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C# Interface Interview Questions and Answers

C# Interface Interview Questions and Answers
May 27, 2023
23 minutes read

Are you ready to strengthen your knowledge of C# interfaces and prepare for your next big interview? Look no further! In this article, we will dive into some of the most challenging and intriguing interface and abstract class interview questions in C# that even experienced developers may find thought-provoking.

Covering a range of topics, these interface C# interview questions will provide you with a deeper understanding of the core concepts and practical aspects of using interfaces in C#.

So, whether you’re a seasoned professional or a determined learner expanding your knowledge, this collection of interface in C# interview questions will give you the confidence and insight to excel in any programming situation.

In C#, can an interface have a constructor? How does this differ from a class constructor, and what are the implications for initializing an interface?


No, an interface in C# cannot have a constructor. Constructors are used to initialize the state of an object when it is created. Since interfaces do not have a state and only define a contract with method signatures, there is no need or reason to have a constructor for an interface. In contrast, a class constructor is used to initialize the object’s state and set default values for its properties when the object is created.

The implication of not having constructors in interfaces is that you cannot create an instance of an interface directly. You can only create an instance of a class that implements the interface, which will then initialize the object according to the constructor of that class.

Explain the concept of explicit interface implementation. What is its purpose, and what are possible use cases for this technique?


Explicit interface implementation is a technique in C# that allows a class to implement an interface method in such a way that it is only accessible through the interface itself and not through an instance of the class directly. This is done by specifying the fully qualified name of the interface along with the method signature in the class.

The main purpose of explicit interface implementation is to provide a way for classes to have multiple interfaces with conflicting method names/signatures without causing collisions or having to redefine the method names. This makes it easier to deal with scenarios where a class needs to implement multiple interfaces that share similar method names.

Some possible use cases for explicit interface implementation are:

  • When a class implements multiple interfaces with the same method name, but the method’s behavior should be different for each interface.
  • To avoid exposing a method that’s meant to only be used in the context of an interface.
  • To have greater control over the visibility of methods in a class, by forcing clients to access methods only through a specific interface type.

How can you implement multiple inheritance in C# using interfaces? Describe a scenario where this would be helpful within a real-world application.


While C# does not support multiple inheritance through classes, it does allow multiple inheritance through interfaces. You can achieve this by implementing multiple interfaces in a single class.

To implement multiple inheritance using interfaces, simply separate the list of interfaces to be implemented by a class using a comma.

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In this example, FlyingCar implements both IFlyable and IDriveable interfaces, thus effectively achieving multiple inheritance.

Real-world scenario:

Suppose you are designing an application that simulates various types of transportation vehicles. Some vehicles can fly, some can drive, and some can do both (like a flying car). Using interfaces to model these capabilities allows you to create classes that inherit multiple behaviors, making it easier to create flexible and extensible vehicle types.

What’s the difference between an abstract class and an interface in C#? In what situations would it be more appropriate to use an abstract class instead of an interface?


An abstract class and an interface in C# have some similarities and differences:

  • Both define a contract with a set of methods that must be implemented by classes that inherit them.
  • Abstract classes can have state (fields, properties), while interfaces cannot have state.
  • Abstract classes can have fully implemented methods, while interfaces only had abstract method signatures prior to C# 8.0 (now they can have default implementation for methods).
  • A class can only inherit from one abstract class, but it can implement multiple interfaces.

In general, you may want to use an abstract class instead of an interface if:

  • You want to provide a default implementation for some (but not all) methods, so derived classes can override or extend them.
  • You need to enforce common behavior across similar classes sharing some state (fields, properties) or implementation details.
  • You want to utilize constructor logic for initialization.

On the other hand, an interface would be more appropriate when:

  • You want to define a contract with method signatures and no state or implementation details.
  • You need to achieve multiple inheritance and support implementing more than one interface in a class.
  • You want a more flexible and loose coupling between components, allowing for easy substitutability and adherence to the dependency inversion principle.

