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Explain the difference between covariance and contravariance.

ICSM Computer 182 16-Jun-2025
c# c# 
Updated on 16-Jun-2025
ICSM Computer

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ICSM Computer

ICSM Computer

16-Jun-2025

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
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