words.

[@johndinovo4168]

Tim, just want to say thanks. My first language was FORTRAN so you can infer my age. I've been programming C# since it first appeared, and I had seen Interfaces but never knew quite what to do with them,. You cleared all that up! I want to rewrite almost every piece of C# code I've ever written :) What always impresses me is the brilliance of the guys who came up with this stuff in the first place and put it into the language. In the great scheme of things there are the theoreticians, like them, who come up with a feature, the engineers like you who figure out how to use the features in powerful and efficient ways, and then the users like me who know a good idea when they steal it. Again, thanks!!!

[@IAmTimCorey]

You are most welcome.

[@GLuiz1982]

You save my life Tim, thank you very much. You are a true educator. I managed to understand everything and I'm not even an English speaker.

[@IAmTimCorey]

You are most welcome.

[@torrvic1156]

You are truly a miracle for anybody learning CSharp Tim! I’ve learned so much stuff from your videos! I loved that you showed advanced technique when one interface can implement the other and then you can see methods and other stuff implemented in some concrete instance implementing one of this interfaces. Brilliant! Thank you so much GREAT teacher! I hope that you are a very rich man because you truly deserve it!

[@IAmTimCorey]

You are welcome.

[@tagyourit3432]

Wow, I finally understand when to use interfaces! Awesome video Tim. Thanks!

[@IAmTimCorey]

Awesome! I am glad it was helpful.

[@cecilchampenois4998]

Thank, Tim. I was not aware that I could control the pixels level. Thank you. It's all very clear now.

[@IAmTimCorey]

You are welcome.

[@videolosss]

7 years of coding C# and now I finally understand why interfaces. Thanks so much.

[@tomthelestaff-iamtimcorey7597]

Thanks for looking to Tim

[@edisonverdejunior2190]

Hi Tim! I just wanna say thank you. I'm Brazilian and kind of English Learner at intermediate level and also a beginner in .Net (C#) and even though, I could understand clearly the lesson. Congratulations to the great didactic way to teach.

[@IAmTimCorey]

I am glad I am so clear to you.

[@chakravarthye]

You are a genius when it comes to teaching.

[@IAmTimCorey]

Wow, thank you!

[@willemvdk4886]

I love these video's Tim! I'm an experienced programmer with various different languages, C# was never one of them, but I'm trying to pick up C# to be able to help a colleague out. Most tutorials are way too basic and beginner-oriented. Your video's exactly hit the spot for me, teaching most of the real C# "gotcha's" instead of how logic operators and if-statements work.

[@IAmTimCorey]

I'm glad they have been helpful.

[@antipodestudios1753]

I love how thorough you are for beginners. Most Unity Devs, such as myself, never wrote in C# before, and tried to find exactly this, but others fly through it, with little explanation. Keep up the great videos! You earned yourself a new sub.

[@jymmorton5612]

Understatement. Like those videos where you can see that they have needed classes in the Solution Explorer but never show you what's in them.

[@IAmTimCorey]

Thanks!

[@karendoran3692]

Tim, I have been struggling with interfaces for the last two months trying to understand how they were advantageous and why I would need them but after watching this video the light bulb went on and I finally see the light. Thank you for presenting this in a way that actually makes sense in a practical example!

[@IAmTimCorey]

Awesome! I'm glad things clicked and you were able to see the value in interfaces.

[@stannisbarracuda5693]

best video i have ever seen on interfaces ever and i've been coding for 3 years

[@IAmTimCorey]

Thanks!

[@T.N.T.15]

say did this cover adding the interface to a startup? i seemed to have missed that if it was added.

[@tommygilchrist7771]

Well explained and helped by a really good practical example, this is the first time I've understood why you would use Interfaces.

[@IAmTimCorey]

Excellent!

[@fakuadegbenga9870]

Thank you Tim, after months of partial confusion, I can fully say I understand interfaces

[@IAmTimCorey]

Awesome!

[@jspesh]

"34:54 - Accessing a Class member outside the initial contract (Interface) in the List of type Interface" was soooo useful, thank you. Nobody has ever explained that to me before. So powerful!

[@IAmTimCorey]

Great to hear!

[@xieulong]

Thanks Tim. This was a very thorough explanation and example.

[@IAmTimCorey]

Glad it was helpful!

