Covariance and contravariance.

covariance-and-contravariance.cs

static void SetObject(object o) { }

void Main()
{
    // Assignment compatibility.
    string str = "test";
    // An object of a more derived type is assigned to an object of a less derived type.
    object obj = str;

    // Covariance.
    IEnumerable<string> strings = new List<string>();
    // An object that is instantiated with a more derived type argument
    // is assigned to an object instantiated with a less derived type argument.
    // Assignment compatibility is preserved.
    IEnumerable<object> objects = strings;

    // Contravariance.
    Action<object> actObject = SetObject;
    // An object that is instantiated with a less derived type argument
    // is assigned to an object instantiated with a more derived type argument.
    // Assignment compatibility is reversed.
    Action<string> actString = actObject;

    // Covariance for arrays enables implicit conversion of an array of a more
    // derived type to an array of a less derived type. But this operation is
    // not type safe.
    object[] array = new String[10];
    // The following statement produces a run-time exception.
    // array[0] = 10;
}