Factory Pattern in C# with Dependency Injection 

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You are welcome. Congratulations on the new job!

I am glad it was so helpful.

Thank you!

You're so welcome!

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Thank you!

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Glad you like it!

Glad it helped!

That gets into the Service Locator anti-pattern, where we aren't declaring our dependencies up front. You could do it, but this is structurally a better way of using the DI system so that you declare your dependencies up front.

@@IAmTimCorey Ahh. So much to learn! Thank you for your time and answer ☺️

Thanks!

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You are not alone -- I think the idea of using/passing/injecting a delegate is usually not most people's first thought. Even the idea of passing a method as a parameter, doesn't seem to come naturally. But with time/experience and presence of thought as you code, it's another tool that works in your favor!

Thanks for sharing!

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I am glad it was so helpful.

Great!

Thanks for the suggestion. Please add it to the list on the suggestion site so others can vote on it as well: suggestions.iamtimcorey.com/

Great job practicing what you are learning.

Excellent!

Great!

This is a great comment -- just as most of us (me included) often miss an opportunity to pass a delegate into a method, I feel like the factory pattern is just a larger version of that. Until you are up against a wall of 'how do I inject parameters into a constructor of a class in my DI container', sometimes you just don't think about... hey wait a minute, I could inject a factory that instantiates my class with params, and then just request that factory be injected! It's brilliant, complex and simple at the same time!

Thanks!

You're very welcome!

Awesome!

It has been a while, but I believe we actually wanted them to be instantiated once per call.

It isn't about the size of the application; it is about the needs of the application. For instance, next week I will show off a small application that needs the factory pattern to properly work.

@@IAmTimCorey thanks for taking your time and respond

I don't believe so. Not yet.

If I had a Nickle for every time I hear that...

@@seandavidson see what you did there.

As with anything else in programming, you use it when it is appropriate and don't abuse it. That's why at the end I covered when to use it and how often to use it. I find that I use the factory pattern rarely in development. However, there are times when it is necessary. For instance, I did this video because I've got an upcoming video on WPF dependency injection where I needed to use a factory pattern.

I have to echo Tim here -- maybe you haven't crossed it yet, but there will be a moment where you need to instantiate an object based on some runtime parameters that you just can't wire up in your DI container. The solution will be a factory pattern -- the idea that you can inject a factory class, that will allow you to instantiate an object at runtime, with the params you need to pass it, in order to instantiate an instance of the class you need.

@@codefoxtrot I think this is an important reason of using factory

a registry or static guid's

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Thanks!

We were just talking about the factory pattern here, but the reality is that all three do something very similar: stackoverflow.com/a/13030163/733798

I cover most C# topics, so probably.

@@IAmTimCorey really appreciate series of videos you post. Thank you. Much love from India.

I'm at a conference right now, but I am working on a fix that should be out soon.

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For the factory? Because we don't need multiple instances of it when one will do.

Thank you!

When you use dependency injection in "normal" classes, you use constructor injection. You just ask for whatever thing you want and dependency injection ensures that you get it (even if the class is multiple levels in).

​@@IAmTimCorey That is where I get stuck. The first class where i often manage to get dependencies injected is a controller. If such controller needs to perform some business logic which is done in another class (lets call it Class A), such class can be instantiated in Controller's constructor, taking advantage of the services injected to it. The problem arises if Class A depends on other resources located deeper in other classes. I don't know how to make the DI container "aware" of Class A needing to instantiate a Class B or Class C for that matter. Unless i'm not getting it right and the whole goal of DI is to setup everyting in the constructor of the first place where dependencies are injected, this is, the controller. Thank you, as always :D

The controller would have a dependency on Class A, so it would ask for Class A in the constructor. Class A would have a dependency on Class B, so Class A would ask for Class B in the constructor. And so on it goes. No one instantiates a class on their own. They all just ask the dependency injection system for the class instances they need.

The thing is the term "abstract factory" is not a C#-specific term. It is an industry term for the pattern that we are applying in C#. So, the term cannot reference C#-specific items (like generics) in the name.

Hmmm, not sure in that scenario.

I'm not sure I understand your question. A "factory" is a design pattern that you implement. The name "Factory" in the class name indicates that the class is a factory.

@@IAmTimCorey Thank you for your answer. I found now tree different design patterns. Factory, factory method and abstract factory. I didn’t know difference between factory an factory method. I thought that only two design patterns exists , factory method and abstract factory. I also thought that factory method and factory are the same in your video and you just name it as factory. But I see that factory is third design patter that i didn’t know that exist.

Thank you!

It can be, if you need those features.

They either don't fit with B or B does rely on A. In general terms, it sounds like you actually have a C, which depends on A. Also, if you have a hard dependency and you can't swap it out for something else, put those in the same library and potentially the same class. The separation doesn't actually give you any value. It just adds complexity without a purpose.

