What is the main difference between in and out modifiers in the context of generic types?

The in and out modifiers in C# are used to indicate covariance and contravariance of generic types:

• out (covariance) allows using a more derived type than specified in the generic. It applies to type parameters that are used only for output (e.g., return values). This allows safely returning more specific types.

• in (contravariance) allows using a more general type than specified in the generic. It applies to type parameters that are used only for input (e.g., method arguments). This allows safely passing more general types.

Example:

interface IEnumerable<out T>  // T is used only for output
{
    T GetItem();
}

interface IComparer<in T>  // T is used only for input
{
    int Compare(T x, T y);
}

Main difference: out — covariance (output), in — contravariance (input).