What is covariance and contravariance?

Question

ICSM Computer · 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 IEnumerableandlt;out Tandgt; or
delegates.
Example:
IEnumerableandlt;stringandgt; strings = new Listandlt;stringandgt;();
IEnumerableandlt;objectandgt; objects = strings; // ✅ Covariant: string → object
public interface ICovariantandlt;out Tandgt;
{
    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 Actionandlt;in Tandgt; or interfaces like
IComparerandlt;in Tandgt;.
Example:
Actionandlt;objectandgt; actObj = o =andgt; Console.WriteLine(o);
Actionandlt;stringandgt; actStr = actObj; // ✅ Contravariant: object ← string
public interface IContravariantandlt;in Tandgt;
{
    void SetItem(T item); // Input only
}
Summary Table
Concept
Keyword
Direction
Applies To
Example
Covariance
out
Output
Return types
IEnumerableandlt;out Tandgt;
Contravariance
in
Input
Parameters
Actionandlt;in Tandgt;
Real-World Use Case
Covariance
You can assign IEnumerableandlt;stringandgt; to IEnumerableandlt;objectandgt; because strings are objects:
IEnumerableandlt;stringandgt; names = new Listandlt;stringandgt;();
IEnumerableandlt;objectandgt; objs = names; // ✅
Contravariance
A method that accepts object can be used where a method accepting
string is expected:
Actionandlt;objectandgt; log = x =andgt; Console.WriteLine(x);
Actionandlt;stringandgt; 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?

Anubhav Kumar

Can you answer this question?

1 Answers

Covariance (Output flexibility — `out`)

Example:

Contravariance (Input flexibility — `in`)

Example:

Summary Table

Real-World Use Case

Covariance

Contravariance

When to Use

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