@@IAmTimCorey Well, here's the context for what I'm suggesting: I want to make a library for Modeling of Rheology, which describe the way materials behave through models that consist of springs, dashpots and masses. From the model we can calculate a number of properties. Then, I want to make a library about Modeling of Dynamic Systems, that can calculate and simulate dynamic behaviour of structures through simplified models. In theory, the simulation of dynamic behaviour depends on the rheology of the material, but for most practical applications there's a standard model to work with, the Kelvin-Voigt model. As such, you can just give the system some properties that you either measured from experiments or got from literature and solve the system considering the Kelvin-Voigt model through a simple differential equation. But, I think it would be cool, and useful, to have a feature where you could give the rheological model you want to use and the library would generate a corresponding dynamic system with the correct properties. Think about it as a constructor overload, where instead of building the DynamicSystemModel with numerical parameters, instead you pass in a RheologicalModel as an argument. My debate here is that those two libraries do very different things, even if one is of theoretical importance to the other, so I would like to keep them separate. Maybe this is a case where the Dynamics of the Systems does really depend on the Rheology and as such the dependency is justified, but from a practical point of view, most systems are modeled trough the Kelvin-Voigt model, so this would be a relatively fringe use-case. For instance, for a regular simulation using the Kelvin-Voigt model, I probably won't even bother generating a RheologyModel, I'd go directly to the governing equation, since it's more efficient. Of course, I'm still on the planning phase, so I'm putting a lot of thought into it, but thanks for the reply.

You could maybe make library C that depends on both A and B and contains extension methods to link them. Alternatively you can make subclasses of A and B classes in C.

Absolutely.

Nope. I don’t work for Microsoft. I own my own business.

Yes, it will work in .NET 5. Actually, it will work in .NET Core 2.1 and above I believe. Dependency injection (DI) is different than declaring a new. When you instantiate a class, you are specifying which concrete type at that spot. That is a dependency. You always have dependencies in code. However, it is hard to change those dependencies if you instantiate them all over. For instance, imagine if you instantiated log4net everywhere that you needed to log something. Then imagine you wanted to switch over to Serilog. How hard would that be? You would need to change every class in your entire project. You may even need to change class libraries where you don't have the source code. With DI, you could change it in one line. With Factories, we can continue to use DI even when we need to instantiate a class immediately instead of once at the constructor. This is not something we do often, but it is valuable in certain situations. As for performance, the performance is basically the same whether you use DI or not.

@@IAmTimCorey Thank you very much Tim for you response. Did Help me with my questions. I follow your channel and I learn of lot with you. You're excelent teacther. So, that I wanted say was that, when you inject the depencey injection for construtor of traditional way, she stay disponible and consume resources immediately and this control is at the discretion of the container. This way is much better, why we be controlling it and adjusting our form. My question final is: when it comes to windows form, which of the options do you recommend for me to use as default? Example 1, 2 or 3? Remembering that I will work with data. Basically entities, controls, etc. Once again, thank you very much for the reply. I'm talking about Brazil, stay with God!

I would not create a factory for that. Just ask for whichever type you wanted. That way your dependencies are clear. You are asking for the SQL version in one class and the Oracle in another.

@@IAmTimCorey Thank you very much. Have a good day.

Yes, you absolutely can. There's nothing stopping you from creating a factory class, that just contains a series of overloaded constructors, where each constructor takes some number of params and returns (creates/instantiates) an object based on the params you provide. The idea being, you inject your factory class into whatever class that needs it... and from the instance of your factory class, you call the appropriate constructor (with runtime params) that instantiates the object you need.

@@codefoxtrot hey, thanks for the reply. Can you please show me an example code?

@@m5s10 I replied to this days ago, but looks like it was filtered, probably because it contained reference to a github repo. Can you put up a posting on Tim Corey's forum, and I'll be happy to direct you to an example... please reply back when you do so I get a notification!

I personally don't recommend source generators for most situations. You are creating source code that you still have to maintain yet isn't created by you directly. When you need to update the code, you need to update the generator, which means you need to debug an extra layer.

A lambda expression is an anonymous function. It is not a delegate. However, it can be converted into a delegate directly. So a lambda can be a delegate, but doesn't have to be. A delegate can be a lambda, but doesn't have to be. They are very close.

You are welcome.

It isn't a great substitution for dependency injection, but that doesn't mean you are doomed or anything. Software development is about growth. You learn new techniques, and then you improve your development going forward.

@@IAmTimCorey what would you see as the challenge/downside to this approach vs DI? It seems to avoid the “rewrite all code” issues if I want to add/change loggers, allows me to string multiple logging actions together, and I think I could use factories to supply the delegate(s) if I wanted… is it just that this approach leads to the possibility of not passing in what might be a required dependency? In my sample, the logging action or actions are optional, if no logger is passed in the any logging calls just return, but if it were a required dependency…. Maybe “optional dependency” is an oxymoron?

You would need to provide those values to the factory and the factory would need to instantiate it with those values.

@@IAmTimCorey I really appreciate you replying. But looking at, for example, the UserDataFactory at 41:45 (ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-2PXAfSfvRKY.html) I'm assuming I'd need to pass them into the Create() method correct? I'm under the UserDataFactory constructor isn't something I would change as it's populated entirely by DI. And if I'm understanding that correctly, I'd need to do something with it at line 21? var output = _factory() Or if you're aware of another video or resource I may have missed I'm happy to rewatch something to save you the explanation, I just wasn't able to find this answer

I do not understand what "English conversation" means. My courses are not also books, though.