[@anhtuanle8318]

Tim Corey, you are one of the best programming teachers

[@IAmTimCorey]

I appreciate the kind words.

[@michalcertik249]

By far the most clear and useful demonstration. Many thanks.

[@IAmTimCorey]

You are welcome.

[@Dumah79]

Your explaning approach is truly amazing. Thanks for your effort Tim.

[@IAmTimCorey]

You are welcome.

[@gregdoucetteguardianangel645]

Your literally a freaking great teacher. Thank you so much your explanation is fantastic. Literally, I understood what they were but didn't realize how interfaces were useful. So I guess I really didn't understand interfaces properly only that it was a contract to derived classes that implemented it. Thank you and btw even though your video is almost 48 mins I swear to god it felt like a few mins. So fun to watch thank you again. I look forward to your other advanced tutorials. I love you!

[@IAmTimCorey]

Glad I could help!

[@Marcel-so4mq]

haha, nice... love your account name:)

[@xtian437]

Great tutorial! I recently shifted to a career in software development using ASP.NET and I'm so happy that I came across this awesome channel. Guess I'll be busy for a while watching your other vids :) Auto subscribed!!

[@IAmTimCorey]

Awesome!

[@MrBombsn]

holy sh1t, finally i understand what interfaces are for!! Everyone tells how they work, how to create them but no one explains what for. Except Tim! Thank you!!!

[@IAmTimCorey]

I am glad it finally clicked!

[@jonathanlusby4213]

Thanks Tim, I have been aware of interfaces for a while but didn't really understand why I should use them. Now I get it. Thanks you for the clear explanation and examples.

[@IAmTimCorey]

I am glad it was helpful.

[@bigdummyhead2162]

Thanks you made this way more helpful then the C# Docs

[@IAmTimCorey]

You are welcome.

[@ranjanadissanayaka5390]

Thanks Sir so much. It feels amazing to have detailed knowledge on those topics.

[@IAmTimCorey]

You are welcome.

[@elliottparedes]

Tim i've been puling my hair out not understanding why we have to have this extra step of making an interfaces in C# but now the lightbulb went off and i get it. Thank you SO much

[@IAmTimCorey]

Awesome!

[@aarondouglas3220]

Almost 3 years later and this video is still teaching folks. I'm a developer who has fallen WAY behind in modern practices and you've been a wonderful teacher to bring me back up to date.

[@IAmTimCorey]

Great!

[@bulent2435]

true.

[@barrythompson3359]

Replies to comments **2 YEARS LATER** We need more people like him

[@IAmTimCorey]

Thanks!

[@rikwouters5043]

These videos of tim are just extremely good

[@IAmTimCorey]

Thanks!

[@freedom13245]

Hi Tim, I have started working as a C# developer and your videos are helping me a lot. Thank you so much for this! I will tell everyone I know about this channel ;) Hands down one of the best if not the best to learn coding on KZbin!

[@IAmTimCorey]

Awesome! I'm glad my content has been so helpful.

[@TwoManyTenors]

Coming from a book learning C# for Unity, this was tremendously insightful and explained the topic in a way that made sense, even if the context was completely different. Thank you for your wonderful work, definitely subscribing to fill in gaps in other areas going forward!

[@tomthelestaff-iamtimcorey7597]

Thank you for continuing your learning with Tim.

[@Layarion]

I enjoy watching his vids at 250% - 400% playback speed with "Video Speed Controller", really helps me pay attention when I can watch at a pace that doesn't let my mind wonder too much.

[@IAmTimCorey]

Sounds good.

[@bruceb8140]

wheere do you find 400% playback? I've been trying to do this forever

[@Layarion]

@@bruceb8140 it's a chrome extension chrome.google.com/webstore/detail/video-speed-controller/nffaoalbilbmmfgbnbgppjihopabppdk?hl=en

[@moonmadness9278]

Such a good and clear explanation! I finally get it ;) AmTimCorey - Class IAmTimCorey - Interface

[@tomthelestaff-iamtimcorey7597]

Thanks for watching

[@PDXpackrat]

You have truly changed my life :) Thank you, thank you. I've been struggling, as a new OO developer, to get a handle around what the purpose of Interfaces are. You made it very easy to understand.

