Drawbacks to Using Interfaces in C# – How to Balance

In the realm of C# programming, interfaces are a fundamental tool, providing an ability to promote clean code, enforce contracts, and support flexible designs. Interfaces allow developers to define a set of methods, properties, and events without specifying how they are implemented. This abstraction is powerful and can lead to highly modular and maintainable code. However, like any tool, there are also drawbacks to using interfaces in C#.

When used without discretion, interfaces can introduce challenges. Over-reliance or misuse of interfaces can lead to a tangled web of dependencies and unnecessary complexity — not to mention additional bloat in code files. It’s essential to strike a balance and recognize when interfaces are beneficial and when they might be overkill.

I try to ensure that I remain as unbiased as possible in the content I put out and the concepts that I talk about. I certainly am from the camp that feels there are many benefits to using interfaces in C#. However, I felt it necessary to discuss the drawbacks to using interfaces in C# as well in order for it to be fair.

Understanding Interfaces in C#

At its core, an interface in C# is a contract. It declares a set of members that a class or struct must implement, but it doesn’t provide the implementation for any of them (until C# 8…). This allows for a form of abstraction where the “what” is separated from the “how”. For instance, if you have an interface named IDrive, it might declare a method StartEngine but won’t specify how the engine starts. Any class that implements this interface will provide its own implementation of StartEngine.

Interfaces are particularly useful in scenarios where multiple classes share common behavior but might have different implementations of that behavior. By defining an interface, you can ensure that each of these classes adheres to a specific contract, even if the way they fulfill that contract varies.

Another key aspect of interfaces in C# is that they support multiple inheritances, a feature not available with classes. This means a single class can implement multiple interfaces, offering a way to inherit from more than one source.

In summary, interfaces in C# provide a robust mechanism for abstraction, ensuring consistency across classes and allowing for flexible design patterns. However, it’s crucial to use them judiciously to avoid the pitfalls associated with overuse.

Drawbacks of Overusing Interfaces in C#

Increased Complexity

While interfaces are designed to simplify and abstract specific functionalities, overusing them can ironically lead to a more convoluted codebase. When there are interfaces for every minor functionality or behavior, developers can find themselves navigating through a maze of contracts to understand the flow of the application.

Instead of clarifying roles and responsibilities, an excess of interfaces can blur them, making the code harder to read and understand. This same type of issue can happen when refactoring large classes into many tiny classes that each have a single method on them. When things are decomposed into many tiny pieces, it can start to increase the cognitive load for which one to use.

Difficulty in Refactoring

Refactoring is a common task in software development, aimed at improving the internal structure of the code without altering its external behavior. However, when interfaces are deeply intertwined and tightly coupled with multiple classes, refactoring becomes a challenge.

And this probably sounds ironic… One of the greatest benefits of using interfaces in C# is that we can get extensibility and decoupled code! But there are always trade-offs!

Changing one interface might necessitate changes in all classes that implement it, leading to a cascading effect. This tight coupling can hinder the flexibility that interfaces are supposed to provide. You may have to get more creative with versioning the APIs provided by interfaces to avoid this.

Overhead in Implementation

Every interface in C# represents a contract that a class must adhere to. When there’s an over-reliance on interfaces, developers can find themselves spending a disproportionate amount of time implementing numerous interfaces.

This overhead becomes especially pronounced when many of these interfaces aren’t crucial to the application’s core functionality. Instead of focusing on implementing vital features, developers might get bogged down fulfilling interface contracts.

Of course, this may be due to poor design in the interface’s required functionality itself. So this isn’t necessarily an inherent problem with interfaces, but with less-than-ideal design, this can absolutely be one of the drawbacks to using interfaces in C#.

Potential for Misleading Abstractions

Interfaces are meant to provide clear abstractions, delineating specific behaviors or functionalities. However, when used inappropriately or excessively, they can create abstractions that are more confusing than clarifying.

