Create an instance of the variable type 'T'

Question

I have a C#-Class Point with two Subclasses ColorPoint and AmountPoint.

Point-Class

public class Point
{
    double x; // Position x
    double y; // Position y

    public Point(double pos_x, double pos_y) // Constructor
    {
        this.x = pos_x;
        this.y = pos_y;
    }
}

public class ColorPoint : Point
{
    double color; // White value (0 to 255)
}

public class AmountPoint : Point
{
    int amount; // Amount of Persons standing at this point
}

Now in my main class I want to create a new Point in the List

This should than look something like this:

public class main
{
    public main()
    {
        List<ColorPoint> colorList = new List<ColorPoint>(4);
        AddPoint<ColorPoint>(colorList);
    }

    public List<T> AddPoint<T>(List<T> pointList)
        where T : Point
    {
        pointList.Add(new T(0, 0)); // DOES NOT WORK (Cannot create instance of variable type 'T')
        pointList.Add(new Point(0, 0)); // DOES NOT WORK (Cannot convert Point to T)
    }
}

The variables coloror amount can be left as null in both cases.

You should define either a parameterless constructor for `Point` class - which it does not make any sense- or define in each of the derived classes a constructor that would take three arguments and call base. If you don't do this, then your code wouldn't compile at all. — Christos, Jul 31 '19 at 09:09
you cannot use `new T(0, 0)` but [there is another way](https://stackoverflow.com/questions/2451336/how-to-pass-parameters-to-activator-createinstancet) — Selvin, Jul 31 '19 at 09:11
@Selvin I would absolutely not recommend doing that. My advice here would be to re-engineer the code above. If you're going to add something to this list, then you should know exactly what type that is beforehand. OPs code is a little smelly. — DavidG, Jul 31 '19 at 09:20

score 1 · Accepted Answer · answered Jul 31 '19 at 09:30

Your code doesn't compile. I can't imagine why you would want to do what you are trying to do. But the closest you can get to a legitimate implementation is:

class Program
{
    static void Main(string[] args)
    {
        List<ColorPoint> colorList = new List<ColorPoint>(4);
        AddPoint<ColorPoint>(colorList);
    }

    public static List<T> AddPoint<T>(List<T> pointList)
        where T : Point, new()
    {
        pointList.Add(new T());
        return pointList;
    }
}

public class Point
{
    double x; // Position x
    double y; // Position y

    public Point() : this(0, 0)
    {
    }

    public Point(double pos_x, double pos_y) // Constructor
    {
        this.x = pos_x;
        this.y = pos_y;
    }
}

public class ColorPoint : Point
{
    double color; // White value (0 to 255)

    public ColorPoint()
    {
    }

    public ColorPoint(double pos_x, double pos_y) : base(pos_x, pos_y)
    {
    }
}

public class AmountPoint : Point
{
    int amount; // Amount of Persons standing at this point

    public AmountPoint()
    {
    }

    public AmountPoint(double pos_x, double pos_y) : base(pos_x, pos_y)
    {
    }
}

Create an instance of the variable type 'T'

1 Answers1