[@IAmTimCorey]

Awesome!

[@FredrikBergman]

Thanks Tim for creating all these well explained and easy to understand videos. They are really great!!

[@IAmTimCorey]

I appreciate the kind words.

[@parko1965]

My god, after watching this, it's just clicked...thanks Tim Corey.

[@IAmTimCorey]

Awesome!

[@tresaidh3y90]

This was the best explanation on interfaces I have ever received. Thank you Tim for all your videos you are really helping me launch my career as a .C# dev

[@IAmTimCorey]

You are welcome.

[@timsaelee3526]

Wow, the only video that made me understand interfaces...I went through like 4 other videos until I found yours. THANK YOU!!!!

[@IAmTimCorey]

Awesome!

[@TonyVirili]

Excellent explanations. Terrific examples. I'm much more confident in knowing when to use these. Thank you!

[@IAmTimCorey]

Awesome!

[@nrich4704]

Great. After years of attempted explorations I finally get it. Thx

[@IAmTimCorey]

Excellent!

[@danielhadad4911]

Thanks! This was enlightening.

[@IAmTimCorey]

Glad it was helpful!

[@Robotron2084Guide]

Thank you so much for these well-thought out and properly paced demo discussions. It helps the abstract feel much more concrete when creating mental images for coding. thanks again! Greatly appreciate your content.

[@IAmTimCorey]

You are welcome.

[@mazingguitar]

Wow Tim. Your materials are real eye-openers. Came here beacuse I had started learning design patterns and could not understand usage of interfaces... Thankfully you made me understand how it works. Subbed. Definitely coming back for more.

[@IAmTimCorey]

Great to hear!

[@gregoWahoo]

OMG Somebody that explains tech as well as Scott Allen (or better ;-))! Amazing!

[@IAmTimCorey]

That’s high praise. Thank you.

[@anthonyj777]

Thank you very much, lots of penny's dropped :)

[@IAmTimCorey]

Great!

[@christellannmataac559]

Thank you for another amazing and easy to understand explanation about interface, Tim! I am really struggling a lot about this concept and your video helped me grasping it in general. Looking forward to more of your videos. God bless.

[@dipanjandey23]

Thanks Tim!! This is truly the best explanation on Interfaces I have seen.

[@IAmTimCorey]

Thank you!

[@RalfsBalodis]

0:00 - Intro 1:06 - Demo application walk through 4:50 - Explaining the problem: putting instances of different classes (product models) in one list (shopping cart) 7:54 - Creating an interface 12:53 - Implementing the interface 20:29 - List of type Interface 23:43 - Note about items in the List of type Interface: Accessing a Class member not in the contract (Interface) 25:07 - recap: basics of the Interface 26:48 - Class Inheritance and Interface implementation 29:02 - Extracting the Interface from a Class 31:19 - Interface implementing another Interface 34:54 - Accessing a Class member outside the initial contract (Interface) in the List of type Interface 40:51 - Power of Interfaces: Adding a new Class to your project, why to use Interfaces

[@nicolasm1664]

Have my like sir

[@RalfsBalodis]

@@nicolasm1664 I hope this was helpful!

[@IAmTimCorey]

Thanks! I added it to the description.

[@rajeshpandhare2638]

I tried to learn and understand Interfaces from Paid courses first, but this lecture from Tim, explains it so cool and easy. Thank you Tim, it really helped me to understand the concept and where to use Interfaces.

[@IAmTimCorey]

Glad it was helpful! That is why I do this, to make learning easier.

[@RayGarza-sr3zj]

Hi Tim, I like using Interfaces and I still had a mind-blowing experience after watching this tutorial. Love learning ...

[@IAmTimCorey]

Great to hear!

[@israell9320]

Thanks!

[@IAmTimCorey]

Thank you!

[@BijouBakson]

I used to wonder what IEnumerable was; now I know it's an interface. Oops! Why does it seem like there's always something that I didn't understand? :) Thanks for this one, it helped clear some clouds.

[@goldenwoof]

I've been a developer for years, but am fairly new to C#. One thing I just couldn't get the grasp of was interfaces, no matter how many videos I watched, or how many tutorials I read. Yours made it very clear and easy to understand. Thank you! I'm a new subscriber to your channel and am very much looking forward to checking out your other stuff.