For instance, having multiple interfaces with overlapping members or very similar purposes can mislead developers, making them wonder about the distinctions and the reasons for separate interfaces. This may not come up obviously in green-field projects when starting things fresh, but it can certainly become more apparent when refactoring code bases. Some patterns start to form in different areas for similar but different code, and APIs on interfaces get created without seeing the holistic picture.

Maintenance Challenges

A codebase littered with redundant or unnecessary interfaces poses maintenance challenges. As the application evolves, developers might struggle to determine which interfaces are still relevant and which have become obsolete. Removing or updating outdated interfaces can be risky, especially if they’re implemented across multiple classes.

Additionally, the presence of superfluous interfaces can make the onboarding process for new developers more daunting, as they try to grasp the purpose and relevance of each interface in the system. This point is mostly a different perspective on the earlier points related to overhead and refactoring challenges but still highlights where there can be drawbacks to using interfaces in C#.

Realistic Scenarios: Drawbacks to Using Interfaces in C#

In the software development world, there are numerous instances where the overuse or misuse of interfaces has led to complications. Here are a few examples:

  1. Over-Abstracted Systems: In an attempt to make a system highly modular, a team creates an interface for every single entity, even for functionalities that are unlikely to have multiple implementations. This can result in a codebase that is hard to navigate, with developers often spending more time tracing through interfaces than writing new code.

    If you reflect on some of the drawbacks to using interfaces in C# mentioned earlier, you can see how the overhead can increase dramatically. And if every single entity needed an interface, could you imagine how wasteful that might be for your Data Transfer Objects (DTOs)?!
  2. Tightly Coupled Modules: Consider a project that aims to use interfaces to decouple modules. However, due to the excessive number of interfaces and their tight integration with multiple classes, any change in an interface leads to changes in several classes, defeating the purpose of decoupling.

    Perhaps a plugin architecture was used (along with Autofac, my favorite package for building software with plugin architectures), and the plugin interface was poorly designed. With many plugins, changing the base plugin interface could be a nightmare of effort!
  3. Performance Overheads: In a real-time application, the overuse of interfaces could introduce slight but critical performance overheads. The constant calls to interface methods, especially in loops, can lead to performance bottlenecks.

    For many of us and for the majority of our code, this may be a very low-risk issue. However, when performance is critical, if this is neglected it can certainly be a big contributor to the drawbacks of using interfaces in C#.
  4. Maintenance Nightmares: A legacy system had numerous interfaces, many of which were no longer relevant. New developers on the team found it challenging to understand the purpose of these interfaces, leading to a steep learning curve and potential errors.

Striking a Balance – Avoiding Drawbacks to Using Interfaces in C#

When to Use Interfaces

  1. Multiple Implementations: If a particular functionality is expected to have multiple implementations, an interface is a good choice. For instance, if you have multiple ways to save data (to a database, to a file, to the cloud), an interface can abstract the save operation.
  2. Decoupling Modules: Interfaces are beneficial when you want to decouple modules or components, ensuring that changes in one module don’t adversely affect others.
  3. Unit Testing: Interfaces are invaluable when writing unit tests. They allow for the creation of mock objects, facilitating testing by isolating the component under test. This may be less of a concern when interceptors are available to us in C# 12, but it’s still a valid point.
  4. Defining Contracts: If you’re developing a library or API that will be used by other developers, interfaces can help define clear contracts that other developers can implement.

When to Avoid or Limit Interface Use

  1. Single Implementation: If a functionality is unlikely to have multiple implementations, introducing an interface might be overkill. Might. The value it might offer for a unit test and providing the ability to mock may be worth it, which has historically been my argument most of the time.
  2. Premature Abstraction: Avoid creating interfaces based on what you think might be needed in the future. It’s better to refactor and introduce interfaces when the need arises rather than anticipating all possible scenarios.
  3. Overhead Concerns: In performance-critical applications, be cautious. While the overhead of interface calls is minimal, in tight loops or real-time systems, it can become significant.
  4. Simplicity: If introducing an interface complicates the design without clear benefits, it’s better to opt for a simpler approach. Remember, the primary goal is to write clear and maintainable code.