How does the introduction of default interface methods in C# 8.0 affect the compatibility and usage of interfaces in the language? What possible issues might arise from this feature?


With C# 8.0, default interface methods were introduced, allowing the interfaces to provide a default implementation for a method. This makes the interfaces more flexible, as they can now have a combination of abstract method signatures and default implementations, making it easier for classes to implement the interface without breaking existing code.

However, there are some potential issues that might arise from the introduction of default interface methods:

  • Versioning Issues: This feature may cause challenges while versioning interfaces. Adding a new method with a default implementation might cause conflicts if a class implementing the interface already has a method with the same signature.
  • Diamond Problem: The introduction of default interface methods can lead to a form of the diamond problem in multiple inheritance, where the class can inherit two conflicting implementations of the same method from different interfaces. While C# does provide rules for method resolution to avoid this problem, it can still lead to confusion for developers.

Despite these potential issues, the introduction of default interface methods has increased the flexibility and capabilities of interfaces in C#, making it easier to work with them in a variety of scenarios.

Now that we’ve delved into the nuances of default interface methods and their possible implications, let’s shift gears and explore some potential issues that may arise from implementing multiple interfaces in a class.

With these impactful interface interview questions for experienced in C# developers, you’ll be well-prepared to address naming conflicts and code optimizations in your real-world projects.

Consider a scenario where you have two interfaces, both with the same method signature. How do you resolve method naming conflicts arising from implementing both interfaces in a class?


To resolve method naming conflicts in C# when a class implements two interfaces with the same method signature, you can use explicit interface implementation. By using an explicit interface implementation, you can provide a separate implementation for each interface method, even if they have the same signature.

Here’s an example:

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In this example, MyClass implements both IA and IB, and uses explicit interface implementation to differentiate between the DoSomething method implementations for each interface.

How would you refactor code that uses multiple inheritance with interfaces so that it adheres to the Interface Segregation Principle?


To refactor code that uses multiple inheritance with interfaces and adhere to the Interface Segregation Principle (ISP), follow these steps:

  • Identify the clients: Identify all the clients (e.g., classes or other components) that depend on the given interfaces.
  • Analyze client needs: Analyze the specific needs of each client and determine which interface methods they rely on.
  • Split the interfaces: Break down the original interfaces into smaller, more focused interfaces that are tailored to the individual functionality needed by each client.
  • Implement the new, smaller interfaces: Update the implementation classes to use the new, smaller interfaces instead of the combined interfaces.

By following these steps, you can ensure that the interfaces are smaller, more focused, and catered to the specific needs of each client, which is the core principle of the Interface Segregation Principle.

How can you utilize extension methods to either enhance or modify the behavior of interfaces without touching the underlying interface code? Additionally, what possible pitfalls should be avoided?


Extension methods allow you to add new methods to existing types (classes or interfaces) without having to modify the original type or create a derived type. They’re static methods in a static class that appear as if they were instance methods on the extended type.

To utilize extension methods to enhance interfaces:

  • Create a static class to hold the extension method(s).
  • Define a static method with the target interface type as the first parameter, preceded by the this keyword.

For example, if you have an IFlyable interface and want to add a method called PrintFlyingStatus:

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Now you can use the PrintFlyingStatus method on any object that implements IFlyable without modifying the interface itself:

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However, be aware of potential pitfalls when using extension methods with interfaces:

  • Overriding Issues: Extension methods cannot be overridden since they are static methods, and they can be hidden by instance methods with the same signature. Be cautious when adding extension methods to not conflict with existing methods in implementing classes.
  • Discoverability: Extension methods may be less discoverable than inherent methods in interfaces since they are not directly visible in the original interface definition. It’s essential to provide clear documentation to assist developers in understanding the available extension methods.

Despite these potential pitfalls, extension methods can be a powerful way to enhance or modify the behavior of interfaces without modifying the original interface code.

In C#, why doesn’t the ‘is’ and ‘as’ operator support casting to and from interface types? What alternatives should be used?


