Question

I have a compiler error when trying to set a generic base collection class member in this derived class.

error CS0029: Cannot implicitly convert type 'System.Collections.Generic.List<IntSegment>' to 'System.Collections.Generic.List<T>'

Here’s the outline of my generic collection

public class Path<T> : IEnumerable<T> where T : Segment
{
    private List<T> segments = new List<T>();

    public List<T> Segments
    {
        set { segments = value; }
        get { return this.segments; }
    }

    public Path()
    {
        this.Segments = new List<T>();
    }

    public Path(List<T> s)
    {
        this.Segments = s;
    }
}

A derived generic class of this collection is then defined for a derived class IntSegment of Segment (for which the base collection is defined)

public class IntersectionClosedPath<T> : Path<T>, IEnumerable<T> where T : IntSegment
{
    public IntersectionClosedPath(List<IntSegment> inEdges)
        : base()
    {
        Segments = inEdges;
    }
}

I can’t understand why this assignment is not allowed. (I don’t need to make a deep copy of the incoming List).

Answer 1

Change List<IntSegment> inEdges to List<T> inEdges and it will work. The problem is that Segments is known as a List<T>, where T : IntSegment, while inEdges is a List<IntSegment>. (For reasons I’ll not go into here unless you ask, such an assignment is not allowed. Look up variance/covariance/contravariance if you’re interested.)