Question

Given the following types:

public interface IPrimary{ void doBattle(); }

// an ISecondary "is" an IPrimary
public interface ISecondary : IPrimary {  }

// An implementation of ISecondary is also an IPrimary:
internal class SecondaryImpl : ISecondary
{
    // Required, since this is an IPrimary
    public void doBattle(){ }
}

Why can I not do this?

List<IPrimary> list = new List<ISecondary>();

This results in the following compilation error:

Argument type ‘System.Collections.Generic.List’ is not assignable to parameter type ‘System.Collections.Generic.List’

I understand the error, and I realize there are workarounds. I just do not see any clear reason why this direct conversion is disallowed. The values contained in a list of ISecondary, should after all, be (by extension) values of type of IPrimary.Why then are List<IPrimary> and List<ISecondary> being interpreted as unrelated types?

Can anyone explain clearly the reasoning for C# being designed this way?

A slightly extended example: I came across the issue when trying to do something similar to the following:

internal class Program
{
    private static void Main(string[] args)
    {
        // Instance of ISecondary, and by extention, IPrimary:
        var mySecondaryInstance = new SecondaryImpl();

        // This works as expected:
        AcceptImpl(mySecondaryInstance);

        // List of instances of ISecondary, which are also, 
        // by extention, instances of IPrimary:
        var myListOfSecondaries = new List<ISecondary> {mySecondaryInstance};

        // This, however, does not work (results in a compilation error):
        AcceptList(myListOfSecondaries);
    }

    // Note: IPrimary parameter:
    public static void AcceptImpl(IPrimary instance){  }

    // Note: List of type IPrimary:
    public static void AcceptList(List<IPrimary> list){  }

}

Answer 1

Why can I not do this? List<IPrimary> list = new List<ISecondary>();

Imagine that you had a method defined like this:

public void PopulateList(List<IPrimary> listToPopulate)
{
    listToPopulate.Add(new Primary());  // Primary does not implement ISecondary!
}

What would happen if you were to pass it a List<ISecondary> as a parameter?

The error that List<ISecondary> is not assignable from List<IPrimary> is the compiler’s way of getting you out of such troubles.