As a person with English far from perfect I remembered funny scene fron Pulp Fiction 😂”English motherducker! Can you speak it!?”

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Thanks for the suggestion. Please add it to the list on the suggestion site so others can vote on it as well: suggestions.iamtimcorey.com/

@@IAmTimCorey I will. Hopefully it was already on your mind :). I would pay for a complete patterns course.

No, the factory pattern is different than the repository pattern.

I'm not sure what you mean.

That's why I included more of the video - you can do more with the factory than just create multiple instances of a class.

@@IAmTimCorey I'm glad you did, it helps a lot.

We don't have a factory of factories. VehicleFactory's job is to return a type of IVehicle. Which instance it returns depends on what the caller wants (Car, Truck, or Van). The end goal of a factory is to create an instance of a requested class. However, just requesting an instance of the interface directly from the constructor won't work for some reason. It may be that we need multiple instances, it may be that we need different concrete types, or it may be that we need the factory to control whether we get a new or recycled instance (like IHttpClientFactory does). The goal isn't to get a factory. The goal is to get the right instance.

@@IAmTimCorey this is a good point. I agree the end user doesn't want to chain calls to create. "factory.Create("Car").Create()" is a bit silly. But I wonder if the vehicle factory should rely on abstract factory to create the car?

Why? How does it add additional value?

@@IAmTimCorey in this example it doesn't but if new Car() needed more setup then maybe the extra layer would be good

There is a lot to thread safety but it sounds like you are well on your way to that.

Yes, it is.

@@IAmTimCorey I watched your video yesterday, and today I had a little time to create the test project and follow along. If I remove the 'class' restriction from 'TImplementation', leaving just 'TInterface', then I get a red squiggly under 'services.AddTransient();' The error is 'The type 'TImplementation' must be a reference type'. So I guess it's not redundant after all. Your videos are great. I really like the way you can explain things in a clear and easy to understand manner.

Ah, good catch. This is more of a compile-time issue rather than a "real" issue.

As you get further into C# and DI (compositional programming), sometimes you find yourself in a scenario where you need to inject constructor parameters, at runitime, that you can't pass when you are wiring up your DI system. It's a rare occasion, but when you need it, you absolutely NEED the Factory Pattern. The idea is that you can add a factory class that can instantiate a new object, based on some runtime parameters that you can fill-in. So, you add the factory class to your DI container, and inject the factory class into any other class that depends on it. Once you inject the factory class, now you can instantiate a new object with whatever runtime params you need, and the factory class returns that created object. It's brilliant, but perhaps you haven't bumped into the need for this pattern just yet. But you eventually will.

It isn't often needed, as Brian mentioned. However, if you watch next Monday's video, you will see one case where it is needed.

No, you just change the appsettings.json file for each implementation. That's the purpose of it - to be externally configurable. When you deploy your application, your build process can change those values. That's what you do to support the Dev/QA/Staging/Prod environments in an automated manner.

@@IAmTimCoreyIn case I have a real abstract factory pattern, how can I inject an abstract factory that creates a family of related products using a parameter obtained from a database? Is it possible to implement this pattern using dependency injection? thank you very much for your work!

Stick with it. If you practice it, sometimes that helps it click.

😀

Sorry, no. I don't cover blockchain.

Patterns are not C#-specific in their names. Design patterns are industry-wide and are named based upon the concept of what they do, not the specific way they are implemented in a language. Here is a link to the description of the abstract factory pattern: www.geeksforgeeks.org/abstract-factory-pattern/

This is a more advanced concept and one that you would benefit from already knowing a lot of the code structures I'm building. It is also one where you would benefit from following along and seeing what the code does yourself.

This code will all work with .NET 8. It will look a little different if you use top-level statements and the new Blazor Web App, but all of the code will still work.

@@IAmTimCorey Somethink has to be missing in Program.cs in ready made Blazor WebApp under .net8 as for some reason the delegate is not trigerred based on your impl - have to check the same ready made example under .net6 and perhaps this will help to figure it out

You have to look at the purpose of my videos to judge their length. It sounds like you want a "just the facts" 5-10 minute video on a topic. I have those. But they don't really prepare you to actually use a topic in the real world. They just give you the start of the information you need. You don't learn how to integrate it into a larger whole, you don't learn about the best practices around the topic, and you don't learn when to use it and when NOT to use it. Those are things that come in my full training videos (like this one). Those can't be short. So yes, I am planning it smarter. My goal isn't to have the most compact video out there. My goal is to prepare my viewers to actually use a topic intelligently in the real world.

Huh?

We had registered multiple IVehicle objects. When you ask for an IEnumerable of an object, DI will give you every object that was registered, not just the last one.

It looks like your local development certificate is expired. I think if you run the following two commands in PowerShell that it will clean it up: dotnet dev-certs https --clean dotnet dev-certs https --trust (from stackoverflow.com/a/70291937/733798 )

This adds all services to dependency injection, but it isn't a factory pattern, and it doesn't solve the problem that the factory pattern solves.