The is and as operators in C# actually do support casting to and from interface types. You can use these operators to perform type checks and safe casting to interface types. The is operator checks if an object can be cast to a specified type (including interfaces) and returns a boolean result. The as operator attempts to cast an object to a specified type (including interfaces) and returns a value of the specified type if successful or null if the cast fails.

Here’s an example using both is and as operators with an interface:

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Can you enforce the ordering of interface implementations within a class? If not, how can you work around this limitation?


In C#, there is no built-in way to enforce the ordering of interface implementations within a class. The interface methods and properties do not have an inherent order that needs to be followed when implementing them. However, there are other ways to document the desired ordering:

  • Comments and Documentation: Document the desired order of interface methods with XML comments and external documentation. This makes it easier for other developers to understand the expected order and implement the interface accordingly.
  • Extension Methods: Use extension methods to provide a default order of execution for interface members. By creating extension methods for an interface, you can define the desired calling sequence without needing to enforce a strict order within the implementing class.

Another approach would be to use a different design pattern that better supports ordering, such as the Chain of Responsibility or the Template Method pattern.

As we continue our journey through these C# interface interview questions, we’ll explore more advanced concepts such as covariant and contravariant type parameters in generic interfaces.

Armed with an understanding of enforcing interface implementation ordering and the ‘in’ and ‘out’ keywords, you’ll feel confident in your ability to design interfaces that make a lasting impact on your application’s architecture and functionality.

Why do we need covariant and contravariant type parameters for generic interfaces in C#, and how do you achieve this using the ‘in’ and ‘out’ keywords?


Covariant and contravariant type parameters in C# fulfill an essential purpose for generic interfaces: they enable the flexibility to assign more derived types (covariant) or less derived types (contravariant) to generic type parameters when implementing interfaces or invoking methods.


  • Allows a method to return a more derived type than specified by the generic type parameter.
  • Implemented using the out keyword in the interface definition.


  • Allows a method to accept a parameter of a less derived type than specified by the generic type parameter.
  • Implemented using the in keyword in the interface definition.

Here’s an example of both covariance and contravariance in a generic interface:

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In this example, ITransformer<TSource, TResult> uses an in keyword for the contravariant TSource type parameter and an out keyword for the covariant TResult type parameter. The different implementations show how covariant and contravariant type parameters provide flexibility when assigning derived or less derived types to generic parameters.

How would you design and implement a custom C# interface that supports both IDisposable and IComparable under the same class hierarchy?


To design an interface that supports both IDisposable and IComparable in the same class hierarchy, you can create a new interface that inherits from both IDisposable and IComparable. This combined interface would inherit all the methods and properties from IDisposable and IComparable and can be implemented by a class to provide the required functionality.

Here’s an example:

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In this example, the IDisposibleAndComparable<T> interface inherits from both IDisposable and IComparable<T>. The CustomType class implements this combined interface and provides the required implementation for both Dispose and CompareTo methods, supporting IDisposable and IComparable functionality.

Explain the concept of an adapter pattern, and how can you use this pattern with interfaces to make two incompatible classes work together seamlessly in C#?


The adapter pattern is a structural design pattern that allows two unrelated, incompatible interfaces to work together by introducing an intermediate object (the adapter) that acts as a bridge between them. The adapter implements one interface and has a reference to an instance of the other interface, translating the method calls between the two.

You can use this pattern with interfaces in C# to make two incompatible classes work together seamlessly by creating an adapter class that implements one or both of the interfaces and forwards the method calls to a class instance of the other interface. Here’s an example:

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In this example, AlphaClass implements IAlpha and BetaClass implements IBeta. These two classes are incompatible. The AdapterClass implements IAlpha and has a reference to an instance of IBeta. It forwards the MethodA call to the MethodB of the BetaClass.

Now, you can use the AdapterClass in a context where an IAlpha is required, and it will internally call the appropriate method from the BetaClass.

How does the use of interfaces affect the Reflection API in C#? Are there any special considerations to be taken while using Reflection on objects implementing multiple interfaces?


