Keyword 'this' (Me) is not available calli

Question

In the inherited class I use the base constructor, but I can't use the class's members calling this base constructor.

In this example I have a PicturedLabel that knows its own color and has an image. A TypedLabel : PictureLabel knows its type but uses the base color.

The (base) image that uses TypedLabel should be colored with the (base)color, however, I can't obtain this color

Error: Keyword 'this' is not available in the current context`

A workaround?

/// base class
public class PicturedLabel : Label
{
    PictureBox pb = new PictureBox();
    public Color LabelColor;

    public PicturedLabel()
    {
        // initialised here in a specific way
        LabelColor = Color.Red;
    }

    public PicturedLabel(Image img)
        : base()
    {
        pb.Image = img;
        this.Controls.Add(pb);
    }
}

public enum LabelType { A, B }

/// derived class
public class TypedLabel : PicturedLabel
{
    public TypedLabel(LabelType type)
        : base(GetImageFromType(type, this.LabelColor))
    //Error: Keyword 'this' is not available in the current context
    {
    }

    public static Image GetImageFromType(LabelType type, Color c)
    {
        Image result = new Bitmap(10, 10);
        Rectangle rec = new Rectangle(0, 0, 10, 10);
        Pen pen = new Pen(c);
        Graphics g = Graphics.FromImage(result);
        switch (type) {
            case LabelType.A: g.DrawRectangle(pen, rec); break;
            case LabelType.B: g.DrawEllipse(pen, rec); break;
        }
        return result;
    }
}

Answer 1

I think as a workaround I will implement this as below:

public class PicturedLabel : Label
{
    protected Image
    {
        get {...}
        set {...}
    }
    ............
}

public class TypedLabel : PicturedLabel
{
    public TypedLabel(LabelType type)
       :base(...)
    {
       Type = type;
    }
    private LabelType Type
    {
      set 
      {
         Image = GetImageFromType(value, LabelColor);
      }
    }
}

EDITED: I make the Type property private for this context, but it can also be public. In fact you can make Type and LabelColour public and whenever user change any of these properties, you can recreate the image and set it to your base class so that you can always guarantee a representative image is used in the picture box

Answer 2

This error does make a lot of sense.

If you were allowed to use this in that way there would be a timing problem. What value do you expect LabelColor to have (ie, when is it initialized)? The constructor for TypedLabel hasn't run yet.

Answer 3

You are trying to access to a member that isn't initialized yet. this.LabelColor calls to the base class member that isn't available : you haven't called the base class constructor yet when you write : base(...)

 public TypedLabel(LabelType type)
        : base()
    {
        pb.Image = GetImageFromType(type, this.LabelColor);
    }

Answer 4

The property LabelColor is not initalized at the moment so it would be null. In fact, "this" is not initalized at that moment because the base constructor gets called before initalizing "this" which is why calling "this" cannot be done.