What is covariance and contravariance?

Question

Ravi Vishwakarma · Answer

Covariance and contravariance are concepts in C# that deal with type compatibility in generic interfaces, delegates, and method parameters/returns, especially when dealing with inheritance hierarchies.

They allow flexibility in assigning types that are related by inheritance — while maintaining type safety.

Covariance (Output flexibility — `out`)

Allows a more derived type to be used than originally specified.
Applies to return types (output).
Declared using the out keyword (for generic type parameters).
Common in interfaces like IEnumerable<out T> or delegates.

Example:

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

public interface ICovariant<out T>
{
    T GetItem(); // Output only
}

Contravariance (Input flexibility — `in`)

Allows a more generic (base) type to be used than originally specified.
Applies to method parameters (input).
Declared using the in keyword (for generic type parameters).
Common in delegates like Action<in T> or interfaces like IComparer<in T>.

Example:

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

public interface IContravariant<in T>
{
    void SetItem(T item); // Input only
}

Summary Table

Concept	Keyword	Direction	Applies To	Example
Covariance	`out`	Output	Return types	`IEnumerable<out T>`
Contravariance	`in`	Input	Parameters	`Action<in T>`

Real-World Use Case

Covariance

You can assign IEnumerable<string> to IEnumerable<object> because strings are objects:

IEnumerable<string> names = new List<string>();
IEnumerable<object> objs = names; // ✅

Contravariance

A method that accepts object can be used where a method accepting string is expected:

Action<object> log = x => Console.WriteLine(x);
Action<string> logStr = log; // ✅

When to Use

Use covariance when your generic type is used only for return values.
Use contravariance when your generic type is used only for method parameters.

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What is covariance and contravariance?

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1 Answers

Covariance (Output flexibility — out)

Example:

Contravariance (Input flexibility — in)

Example:

Summary Table

Real-World Use Case

Covariance

Contravariance

When to Use

Covariance (Output flexibility — `out`)

Contravariance (Input flexibility — `in`)