[@IAmTimCorey]

I'm glad it was clear and helpful. Welcome to the channel.

[@RanVargas]

Your videos are always so instructive, and so easy to understand. Thanks for doing this man.

[@IAmTimCorey]

You are most welcome. Thanks for watching.

[@mizantednioc]

Thank you

[@IAmTimCorey]

You are welcome.

[@neveobardo1853]

Hey Tim! I got the idea of how to use Interfaces but i still didn't get why it works different from a super asbtract class with abstract methods. On this example, we could make the ProductModel class and put the abstract statement on ShipItem method. So, in the child classes we would override it to what they should really do. Why use Interfaces on this case?

[@IAmTimCorey]

Couple of reasons. First, an abstract class implies a relationship (object a "is a" b). That isn't always true. Second, you can only inherit from one class. You can implement multiple interfaces. Here is a quick example: IDisposable. This is an interface created by Microsoft. It has one method definition in it: Dispose(). If you implement that class, you can us a "using" statement to ensure that your class gets disposed of properly. Microsoft does not know about your class, yet you can implement their interface and they can call a method on your class. If you tried to do this with inheritance, you could never inherit from another class if you wanted to be disposable.

[@s4p1d3rm4n]

Your video make me see clearly what is interface is for how to use and how to implement it, I've google for clear explenation about interface and none of them is as clear as yours. Thanks again for making this video

[@IAmTimCorey]

Glad it helped!

[@m.t2245]

Amazing Explanation (Best so far) , especially the why part.

[@IAmTimCorey]

Thank you!

[@steamerSama]

If we are going to cast the object as IDigitalProductModel to access TotalDwnloadsLeft anyways at 36:00, we might as well store List and cast to IProductModel as well. If we think of a case where there are 10 interfaces implementing IProductModel, then we'd have 10 _if (prod is InterfaceImplementingIProductModel variableName)_ assignments, more in some cases. Shouldn't there be a better way to encapsulate this logic inside the interface themselves?

[@steamerSama]

I kind of figured out why this example has been bothering me. Technically speaking, IProductModel should actually be IPhysicalProductModel, because it's ShipItem is unique to physical products and therefore should not be a part of the base interface. The base should have something like _void Checkout();_ and IPhysicalProductModel should implement it as ShipProduct() and IDigitalProductModel should implement it as DownloadItem() or somewhere along those lines. This way, the list can have the base class, just call Checkout() on it and each item would call it's defined method. interface IProduct { string name { get; } void Checkout(); } interface IPhysicalProduct : IProduct { void ShipItem() { Debug.Log("Shipping item"); } void IProduct.Checkout() { ShipItem(); } } interface IDigitalProduct : IProduct { int totalDownloadsLeft { get; } void DownloadItem() { Debug.Log($"Downloading item; {totalDownloadsLeft} downloads left"); } void IProduct.Checkout() { DownloadItem(); } } class PhysicalProd : IPhysicalProduct { public string name { get { return "productName"; } } }

[@drift9365]

Hi Tim, been struggling to understand Interfaces, this is about my 4th video on them, "it's a contract" doesn't help me :). Would it be correct to say "an interface is structural inheritance, that adds a layer of abstraction"?

[@IAmTimCorey]

It sounds like you are thinking too abstractly. An interface says that a class will have certain attributes. When you use an interface as an object type, you will only have access to those attributes because that is all you know that it has. However, that is a benefit because any class can go into that object, as long as it implements the interface. The IDisposable interface is probably the best example of this. That interface says that a class implementing it will have a Dispose method. That's it. The benefit of having that interface is that we can then apply it to any class we want and know that the class will have a Dispose method. We use this in using statements. For example, we might have this code: using (MyClass test = new MyClass()) { } Now, when the code inside the curly braces completes, the "using" statement will call the Dispose method in our "test" object. How does it know to do that? Because it implements IDisposable (otherwise, we cannot use the using). Microsoft wrote the using statement. They didn't write your code. Yet, Microsoft's code can call your code's Dispose method because it implements an interface Microsoft created. That's why a contract is the most common way of visualizing it. By implementing IDisposable, you are agreeing that you will have a Dispose method. That is the contract you are signing.

