Drag and drop rectangles in list

Question

I have made a simple app in c# that lets me draw a rectangle and than move it by mouse. Now I want to draw multiple rectangles I also add them to a list, this also works, but I want to be able to move each rectangle by itself. This goes wrong. I can only move the first rectangle I created. If I try to move the other rectangle the first rectangle teleports to my mouse but only if I click on the second rectangle, if i click anywhere else it crashes with a nullpointer (I know how to fix this but its not the problem) What I cant figure out is why cant I move the second rectangle

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Drawing;
using System.Linq;
using System.Windows.Forms;

namespace TekenTest
{
    public partial class Form1 : Form
    {
        bool isMouseDown;
        List<Item> _Items;
        Item i;

        public Form1()
        {
            InitializeComponent();
            _Items = new List<Item>(); 
            isMouseDown = false;
        }

        private void tekenVel_Paint(object sender, PaintEventArgs e)
        {
            Graphics g = e.Graphics;

            foreach (Item i in this._Items)
            {
                i.drawItem(g);
            }
        }

        private void tekenVel_MouseDown(object sender, MouseEventArgs e)
        {
           this.i = _Items.Find(Item => ((i.X <= e.X && (i.WIDTH + i.X) >= e.X) &&
                                         (i.Y <= e.Y && (i.HEIGTH + i.Y) >= e.Y)));

            i.note = Color.Azure;
            isMouseDown = true;
        }

        private void tekenVel_MouseMove(object sender, MouseEventArgs e)
        {
            if (isMouseDown == true)
            {
                i.X = e.X;
                i.Y = e.Y;
                Refresh();
            }
        }

        private void tekenVel_MouseUp(object sender, MouseEventArgs e)
        {
            isMouseDown = false;
        }

        private void itemToolStripMenuItem_Click(object sender, EventArgs e)
        {
            this.i = new Item();
            this._Items.Add(i);
            this.Refresh();
        }
    }
}

Object class

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace TekenTest
{
    class Object
    {
        public int X
        {
            get;
            set;
        }

        public int Y
        {
            get;
            set;
        }

        public int HEIGTH
        {
            get;
            set;
        }

        public int WIDTH
        {
            get;
            set;
        }

        public Object()
        {

        }
    }
}

Item class

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace TekenTest 
{
    class Item : Object
    {
        public Rectangle rect;
        public String text;
        public Font font;
        public Brush textb;
        public Color note;

        public Item()
        {
            this.X = 200;
            this.Y = 200;
            this.WIDTH = 200;
            this.HEIGTH = 200;
            font = new Font("Arial", 12, FontStyle.Bold, GraphicsUnit.Point);
            text = "Ik ben tekst";
            note = Color.Yellow;
            textb = Brushes.Black;
        }

        public void drawItem(Graphics g)
        {
            this.rect = new Rectangle(X, Y, WIDTH, HEIGTH);
           // g.DrawRectangle(new)
            g.FillRectangle(new SolidBrush(note), rect);
            g.DrawString(text, font, textb, rect);
        }
    }
}

Answer 1

I would suggest a different approach. I would not make the Item a class variable or a property. Every control has a Property named Tag of type object. It can be used for any purposes. What I would typically do is to upon creation set the item as the tag of the represented control. Then In the Move triggers I would handle extract the Item from the Tag property, cast it from object and then manipulate it directly via values.

private void itemToolStripMenuItem_Click(object sender, EventArgs e)
{
    var i = ((Control)sender.Tag) as Item;

    this.i = new Item();
    this._Items.Add(i);

    // you dynamically create a control and set the Tag property
    someControl.Tag = i;

    this.Refresh();
}


private void tekenVel_MouseDown(object sender, MouseEventArgs e)
{
    var i = ((Control)sender.Tag) as Item;
    if(i!=null)
    {
        i.note = Color.Azure;
        isMouseDown = true;
    }
}

private void tekenVel_MouseMove(object sender, MouseEventArgs e)
{
    if (isMouseDown == true)
    {
            i.X = e.X;
            i.Y = e.Y;
            Refresh();
    }
}

You have probably the problem that the instance is not being found by the find method and you are trying to manipulate a null object. This way this is always going to work on a specific object and you you do not have to search for it. It cleans up the code and it runs much smoother.

EDIT Also, I would Suggest renaming your class from Object to something else. just so it does not get confused with the .NET object root type

Answer 2

Problem can be because of in function tekenVel_Paint in foreach loop you are using name for Item the same as for instance variable i . Change it to other like here:

    private void tekenVel_Paint(object sender, PaintEventArgs e)
    {
        Graphics g = e.Graphics;

        foreach (Item obj in _Items)
        {
            obj.drawItem(g);
        }
    }

And similar problem in tekenVel_MouseDown . You also should change name in conditional statement in function Find .

    private void tekenVel_MouseDown(object sender, MouseEventArgs e)
    {
       this.i = _Items.Find(item => ((item.X <= e.X && (item.WIDTH + item.X) >= e.X) &&
                                     (item.Y <= e.Y && (item.HEIGTH + item.Y) >= e.Y)));

        i.note = Color.Azure;
        isMouseDown = true;
    }

Also you don't need isMouseDown variable. You can check if mouse button is pressed using MouseEventArgs e in tekenVel_MouseMove function. On MouseUp event you should set usual color for your active item i and set it to null . And check in OnMouseMove event if i is not null in case if user click inside your control but not inside any object.

Answer 3

I fixed by changing my List searching method. I now the for each loop isnt the best way to go so I will change that later:

        private Item selectItem(MouseEventArgs e)
    {

        IEnumerable<Item> itemQuerry =
        from it in _Items
        where it.X <= e.X && it.WIDTH + it.X >= e.X && it.Y <= e.Y && it.HEIGTH + it.Y >= e.Y
        select it;

        foreach (Item foundItem in itemQuerry)
        {
            this.mItem = foundItem;
        }

        mItem.note = Color.Azure;
        return mItem;
    }