Use generic type in abstract class constructor

Question

I have a problem similar to this thread but mine is a bit different.

I want to create something like this

public abstract class Plot
{
    protected GameObject plotModel;
    protected IDataPoint[] data;
    protected GameObject[] plotModelInstances;

    protected Plot<TDataPoint>(TDataPoint[] data, GameObject plotModel, Vector2 plotCenter = default) where TDataPoint : IDataPoint
    {
        this.data = data;
        this.plotModel = plotModel;
        plotModelInstances = new GameObject[data.Length];
        this.plotCenter = plotCenter;
    }
}

A base class that takes in a data array of a generic type which implements the interface IDataPoint. The child class is now supposed to be constructed with a data array of a struct that implements this interface

public BarPlot(BarDataPoint[] data, GameObject plotModel, float barWidth = 1, float barHeight = 1, Vector2  = default) : base(data, plotModel, plotCenter) 
    {
        this.barWidth = barWidth;
        this.barHeight = barHeight; 
    }

One answer in the thread linked above said that constructors can't use generics in C# and suggested a combination of a generic class and a static class. However, I don't want a whole class but only one parameter to be generic. Any ideas how to achieve that?

Why does the constructor need to be generic? `data` is simply setting a field, so what's wrong with: `protected Plot(IDataPoint[] data, GameObject plotModel, Vector2 plotCenter = default)`? — Johnathan Barclay, Jul 22 '20 at 12:15
The constructor itself can't use generics directly. Either you make the abstract class generic or you use an array of `IDataPoint` in the constructor. — Zohar Peled, Jul 22 '20 at 12:18
because if I try to pass `BarDataPoint[]` to the `Plot` constructor it says it can't convert `BarDataPoint[]` to `IDataPoint[]`. But I also don't want to pass in `IDataPoint[]` into the `BarPlot` constructor since this wouldn't be the correct type — tadeseus, Jul 22 '20 at 12:20
Does `BarDataPoint` implement `IDataPoint`? If it does, then there shouldn't be a problem. If it does not, generics isn't going to help anyway. — Johnathan Barclay, Jul 22 '20 at 12:24

score 4 · Accepted Answer · answered Jul 22 '20 at 12:27

Your best option is probably something like this:

public abstract class Plot<TDataPoint>  where TDataPoint : IDataPoint
{
    protected GameObject plotModel;
    protected TDataPoint[] data; // Note: Changed IDatePoint[] to TDataPoint[]!
    protected GameObject[] plotModelInstances;

    // Note: Changed IDatePoint[] to TDataPoint[]!
    protected Plot(TDataPoint[] data, GameObject plotModel, Vector2 plotCenter = default)
    {
        this.data = data;
        this.plotModel = plotModel;
        plotModelInstances = new GameObject[data.Length];
        this.plotCenter = plotCenter;
    }
}

And then, in the child class:

public class BarPlot : Plot<BarDataPoint>
{

    public BarPlot(BarDataPoint[] data, GameObject plotModel, float barWidth = 1, float barHeight = 1, Vector2  = default) 
        : base(data, plotModel, plotCenter) 
    {
        this.barWidth = barWidth;
        this.barHeight = barHeight; 
    }
}

Use generic type in abstract class constructor

1 Answers1