[@rconte]

Let me state my vision on that (does not mean it is correct, just the way I see it... A property is something like... (3 below) public string Make {get; set;} public string Model { get; set; } public string Year { get; set; } A method is something like... public bool Equals(Car car) { return (this.Make, this.Model, this.Year) == (car.Make, car.Model, car.Year); } A method SIGNATURE is something like... public bool Equals(Car car) (looks like a method without the instructions, just the header...) An interface is a set composed of --some properties and --some method signatures, for example public int xxxx {get;set} public bool yyyy {get,set} public string zzzz (int number) Normally with a name starting with I, say ISomething (but not a must) One may write classes (ex Class CCC) that implement the properties and the methods, witch means it has the same properties and method WITH the "signature" (top line) of the interface. Thats free. (and nobody needs to know) Besides that, if you write "CCC : ISomething" - the name of an interface after the class name, the C# compiler will now you intend to implement the interface ISomething and check for that to help you do it right, and complain if there is some properties or some methods missing... By the way, this is one common thing to do... And what is the use for that? Well one example is the example in the video above... In a nutshell you write code referring to the named interface (ISomething) and that named interfaced is replaced by classes that the compiler knows (you told it) that implements it... ----------------------------- Bur remember, this is only my view of the thing that I dare to show. I hope those that in fact know the subject may correct it... until then...

[@rconte]

One more note: I did not know what an interface was until I saw this video, maybe I still do not know...

[@TomRaf]

Very good explained! I've been learning C# for a long time with some breaks, mostly by reading a MS Visual C# Step by Step book. It's pretty good written but sometimes it's just not enough and there comes the video tutorials like this one. Thanks :)

[@IAmTimCorey]

I am glad it was helpful.

[@Barrosy]

42:25 I like how you explain this very basic principle and a lot of new (and even experencied) developers could learn from this.

[@IAmTimCorey]

Thank you!

[@ardavaztterterian1211]

Great explanation. You are one of the few tutors who are clear in their explanation. Thank you, but why your online courses are so damn expensive? It would be amazing if you do some sales or provide sale coupons.

[@IAmTimCorey]

I do offer sales to my mailing list members and occasionally to the public as a whole once or twice a year. The cost is relative. If you are looking at it compared to a free course or a $10 Udemy course then yes, they are expensive. If you are looking at them in comparison to a college course then they are cheap. My Foundation in C# course series is essentially about 3 semesters worth of teaching (two and a half if you pushed it). Besides, not all training is created equal, just like not every car is the same. Just because a run-down Ford has four wheels and goes when you push the gas doesn't mean that a Mercedes should cost the same because it too has four wheels and goes when you push the gas.

[@fordfiveohh]

Because he doesn't suck .... And this one video probably saved me 5 hours of spinning....

[@abenetbeyene3398]

why i like Mondays..Thanks to you

[@IAmTimCorey]

Awesome! I'm glad I can improve Mondays for you.

[@harag9]

Agree, it's the best thing about Mondays ! Thanks Tim!

[@gunjanshrestha782]

This is great. I have used interface but wasn't always clear on the conceptual level, but your example helped me understand interface more clearly. Thank you!!

[@IAmTimCorey]

Awesome!

[@MJPlaysMusicForYou]

Your doing gods work! Basically just mastered interfaces in one video.

[@IAmTimCorey]

Excellent!

[@HaileeMiu]

minute 21:00 - "OOOOOOOOOOOOOOO" finally I get a real like reason for interfaces

[@IAmTimCorey]

Excellent!

[@rinzler9775]

A lot of people will jump to inheriting from a base class rather than use an interface, the more you inherit, the harder the code gets to maintain as a general rule..

[@IAmTimCorey]

Correct.

[@matterece3164]

Thanks! Because of this video I now understand what's the point of interfaces in creating applications

[@IAmTimCorey]

Awesome!

[@beri4138]

Excellent video. I only watched half of it and was already able to implement an interface in my game.

[@IAmTimCorey]

Awesome!

[@Mohie.Essingery]

Wouldn't if(prod is DigitalProductModel digital) works too? Without needing an extra interface 35:40

[@IAmTimCorey]

It would work, but you would then be hard-coding it to only work with DigitalProductModel. If you had mocked an IDigitalProductModel for unit testing, it would not work here.