Summarizing the Drawbacks to Using Interfaces in C#

Interfaces, like many tools in a developer’s arsenal, come with their own set of advantages and potential pitfalls. While they play a pivotal role in crafting modular, maintainable, and scalable software in C#, it’s essential to use them purposefully.

Over-reliance or misuse can lead to unnecessary complexities and challenges in both the development and maintenance phases. As with many aspects of software design, the key lies in striking a balance. By understanding the core principles of interfaces and being aware of the drawbacks of overuse, developers can make informed decisions, ensuring that their C# projects remain robust, adaptable, and efficient.

Always remember: the goal is not just to use a feature but to use it effectively for the betterment of the software and its stakeholders. So while there are certainly benefits, there are also drawbacks to using interfaces in C#.

author avatar
Nick Cosentino Principal Software Engineering Manager
Principal Software Engineering Manager at Microsoft. Views are my own.

This Post Has 4 Comments

  1. Kerry

    Can you drill in a bit on the performance overhead argument above, maybe with an example showing the IL created by some code with an interface compared to code with no interface and how that actually impacts performance? I’ve never heard that particular argument against using interfaces before and I’d be curious to understand more about the overhead.

    1. Nick Cosentino

      I can try to dig some information up on this! Personally, in my own experiences, I have *never* cared about this. I don’t have the benchmarks to prove it, but I think for anything I’ve ever written (in 20 years) it’s never come down to a micro-optimization like this.

      Now, yes, there are optimizations that newer dotnet versions have on things like arrays and List that it cannot perform on just an ICollection, for example… But other than obvious things like that, there’s supposed to be a bit of overhead for the scenario described. There’s an ex-performance architect from Microsoft that has mentioned this multiple times on social media, so I *feel* like there has to be some truth to it 🙂

      I’ll see what I can come up with!

  2. Fred Blogs

    I’d like to point out a couple of problems:
    1. You are making the (sadly, extremely common) mistake of confusing decoupling and loose coupling. If ClassA never ever calls ClassB, either directly or indirectly, they are decoupled. If ClassA calls an interface implemented by ClassB, then they are loosely coupled. If ClassA calls ClassB directly then they are tightly coupled. I think using the terms directly coupled and indirectly coupled would be less confusing, myself. But you cannot, almost by definition, decouple classes by putting an interface in between.
    2. You should not use interfaces to “isolate a component under test”. The “Isolation” in the FIRST acronym is intended to convey the isolation between individual tests (mainly to ensure that the order of execution does not matter). Often people take it to stand for “Independent”, which is somewhat clearer. It is perfectly acceptable, even desirable, that code under test calls other code (that should itself be under test, of course). The idea that you should mock all code not under test was put to the test and thoroughly debunked back in the early/mid 2000s. It leads to
    – tests that pass and code that fails (because the mocks do not accurately reflect the current behaviour of the actual code)
    – flaky and difficult to maintain code, as you pointed out
    – risking having tests that end up only executing the code in mocks, proving nothing
    You should only mock code you cannot execute (eg database calls, web service calls etc.)

    1. Nick Cosentino

      Thanks for the comment!

      1) Apologies for the terminology concerns there. I appreciate the clarity.
      2) I’m going to agree to disagree on this one. For the most part, I can align with it. I think we can write tests that get great coverage, and if the code is written in a particular way, easy to setup + validate without requiring mocks. More and more I find myself writing more test coverage like this, so I mostly align with that. As for “…thoroughly debunked back in the early/mid 2000s” – I led a team of software engineers that took a strong approach to this idea and had a great deal of success. Now, there were *absolutely* other types of tests that we would layer in without mocks but “only mock code you cannot execute” was better framed as “mock code that you want to control for a unit test” for us. So I’m not going to argue that this approach is somehow superior or something, but everything we have as software engineers are tools for us to use. This tool worked well when we used it a particular way, for us. The goal was confidence in our code, and we had a great deal of it and could iterate incredibly fast.

      I do appreciate the thoughts and perspective, so thank you for sharing!

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