C#

public void myPanel_Paint(object sender, PaintEventArgs e) {
    Graphics g=e.Graphics;
    g.DrawImage(myImage, 0, 0);
    int w=myImage.Width;
    int h=myImage.Height;
    int GRIDSTEP=20;
    for (int x=0; x<w; x+=GRIDSTEP) g.DrawLine(Pens.Yellow, x, 0, x, h);
    for (int y=0; y<h; y+=GRIDSTEP) g.DrawLine(Pens.Yellow, 0, y, w, y);
}

Image myImage=Image.FromFile(filepath);

private void myPanel_MouseMove(object sender, MouseEventArgs e) {
    color=myImage.GetPixel(e.X, e.Y);
    myLabel.Text="Color at ("+e.X+", "+e.Y+") = "+color.ToString();
}

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        Bitmap DrawArea;
        Bitmap myImage;
        public Form1()
        {

            DrawArea = new Bitmap(pictureBox1.Size.Width, pictureBox1.Size.Height);
            pictureBox1.Image = DrawArea;
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            
        }

        private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                OpenFileDialog open = new OpenFileDialog();
                open.Filter = "Image Files(*.jpg; *.jpeg; *.gif; *.bmp)|*.jpg; *.jpeg; *.gif; *.bmp";
                if (open.ShowDialog() == DialogResult.OK)
                {
                    pictureBox1.Image = new Bitmap(open.FileName);
                    Image myImage = Image.FromFile(open.FileName);

                }
            }
            catch (Exception)
            {
                throw new ApplicationException("Failed loading image");
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            Graphics g = Graphics e;
            g.DrawImage(myImage, 0, 0);
            int w = myImage.Width;
            int h = myImage.Height;
            int GRIDSTEP = 20;
            for (int x = 0; x < w; x += GRIDSTEP) g.DrawLine(Pens.Yellow, x, 0, x, h);
            for (int y = 0; y < h; y += GRIDSTEP) g.DrawLine(Pens.Yellow, 0, y, w, y);
        }

Daniel Grunwald wrote:

What your code is doing is:
- read a
- read b
- perform the comparison

Any other thread may write between those two reads. Being "atomic" doesn't apply to the comparison as it only deals with local values.

Daniel Grunwald wrote:

There is no Interlocked.ReferenceEquals, you're just calling the inherited Object.ReferenceEquals.

Daniel Grunwald wrote:

AFAIK there is no way to read two references atomically (so that no other thread can write between the two reads).

lock(myLock) {
    if (a==b) doSomething();
}

void Button_OnClick(object sender, eventargs args)
{
  //......
}

void Button_OnClick(object sender, eventargs args)
{
  Button buttonClicked = sender as Button;
  if(buttonClicked == null)
    return;
  string buttonText = buttonClicked.Text;
  // use buttonText here or wherever....
}

private void ButtonEventHandler(object sender, EventArgs e)
{
  Button button = sender as Button;
  if (button == null) return;
  txtClicked.Text = button.Text;
}