Interfaces in C# have an impact on the Reflection API, which is used to inspect and interact with types, objects, and their metadata during runtime. In general, using Reflection on objects implementing multiple interfaces is straightforward; however, some special considerations should be taken into account.

  • When querying interfaces implemented by a class, the Type.GetInterfaces method returns an array of Type objects representing all implemented interfaces, including inherited interfaces. This information may be useful to inspect which interfaces a class is implementing or to create instances based on interface types.
  • If a class uses explicit interface implementation, the method names would be prefixed with their interface name, separated by a dot (e.g., IInterface.Method). In this case, you might need to search for methods with the appropriate naming scheme.
  • When using the Type.GetInterface method, be aware of namespace and assembly differences, as the method takes a string representing the fully qualified interface name.
  • For reflection-based operations (such as dynamic invocation or property value access), you may need to first cast the object to the respective interface before performing the operation. Otherwise, you might encounter an AmbiguousMatchException if there are multiple methods or properties with the same name across different interfaces or in the class itself.

By taking these considerations into account, you can reliably use the Reflection API with objects implementing multiple interfaces in C#.

What is the term for the phenomenon that occurs when you define a method with the same signature as the method inherited from the interface? How does this affect the way your method is called?


The phenomenon where a method in a class shares the same signature as a method inherited from an interface is known as “interface method reimplementation” or “interface method hiding”.

When a method in a class shares the same signature as an inherited interface method, the following rules apply:

  • If the class method does not include the new keyword or the interface method is not explicitly implemented, the class method implicitly re-implements the interface method. The implementing class will use the method in the class to fulfill the interface contract, and calls to that method through the interface reference will execute the class method.
  • If the class method includes the new keyword, the class method hides the method inherited from the interface, and the class method will not be considered an implementation of the inherited interface method. In such a case, an explicit implementation of the interface method must be provided in the class.

It’s important to pay attention to these cases and implement the inherited methods correctly to ensure the desired behavior.

Having explored the intricacies of method signatures and hiding in relation to interface inheritance, let’s now focus on how to handle thread-sensitive resources while implementing an interface method.

By mastering these intricate scenarios, you’ll have an even more comprehensive understanding of C# interface interview questions, bringing you closer to becoming an expert in the language.

How can you safely implement an interface method within a C# class that accesses thread-sensitive resources? Are there any specific patterns or best practices to follow?


To safely implement an interface method within a C# class that accesses thread-sensitive resources, you must use proper synchronization, locking or other mechanisms to prevent race conditions and ensure data integrity. There are several patterns and best practices to follow:

  • Locking: Use a lock statement with a dedicated synchronization object to ensure that only one thread at a time can execute a specific block of code. For example:

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  • Monitor class: Use the Monitor class to acquire and release locks explicitly. This allows finer-grained control over the synchronization process.

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  • Concurrent Collections: Use the concurrent collections provided by the System.Collections.Concurrent namespace, which are designed for safe and efficient access by multiple threads.
  • Immutable data structures: When possible, use immutable data structures to represent thread-sensitive resources. Immutable data structures cannot be modified after they’re created, which eliminates the need for synchronization and thread safety concerns.
  • Atomic Operations: Utilize atomic operations (e.g., Interlocked) to perform operations on shared resources without the need for a lock, providing better performance under certain scenarios.

By following these patterns and best practices, you can safely implement an interface method within a C# class that accesses thread-sensitive resources while avoiding race conditions and ensuring data integrity.

How do event handling and delegate invocation differ within an interface implementation compared to a concrete class?


Event handling and delegate invocation within an interface implementation and a concrete class mainly differ in terms of the way they are declared and defined.

In an interface, you declare an event using the event keyword followed by a delegate type without providing any accessor logic (i.e., add, remove) or raising the event:

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In a concrete class implementing the interface, you define the event with standard event accessors (if required) and provide code for raising the event when necessary:

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In both cases (interfaces and concrete classes), the event handling process and delegating the invocation to the subscribed methods remain the same. Classes implementing the interface or even inheriting from the concrete class can subscribe and unsubscribe from the event using the += and -= operators or be assigned a delegate method directly, respectively.

