Explain the difference between covariance and contravariance.

Question

ICSM Computer · Answer

Covariance vs Contravariance in C#

These concepts apply to generic type parameters and determine how you can use inheritance in delegates, interfaces, and generics.

1. Covariance (output)

Allows a method to return a more derived type than specified by the generic parameter.

Works with out keyword
Applies to return types
Preserves assignment compatibility

Example: Covariance with `IEnumerable<out T>`

IEnumerable<string> strings = new List<string>();
IEnumerable<object> objects = strings; // ✅ allowed: string → object

Here:

string inherits from object
IEnumerable<out T> is covariant, so you can assign a more derived type (IEnumerable<string>) to a base type (IEnumerable<object>)

Custom Example:

public interface ICovariant<out T>
{
    T Get();
}

public class CovariantExample : ICovariant<string>
{
    public string Get() => "hello";
}

ICovariant<object> obj = new CovariantExample(); // ✅ Covariance

2. Contravariance (input)

Allows a method to accept a more general type than specified by the generic parameter.

Works with in keyword
Applies to parameter types
Reverses assignment compatibility

Example: Contravariance with `Action<in T>`

Action<object> actObj = o => Console.WriteLine(o);
Action<string> actStr = actObj; // ✅ allowed: object ← string

string is a subtype of object
Action<in T> is contravariant, so Action<object> can be assigned to Action<string>

Custom Example:

public interface IContravariant<in T>
{
    void Set(T value);
}

public class ContravariantExample : IContravariant<object>
{
    public void Set(object value) => Console.WriteLine(value);
}

IContravariant<string> contravariant = new ContravariantExample(); // ✅ Contravariance

Summary Table

Feature	Covariance	Contravariance
Keyword	`out`	`in`
Works on	Return types	Method parameters
Type direction	Derived → Base (`string` → `object`)	Base → Derived (`object` → `string`)
Example interface	`IEnumerable<out T>`	`IComparer<in T>`, `Action<in T>`
Real use case	Reading/producing	Consuming

Real-Life Analogy

Covariance: “I can treat a Cat like an Animal when returning it from a method.”
Contravariance: “I can give an Animal to a method that expects a Cat because the method knows how to handle any Animal.”

Let me know if you'd like:

A diagram to visualize this
Covariance/contravariance in delegates
How this applies to real-world APIs or event handlers

interview

Explain the difference between covariance and contravariance.

Can you answer this question?

1 Answers

Covariance vs Contravariance in C#

1. Covariance (output)

Example: Covariance with IEnumerable<out T>

Custom Example:

2. Contravariance (input)

Example: Contravariance with Action<in T>

Custom Example:

Summary Table

Real-Life Analogy

Example: Covariance with `IEnumerable<out T>`

Example: Contravariance with `Action<in T>`