In summary, the main difference lies in the way events are declared in interfaces (without accessors or raising logic) compared to concrete classes (with complete event accessors and raising logic). The actual event handling and delegate invocation processes are largely the same for both interface implementations and concrete classes.

Describe the use of ‘variance’ and how it applies to interface implementations with arrays? What are the scenarios where variance might cause runtime exceptions?


In C#, ‘variance’ refers to the ability to use a more derived or a less derived type than originally specified in a generic type parameter. Variance comes in two forms: covariance (more derived types) and contravariance (less derived types). Variance mainly affects arrays, delegates, and generic interface types (as discussed in the previous questions).

In the context of arrays with interface implementations, variance allows you to assign an array of a derived class to an array of an interface type or an array of a less derived class. For example:

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However, there are scenarios where this use of variance with arrays may lead to runtime exceptions:

  • Attempting to store an object of a different type in the array will raise an ArrayTypeMismatchException. For example:

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To avoid such exceptions, always ensure that the object being stored in the array is of a compatible type. You can also utilize generics with proper covariance or contravariance support to reduce the risks of runtime exceptions while preserving type safety.


C#’s support for multiple interface inheritance primarily affects the generated Intermediate Language (IL) code in terms of method resolution and the generated method tables. When a class implements multiple interfaces, the C# compiler generates IL code that includes the necessary metadata for each implemented interface and the corresponding method implementations in the class.

The main performance implications related to multiple interface inheritance are primarily during runtime method resolution:

  • For virtual dispatch, the CLR runtime needs to identify the correct method implementation to call when accessing the method via an interface reference. In cases where the class implements multiple interfaces, it may take slightly longer to resolve the method in the virtual dispatch table.
  • When using explicit interface implementation or when method signatures conflict across multiple interfaces, the compiler generates separate IL code for each explicitly implemented method.

While these performance implications exist, they tend to have a negligible impact on runtime performance in most real-world scenarios. Therefore, C#’s support for multiple interface inheritance should not be considered a performance bottleneck for most applications.

What are the advantages and disadvantages of using method hiding (via the ‘new’ keyword) when implementing an interface within a derived class? How does this technique compare to other methods of method resolution?


The technique of method hiding (using the new keyword) when implementing an interface within a derived class has advantages and disadvantages.


  • Allows you to create a new method in a derived class with an identical signature as the method from the base class or implemented interface, decoupling the method in the derived class from the base class method.
  • Simplifies the design of your classes when there is no need to override the original method, but you still want to provide a different implementation in the derived class.
  • Offers an alternative to implement an inherited interface method with a different behavior.


  • Increases the complexity and inconsistency in the code since both the hidden method and the base/interface method can be called, leading to potential confusion and maintainability issues.
  • Can weaken polymorphism since the hidden method in the derived class does not participate in the base class or interface method resolution when working with base class or interface references.
  • Can lead to bugs in case an object of the derived class is treated as the base class or interface type, causing the original method to be called instead of the hidden method.

In comparison to other methods of method resolution, such as method overriding or explicit interface implementation, method hiding is often considered a less preferred choice. Overriding promotes polymorphic behavior, enabling derived classes to provide alternative implementations for base class or interface methods without increasing code complexity, while explicit interface implementation helps avoid naming conflicts and provides separate implementations for each interface method.

Use method hiding with caution and consider only when it is absolutely necessary to decouple a method from its base class or interface without overriding its behavior.

Congratulations on making it through these challenging interface interview questions in C#! Now you can confidently tackle any topic related to interfaces and abstract classes in your next programming interview or ongoing development projects.

Remember, the key to success is continually refining your skills and deepening your understanding. As you advance in your programming journey, don’t hesitate to revisit these questions and challenge yourself with new problems. By applying the lessons learned here, you’ll be well on your way to becoming a C# master and solving complex problems with ease.

Good luck and happy coding!

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